Pollination and fruit set – additional information

Site altitude, aspect and slope

In the UK the most favourable sites are likely to be those at altitudes between sea level and 125 metres above sea level.
Depending on site topography, wind speed tends to increase whilst temperature and sunshine decrease with increasing altitude.
Although it would seem that crops such as sweet cherry grow better on south or south westerly aspects, there is no supporting evidence to suggest that apples are similar in this respect.
A site with a slight slope is recommended, as this allows the steady flow of cold air down the slope away from the trees and gives some protection to their vulnerable flowers during frosty weather at flowering time.
No barriers to this flow of air should be permitted, such as ill-sited windbreaks.
Sites in valley bottoms (often called ‘frost pockets’) should be avoided, as these tend to be prone to frost damage in the spring.
Sites close to lakes or other large bodies of water can reduce the incidence of frost damage.

Meteorological records taken in previous years can aid greatly in site selection.

Particular attention needs to be given to the incidence of frost, likely wind speeds and daytime temperatures during the April and May flowering period.
Temperatures at flowering time of less than -2^oC, (at approximately 0.5m above ground level) cause significant damage to the flowers if sustained for one hour or more.
Although some varieties will set fruits when daytime temperatures are as low as 5^oC (e.g. Falstaff), the majority of scion varieties require daytime temperatures of 12-15^oC for successful pollen germination and growth.

Previous successful fruit production on a site is not always a good guide for the future.

Destruction of woodland or the removal of surrounding plantings of large trees or hedgerows in an area can change its climatic suitability significantly.
Also, global warming is now recognised as a real phenomenon and this may, in future, change the suitability of traditional sites for apple production.
Unfortunately, the anticipated consequences to weather patterns in the UK are less well known, currently.
If future predictions suggest the incidence of stronger winds and/or more spring frosts, even more care will need to be taken when selecting sites.

Summary

Choose a site which is between sea level and 125 metres above sea level.
Choose a site which has a slight slope to allow the escape of cold air flows.
Ensure that there are no barriers (buildings or windbreaks) which impede the movement of cold air off the site and create ‘frost pockets’.
Sites close to large bodies of water tend to be slightly warmer and less sensitive to frost damage.
Choose a site which is sheltered from strong winds.
If available, study meteorological records taken on the site in previous years. Avoid low temperature sites.

Provision of adequate shelter in the orchard

Most sites benefit from planting strategically positioned living windbreaks or the erection of artificial windbreaks. These reduce the speed of wind flows across the site in the spring, so reducing the desiccating and chilling influence on the delicate and vulnerable floral organs. Improved shelter and the associated reduced wind speed and higher temperature also encourage the activity of the essential pollinating insects.

Living windbreaks should ideally be established several years ahead of planting the orchard if the early yields on young trees, that are vital to orchard profitability, are to be secured. When planting living windbreaks, it is recommended that deciduous species, which compete strongly for water and nutrients, such as willow (Salix) and some types of poplar (Populus), are avoided.

Alders are one of the best genera for use as windbreaks in apple orchards and Alnus cordata and Alnus incana are especially suited to the purpose.
Windbreak trees are generally planted in single rows with plants spaced 1.0 to 1.75m apart.
In particularly exposed locations, a double row of trees may be warranted, with the rows spaced 2m apart.

Evergreen species are occasionally included as a proportion of the trees in a windbreak.

These may be of value where the main windbreak species is particularly slow to leaf out in the spring when the protection for flowers is most critical.
However, windbreaks made up entirely of evergreen species (e.g. Chamaecyparis spp) are, like very dense deciduous windbreaks, not to be recommended.
This is because they cause excessive air turbulence both up and down wind of the windbreak.

Artificial windbreaks, made from plastic netting on poles and wire, are only rarely used in UK orchards.

They are relatively expensive and have a limited lifespan.
However, in the absence of living windbreaks or where the latter are too young to provide adequate shelter to young trees, they can be of transient value.

It has always been recommended that windbreaks are planted/erected at approximately 100m intervals around and across level orchard sites so as to provide adequate shelter for pollination, but not impede the escape of cold air flows.

They should be kept regularly trimmed and may be allowed to grow to approximately 7m in height.
A well-managed windbreak will provide maximum shelter for a distance of approximately 5 times its height, but some reduced shelter will be achieved up to a distance of 20 times its height.
On sloping sites, with windbreaks grown across the slope, the base of the windbreak must be kept free of vegetation for a height of 1.5 to 2.0m to allow the unimpeded escape of cold air down the slope in the spring and avoid the creation of mini frost pockets.

Choose species such as alders or hornbeam, which are less competitive for water and nutrients than willows or poplars.
Plant at spacings of 1.0 to1.75m apart in single or, if very good shelter is needed, in double rows. Irrigation and fertigation will improve establishment.
Trim windbreaks regularly and cut to approximately 7m in height.
Plant windbreaks in advance of planting the orchard trees to ensure adequate shelter when the young apple trees begin to flower.
Where living windbreaks are not available use artificial windbreaks.

Frost damage

The nature of frost damage to flowers and fruitlets

At freezing temperatures cells are destroyed in the flower.
Usually the ovule and style are more susceptible to freezing damage than the pollen (Lommel and Greene, 1931).
Also, temperatures just above freezing inhibit the fusion of the nuclei in the embryo sac, resulting in only the egg and not the endosperm being fertilised (Konstantinov, 1960).

Research conducted at Long Ashton Research Station in the 1960s (Williams, 1970b) showed that pre-blossoming frosts could have a very damaging influence upon potential fruit set of Cox apples.

When opening, these flowers appeared undamaged but internal studies showed arrested or abnormal ovule development, leading to an inability to set fruits.
Further work is needed to enable growers to forecast potential problems caused by frosts that occur prior to flowering.

Poor weather conditions shortly before, during and shortly after flowering of apples can cause significant reductions in fruit set and harvestable yield.

Cold temperatures, especially frost, are the most damaging, although consistent strong winds (>10 mph) will inhibit bee foraging and reduce pollination.
Hail, although unusual at flowering time in the UK, can also, if severe, damage blossoms and young fruitlets.

Two types of frost occur in UK orchards – radiation frosts and wind frosts.

Radiation frosts

These occur on cloud-free, dry and relatively wind-free nights when the soil, trees and surrounding vegetation lose heat by long wave radiation.
Cold air builds up from ground level and soon reaches the height of the lowest flowering branches on modern slender spindle trees.
On sloping sites this cold air flows away down the slope and damage can be avoided by facilitating the free flow of this cold air down the slope.
Windbreaks must be positioned so as not to impede this air flow.
In foggy, misty conditions or where there is significant cloud cover this long wave radiation heat loss is reduced and frost damage is less frequent.

Wind frosts

These occur when air at temperatures below freezing is carried onto the orchard site by winds of varying strengths.
They are much more common on hilly sites or close to the coast.
They are best avoided by appropriate choice of site and by the provision of suitable windbreaks.

There are several possible strategies for reducing the worst effects of poor weather conditions. These are:

Avoid sites at high altitudes or the tops of exposed hills where wind frosts are a likely occurrence.
Ensure flows of cold air down slopes and out of orchards in times of radiation frosts.
Do not plant windbreaks which impede this flow.
Orchard management aids to reducing frost damage.
Installation of a frost protection system.
Protecting the trees within canopy structures.
Use of chemical sprays to trees aimed at providing some protection from frost.

Orchard management aids to reducing frost damage

Soil management

If frost damage is to be reduced, the soil should be kept free of weeds and be uncultivated, i.e. firm and be moist down to approximately 15cm.

Most heat loss during radiation frosts (50%-80%) is from the soil surface.
To help in avoiding damage from radiation frosts, keep soil surface free of weeds and grass, keep soil compact and moist.
Although covering the surface during the day with materials aimed at increasing heat absorption and then removing these in the evening is beneficial, it is also much too labour intensive.

Tree management

Although contrary to modern systems of management, growing apple trees taller can reduce radiation frost damage.
A difference of only 30 cm in height in the tree canopy can, on occasions, mean a 1 or 2ºC difference in temperature.

Protecting the trees within canopy structures

The low profit margins usually associated with apple production have deterred growers from protecting trees from frost damage using canopies or polythene tunnels. Also, it is not easy to provide the required amount of heat rise (1-2^oC) under covering materials of low cost.

On cold nights of radiation frosts, radiant heat is lost from the soil in the infra red end of the spectrum.
To be effective in reducing this heat loss, therefore, the covering material should have low transmittance to infra red long wave radiation.
Trials conducted at East Malling in the 1970s showed that whilst polyvinyl chloride (PVC) exhibited low infra red transparency, polyethylene and polypropylene showed high transparency (Hamer et al., 1973) and were, therefore, of little value in this respect.

Trials attempting to reduce frost damage using covers of plastic ‘Rokolene’ netting, erected 3m above ground level, were not very successful (Hamer, 1974).

Firstly, it was necessary to only put the netting in position when frosts were expected, as otherwise light levels to the trees were reduced excessively.
Secondly, although the netting reduced the heat loss from the soil to the surrounding air (by 20% to 30%) and flower buds at 90cm from ground level were slightly warmer than those on trees without nets, at 210cm above ground there were no benefits. This was attributable to the net itself lowering the air temperature at this level.

Protecting apple trees from frost or other poor weather conditions at the time of flowering, by enclosing them within plastic or other structures, is not considered to be economic in the UK.

The costs of applying and removing covers is prohibitively high.
Leaving trees under covers for extended periods of time causes problems of low light levels, atypical growth, poor fruit quality and reduced flower production.
Polythene covers provide almost no protection from frost, as they are permeable to long wave radiation. They may prevent damage from wind desiccation, however.

Suitable pollinating varieties

Most commercial varieties of apple are self-sterile and require pollen from another variety in order to ensure adequate fruit set. Self-fertile clones of Cox are available and a few other varieties show a level of self-fertility, if given favourable climatic conditions.

The attributes of an ideal pollinating variety are:

Compatibility with the main commercial variety.
Synchrony of flowering time with the main commercial variety.
Production of adequate quantities of viable pollen.

Another important factor is the influence of temperatures and stylar receptivity on pollination efficiency.

A few varieties of apple are self-fertile and able to set a crop of fruits with no need for cross pollination. Also, in a few special circumstances, (usually very high spring temperatures during flowering time), certain varieties of apple which are normally self-sterile exhibit a level of self compatibility and are able to set fruits with their own pollen.

Similarly, some varieties of apple are able to set fruit without the need for fertilisation of the female egg cell (ovule) by pollen. Fruit set of this type is known as Parthenocarpic Fruit Set.

Compatibility with the main commercial variety

All commercial varieties of apple are of the species Malus pumila Miller (also sometimes referred to as Malus x domestica Borkh.). All the cultivated varieties are very similar and probably the same species as the wild apples found in the forests of several Eurasian countries. These are often referred to as Malus sieversii.

The majority of varieties are self-sterile (cross compatible) i.e. the pollen of one variety is capable of germination, growth and fertilisation of the flower ovules of the other variety.

There are however some significant exceptions to this rule:

Triploid varieties (e.g. Bramley’s Seedling and Jonagold).
Varieties exhibiting cross incompatibility.
Varieties that are semi- or partially cross incompatible.

It is also necessary to consider the climatic influences on the compatibility/incompatibility relationships.

Triploid varieties of apple

All varieties of apple which have three sets of chromosomes (triploids), rather than the normal two sets (diploids), produce almost no viable pollen and should not be planted as pollen donors for another variety in the orchard.

In most cases, these triploids can be pollinated efficiently by most diploid varieties.
However, recent results would suggest that Golden Delicious could prove a very poor pollinator for Jonagold, Crispin and Belle de Boskoop, whilst Summered will also prove a poor pollinator for Jonagold.

Taking account only of compatibility, and not considering synchrony of flowering times, the best pollinators for Jonagold (amongst those so far characterised) appear to be:

Alkmene, Arlet, James Grieve, Idared and Tydemans Early Worcester.
Many other varieties, such as Golden Delicious, Cox, Fiesta, Gala, Summered, Falstaff and Worcester Pearmain, exhibit partial compatibility with Jonagold.

Pollen compatibility characterisations on Bramley’s Seedling are still incomplete. However, preliminary evidence suggests that no variety is fully incompatible with it. Partial incompatibility with Bramley is shown by:

Idared, Golden Delicious, Fiesta, Elstar and Kent.
Full compatibility with Bramley is shown by Braeburn, Alkmene, Arlet, Cox, Delbard Jubilee, Gala, James Grieve, Worcester Pearmain and Falstaff.

Some popular triploid varieties of apple

Variety Name

Bramleys Seedling

Jonagold

Belle de Boskoop

Ribston Pippin

Gravenstein

Blenheim Orange

Crispin/Mutsu

Sir Prize

Do not use triploid varieties such as Bramley’s Seedling or Jonagold (or its various sports) as pollinators for other varieties.
Do not choose Golden Delicious as a pollinator for Jonagold.

Varieties exhibiting cross incompatibility

Most varieties of apple have two sets of chromosomes (diploids) and are capable of pollinating/fertilising each other, providing they flower at approximately the same time and produce viable pollen.

However, studies at East Malling characterising what are known as the ‘incompatibility alleles’ in apple varieties, have shown that a few varieties are, in most environmental conditions, incompatible with each other.
This means that in most circumstances when planted together they will fail to pollinate each other effectively.
This phenomenon has been recognised in sweet cherry varieties for many years, but it is only recently that it has been noted with apples.
When pollen from one variety is transferred to the stigma of another with which it is incompatible, the pollen tubes fail to grow down the style into the ovary and no fertilisation of the ovule and fruit set is possible.
This work also shows that some varieties may prove only partially efficient as pollinators for each other.

Definitive proof of the incompatibility of apple varieties in the orchard situation is, as yet, lacking.

However, the research suggests that Elstar and Fiesta, in some environmental conditions, will not be able to pollinate each other efficiently.
The same is true for Falstaff, Gala and Greensleeves. Golden Delicious and Jester will also not fertilise each other efficiently.

Even in apparently incompatible combinations fruit set is often able to occur.

For instance, trials have shown that when Fiesta received pollen of another variety, with which it should be incompatible, it was able to set a reasonable crop of fruits.
However, when the reciprocal cross was made, using pollen from Fiesta, no fruit set occurred.

Avoid using ‘incompatible’ varieties to pollinate each other:

Do not choose to plant Elstar and Fiesta (Red Pippin) with the aim of them pollinating one another effectively.
Falstaff, Gala and Greensleeves are not fully compatible and in situations unfavourable to pollination and fruit set should not be planted with the aim of mutual pollination.
Only in situations of warm temperatures at blossom time will the above combinations be successful in pollinating each other.

Varieties which are semi or partially cross incompatible

With any two semi-incompatible or partially compatible varieties, only 50% of the pollen grains produced are able to germinate, grow down the style and set fruit when pollen is transferred between the two varieties.

Cox’s Orange Pippin and its sports are only partially compatible with Alkmene, Elstar, Fiesta, Gala, Kent, James Grieve, and Falstaff.

However, as several of these varieties have been used quite successfully as pollinators in Cox orchards.
This partial constraint is probably only critical when conditions for favourable pollen production and transfer are severely limited by unfavourable weather conditions.

Similarly, Gala and its sports are only partially compatible with Alkmene, Arlet, Cox, Elstar, Fiesta, Golden Delicious, Summered, James Grieve and Worcester Pearmain.

In conditions very unfavourable for pollination these varieties may be slightly less efficient pollinators than fully compatible varieties.

Semi-compatibility usually presents little or no problem to the majority of apple growers.

Studies have shown little difference between the pollen tube growth and fruit set of fully compatible and semi-compatible combinations (Alston, 1996) given reasonable environmental conditions at the time of flowering.
Indeed, semi-compatible combinations of apple varieties have been recommended for planting in areas experiencing warm temperatures and other favourable climatic conditions at flowering time, as they can help avoid overset and biennial bearing and may reduce the need for fruit thinning (Alston and Tobutt, 1989).

The potential incompatibility relationships between all the popular varieties of apple have not been characterised. For instance, further research is needed to characterise Braeburn and Bramley’s Seedling.

Some apple varieties are only partially compatible with each other.
In situations unfavourable to pollen transfer and germination (cool windy sites) these varieties should not be planted together in low ratios of pollinator to main variety for pollination purposes.

Climatic influences on compatibility/incompatibility relationships

Temperature at the time of flowering can have a significant influence on pollen germination and at high temperatures (>25^oC) incompatible combinations may set fruits quite satisfactorily and many varieties become partially self-fertile. One example is the variety Fiesta, which exhibits increasing levels of self-fertility as temperatures increase up to 15^oC (Petropoulu, 1985).

Frost damages both the male (pollen) and female (stigma, style and ovary) parts of the flowers.
Improving shelter in orchards and raising ambient temperatures at flowering time can influence the type and number of pollinators required in an apple orchard.

Pollen germination and growth down the style is greatly aided at temperatures of >15^oC.
At high temperatures (e.g. 25^oC) pollination efficiency is improved with variety combinations, which are normally incompatible or show only partial compatibility.
Winds cause pollen desiccation and often death.
Frost causes death of pollen and the female parts of the flowers. The damage is not always visible.

Influence of temperatures and stylar receptivity or pollination efficiency

Low temperatures (<10^oC) can seriously reduce the effectiveness of pollen germination and speed of pollen tube growth down the style (i.e. stylar receptivity).

Pollen of varieties such as Redsleeves gives high germination percentages and speed of growth, even at temperatures of 8-10^oC, whilst the pollen of Spartan and Falstaff are also good in this respect (Petropoulu and Alston, 1998).
At these same low temperatures, the pollen of Cox and Fiesta germinated poorly.
At temperatures as low as 5^oC, Redsleeves and Spartan were very effective pollinators for Cox, whilst many other varieties performed very poorly.
Although Fiesta pollen grew poorly at low temperatures, the styles of flowers of this variety were very receptive to pollen of other varieties applied at low temperatures.
At 15^oC, flowers of Fiesta exhibit partial self fertility.

In orchards where temperatures at flowering time are frequently less than ideal for pollination, choose pollinating varieties such as Falstaff or Redsleeves, which produce pollen that can germinate and grow at quite low temperatures.

Synchrony of flowering times

If pollinating varieties are to be efficient, it is essential that their flowering periods overlap sufficiently with that of the main variety planted in the orchard.

In several countries, varieties of apple have been divided into three categories of early, midseason and late flowering types.
The assumption usually made is that varieties in the same grouping pollinate each other best, there being no other constraints on pollination (e.g. compatibility).

It has also been assumed that varieties in the early and midseason groups, and also in the midseason and late flowering groups, overlapped each other sufficiently to ensure adequate pollination and fruit set.

These assumptions are not always true.
In some seasons certain varieties classified in the early and midseason categories do not overlap sufficiently; the same is true for varieties in the midseason and late categories.
It has been suggested that, unless the flowering date ranges of any two varieties overlap by a minimum of 6 days, they should not be considered as suitable pollinators for each other (Kemp and Wertheim, 1992).

Therefore choose pollinating varieties which, according to records, have flowering periods that overlap by a minimum of 6 days with the main apple variety in the orchard.

This overlap should be consistent and judged from records collected over a number of years.

The average dates of full bloom since 1936 at East Malling for the scion varieties Cox’s Orange Pippin and Bramley’s Seedling are displayed in a graph.

Self fertility

Most varieties of apple are self-sterile requiring pollination by another variety for successful fruit set. The exceptions are

Self-fertile clones
A number of varieties which seem partially self-fertile in specific climatic conditions
Self-fertility induced by pollen mixtures

Self-fertile clones

A programme of breeding using irradiation techniques, begun by Long Ashton Research Station in the late 1960s and transferred to East Malling in the early 1980s, has been successful in producing self-fertile clones of the commercial apple varieties Cox’s Orange Pippin and Queen Cox.

Two self-fertile clones have been released commercially. Self-fertile Cox Clone 8 is mainly sold into the home garden market.
Of more interest to commercial fruit growers is self-fertile Queen Cox clone 18, which now accounts for a proportion of the sales of Queen Cox trees in the UK.

In seasons when climatic conditions at flowering time are unfavourable for pollen transfer by bee vectors, these self-fertile clones yield better than the more conventional self-sterile clones.

Fruit size, colour and storage potential are similar on the self-fertile and self-sterile clones of Queen Cox.

Self-fertile Queen Cox clone 18, which is available from UK nurseries, gives more reliable cropping than the traditional self-sterile clones, in years unfavourable for pollen transfer between varieties by bees.
This self fertile Queen Cox clone should not be used to pollinate other varieties, as it produces insufficient viable pollen.
Growers considering purchasing Self-Fertile Queen Cox clone 18 are recommended to obtain this only from a verified UK source.

Partially self-fertile varieties

Several varieties, such as Braeburn and Fiesta, although not self-fertile in the true sense, seem able to set adequate crops of fruits with their own pollen (Volz et al., 1996; Petropoulu and Alston, 1998).

Such varieties are thought to combine high stylar receptivity and support strong pollen tube growth, even at relatively low temperatures (e.g. 15 degrees C).
When planting such varieties on sites favourable to consistent fruit set, use of fully compatible varieties as pollinators should be avoided if overset is not to be a problem.
However, relying on this partial self-fertility and planting no other pollinators is not to be recommended.
Studies on Braeburn in New Zealand have shown that, when bees were excluded from trees and set was only by selfing, although yields were not reduced, seed numbers and calcium concentrations in the fruits at harvest time were reduced.
This could have severe implications in terms of bitter pit incidence in some situations and planting Braeburn without pollinating varieties cannot be recommended.

Work in Switzerland (Kellerhals and Wirthner-Christinet, 1996) has shown that Gala may set substantial crops of fruits when pollinated with its own pollen or even when left unpollinated.

However, most of the fruits had no seeds and fruitlet drop was much more severe than on trees receiving cross-pollination.
Most of the few fruits that persisted were thought to be parthenocarpic.
However, a few fruit with seeds were produced and partial self-fertility has been reported elsewhere with Gala (Kemp and Wertheim, 1992).
Also, experiments in the Czech Republic (Papštrein and Blažek, 1996) have shown that although a few varieties, such as James Grieve, Wagener and Ontario are partially self-fertile, they rarely set a full crop of fruit without cross-pollination.

The variety Golden Delicious, which is often considered partially self-compatible in other countries, set fruits very poorly when selfed in trials at Long Ashton Research Station (Bennett et al., 1973).

However, it was noted that some clones of Golden Delicious may prove more self-compatible than other clones and that virus infection may influence this.

Even the variety Cox and its clones seem capable of a level of self-fertility if given suitable climatic conditions.

Trials many years ago at Long Ashton Research Station (Williams and Maier, 1973), showed that at temperatures of 20-25⁰C Cox flowers were capable of setting fruits with their own pollen.
Using detached flowers held at 20⁰C, 8% of styles had pollen tubes from selfed pollen that had penetrated their full length and into the ovary after only 2 days and by the third day 70% of the styles had been completely penetrated.
Pollinating the flowers 2-3 days after anthesis and use of high pollen densities on the stigma were also beneficial in inducing this self-fertility.
The term ‘pseudocompatibility’ was coined for this phenomenon.
In areas of the world with more favourable spring temperatures, such as North Island, New Zealand, it is quite common for Cox orchards to be planted with few, if any, pollinating varieties.

Although several popular apple varieties, such as Red Pippin and Braeburn, show a level of self fertility if climatic conditions at flowering time are favourable, this cannot be relied upon to ensure consistent and high yields of fruits.

Although Braeburn planted without pollinators will set good yields of fruits, these will contain few seeds and will have low levels of calcium.
The seeds are essential in the uptake of calcium into the fruits and the reduction in bitter pit incidence.
Varieties such as Gala and Golden Delicious also often set fruits with their own pollen when weather conditions are particularly favourable.
However, most of the selfed fruits usually drop off at the time of June Drop.

Self-fertility induced by pollen mixtures

Research conducted in the 1970s showed that the variety McIntosh was fully self-sterile and, when pollinated with its own pollen, no fruit set was achieved.

However, if dead pollen of the variety Delicious was mixed with the McIntosh self pollen, successful fruit set was achieved (Dayton, 1974).
This dead pollen, termed ‘recognition pollen’ overcomes in some way the normal rejection of the self-pollen both on the surface of the stigma and in the style.
Seed numbers in the fruits are, however, generally less than if the flowers were pollinated with live pollen of another variety.
The self-fertile clones of Cox and Queen Cox both produce much dead pollen and it is possible that this may account partially for their self-fertility.

Parthenocarpic fruit set

Parthenocarpy is the ability of a variety to set and develop fruits without the need for flower pollination or fertilisation.

No commercially important varieties of apple show natural parthenocarpy.
A few varieties of apple are able to set fruits and grow these to maturity without need for fertilisation of the female egg cells (ovules) in the flowers and the formation of seeds.
In some instances, pollination is necessary, but not fertilisation; in others no pollination stimulus is required. This fruit set without ovule fertilisation is called ‘parthenocarpy’.
Parthenocarpy in varieties could have significant benefits to fruit growers, as the pollination and fertilisation of flowers is not necessary for fruit set.
This could be a significant advantage in springs when climatic conditions are unfavourable for bee activity and pollen transfer.

Also, trials with a naturally parthenocarpic variety, Spencer’s Seedless, show that when it sets fruits parthenocarpically without seeds there is no problem with return bloom (i.e. bienniality).

In contrast, if this variety is hand pollinated with pollen from another variety seeded fruits and biennial cropping are induced.
This supports the long held hypothesis that it is the seeds in apples not the fruits per se that cause the reductions in flowering in the subsequent season when yields are too high.

Breeding for parthenocarpic fruit set has been undertaken as a small part of a programme based at East Malling (Tobutt, 1994).

Trials conducted in the 1970s showed that parthenocarpy could be induced in Cox and other UK apple varieties using sprays of growth regulators.
The seedless fruits sometimes produced following self-pollination of the apple variety Kent are thought to be due to a phenomenon called ‘stenospermocarpy’ (Spiegel-Roy and Alston, 1982), where fertilisation, followed by early seed abortion, results in seedless fruits of parthenocarpic appearance.

Parthenocarpic fruit set can be induced by spraying mixtures of a gibberellin and an auxin. These mixtures are not, however, approved for use in UK orchards.

Pollination using ornamental crab apples or other Malus species

In the 1960s and 1970s, research at Long Ashton Research Station showed that several varieties of ornamental ‘crab apples’ performed well as pollinators for commercial varieties of apple such as Cox and its clones (Williams, 1977). Malus aldenhamensis, M. ‘Golden Hornet’, M. ‘Hillierii’ and M. ‘Winter Gold’ were all suggested as suitable pollinating varieties.

Since this initial research, several other varieties such as ‘John Downie’, ‘Evereste’ and ‘Golden Gem’ have been added to the recommended list, following research conducted in France (Le Lezec and Babin, 1990) and the UK.

Flowering crab apples with proven value as pollinators:

Malus aldenhamensis has a weak growing habit with slender twiggy branches easily distinguishable from most fruiting varieties of apple. It flowers regularly and abundantly and the flowers produce large quantities of viable pollen.
Malus floribunda Hillierii is another weak growing crab with thin drooping branches, very distinct from most fruiting varieties. Flowering is abundant and regular and flowers on one-year-old wood help provide a long flowering period. Pollen production is also good. This crab exhibits high resistance to disease.
Malus Winter Gold. In trials at Long Ashton, pollen of Winter Gold, proved poor in pollinating Spartan, Golden Delicious and Cox in 1981 (Williams et al., 1982).
Malus Golden Hornet. A popular pollinating variety in orchards of Cox planted in the 1980s, Golden Hornet does tend to release its pollen prior to opening its flowers and this may occasionally prove problematic (Anon, 1972b).
Malus Evereste and Malus Professor Springer. These varieties have become popular in Holland and Belgium and are being used successfully in UK orchards though no efficacy studies have been done in the UK.

Use of these crab apple pollinators has advantages:

They take up minimal space in the orchard and they require no harvesting.
The traditional planting ratio for Malus pollinators was one tree every three trees of the main variety in every other row of the latter.
No extra space in the row was provided for the pollinator; the small tree was fitted between two trees of the main variety at their standard spacing.

It is fortunate that most species of Malus are cross compatible with dessert and culinary apple scion varieties and no problems of pollen compatibility are likely when using these species.

To ensure adequate overlap of flowering dates with the main orchard fruiting variety, use of 2 to 4 different Malus types is recommended in the same orchard.

In studies undertaken in 1979, scientists at Long Ashton calculated the amount of pollen produced by either normal pollinating varieties or Malus selections, and also the time of flowering relative to Cox (Church and Williams, 1980).

The records show that when weather conditions in the spring are unfavourable poor synchrony of flowering may be a problem with some of the Malus species.
The best pollen yields per tree on trees planted in 1975/6 came from Malus Winter Gold, Hillieri and Aldenhamensis.
However, good pollen yields were also obtained from trees of the more conventional pollinators James Grieve and Golden Delicious.

Flowering time and pollen production in 1979 on apple varieties and ornamental Malus planted at Long Ashton Research Station (from Church and Williams, 1980)

Variety or Malus species	Flowering time, days before (-) or after (+) Cox								Mean wt. of viable pollen (g)
	50% open				80% open				/cm2 CSA of trunk		/tree
	Spurs		Axillary		Spurs		Axillary
Discovery	-5	+3		-9		+3		0.11		1.49
Egremont Russet	-5	+4		-9		+7		0.05		0.79
Emneth Early	-1	+11		-5		+6		0.07		0.88
Golden Delicious	0	+8		-3		+6		0.13		2.19
Grenadier	-2	+10		-5		+6		0.02		0.20
James Grieve	-5	+9		-9		+6		0.15		2.26
Lord Lambourne	-5	+1		-9		-1		0.11		1.30
Worcester Pearmain	+1	+9		-3		+7		0.03		0.32
Golden Hornet	-1	0		-3		-1		0.34		3.95
Hillierii	+3	+9		0		+6		0.60		3.56
Aldenhamensis	+4	+7		+1		+4		0.30		1.97
Winter Gold	+1	+6		-1		+4		0.45		5.82

Use of ornamental crab apples as pollinators in overseas orchards

Research in other countries has also resulted in recommendations for use of ornamental crab apples in USA, German and Dutch apple orchards (Crasweller, et al., 1978; Stainer and Gasser, 1982; Witteveen, 1973).

In research conducted in Sweden in the early 1980s the flowering times of a range of ornamental crab apples and several fruiting varieties were recorded over a four year period (Goldschmidt-Reischel, 1993).
The results are shown below.

Rankings of earliness of flowering of ornamental crab apples used as pollinators in apple orchards in Sweden

Variety	Earliness Rank	Variety	Earliness Rank	Variety	Earliness Rank
Dolgo	3	Akero	36	Golden Hornet	49
Robin	4	Silva	36	White Angel	50
Hopa	25	Kim	39	Aroma	51
M. sieboldii	29	M. baccata	40	M. floribunda	51
M. zumi caloc	30	Snowdrift	42	Royalty	52
John Downie	31	Katy	42	Van Eseltine	54
Hyslop	31	Mio	44
Kerr	33	M. Eleyi	47
Rosencrab	35	Ingrid Marie	49

Rankings of stage of flowering (all recorded on the same day) range from 1 to 71. 1 is very early (all flowers at petal fall stage) and 71 very late (all flowers still in bud). (From Goldschmidt-Reischel, 1993).

Several species of ornamental Malus can prove effective pollinators for commercial varieties of dessert and culinary apples.

They have the advantage of taking up less space in the orchard than normal pollinating varieties.
The tried and tested species/varieties are M. hillierii, M. aldenhamensis, M. Golden Hornet, M. Winter Gold and M. Evereste.
Always plant several of these ornamental crab pollinators in an apple orchard, not just one.
Their winter chilling and spring forcing temperature requirements are different from those of the commercial apple varieties and this often leads to lack of synchrony in flowering times.
Do not neglect the pruning and, where necessary, the thinning of ornamental crabs, or they may go biennial and fail to produce the required flowers in sufficient abundance.

Production of adequate quantities of viable pollen

Pollinating varieties for use in orchards of self-sterile varieties of apple must produce adequate quantities of viable pollen. The quantities produced are influenced by:

Scion varieties and their production of viable pollen.
Rootstock influence on the production of viable pollen by scions.
Crop loading in the previous year and its influence on pollen production by the scion.
The orchard environment and its influence on pollen quality.
The number of trees of the pollinating variety that are planted in relation to the number of trees of the main variety.
The management of the pollinating variety in the orchard.

Scion varieties and their production of viable pollen

Varieties such as Golden Delicious have been shown to produce abundant quantities of viable pollen. This is a combined influence of the high numbers of flowers produced and the large quantities of viable pollen produced in each flower.

Cox and its clones are poorer in this respect, producing fewer flowers per tree and less viable pollen per flower.
The self-fertile clones of Cox and Queen Cox produce even less viable pollen per flower and should not be used as pollinators in orchards where the main variety needs pollination.
Triploid varieties, such as Bramley’s Seedling, Jonagold and Crispin, produce almost no viable pollen and are, therefore, of no value as pollinators for other varieties.
Take account of the pollen producing potentials of the varieties chosen as pollinators.
Varieties such as Golden Delicious produce copious quantities, whilst Cox and its clones produce much less.
Triploid varieties, such as Bramley and Jonagold, produce almost no viable pollen and should not be used as pollinators.

Rootstock influence on the production of viable pollen by scions

Most apple varieties and crab apples planted only for the purpose of pollinating the commercial apple variety are planted on dwarfing rootstocks, so as to take up the minimum land space within the orchard. This choice of rootstock has the additional advantage of inducing precocious and abundant flowering on the pollinator.

The increased flowering induced usually also equates with the production of more viable pollen.
There is no reliable evidence suggesting that rootstocks influence the percentage viability of the pollen produced by flowers of scions worked upon them.

Pollinating varieties grown on dwarfing rootstocks, such as M.9 produce more flowers per unit tree size than the same variety on a more invigorating rootstock.
Trees on dwarfing rootstocks also take up much less valuable space in the orchard.

Crop loading in the previous year and its influence on pollen production by scion varieties

It is essential that pollinating varieties are not allowed to crop excessively.

Excessive set and retention of fruits on pollinating varieties can lead to reduction in the number of flowers produced in the subsequent season.
Occasionally, the flower quality, including pollen viability, may also be influenced deleteriously.
In extreme cases, the pollinating varieties may go into a cycle of biennial bearing.
Where this is a potential problem take time to thin the pollinating varieties.

Research conducted on the variety Golden Delicious growing in the Czech Republic (Blažek) showed that, where pollen donors were abundant, seasonal cropping was inconsistent.

In contrast, where the distances between the Golden Delicious trees and a source of compatible pollen were as much as 500–1000m, cropping was lower but quite consistent, season to season.
The Czech research suggested that a critical limit, above which symptoms of biennial bearing were noticed, was 5000 to 6000 apple seeds per 100 cm of trunk cross sectional area.
For many varieties of apple this would mean approximately 1000 fruits per 100 cm of trunk cross sectional area.

Manage the crop loads on the pollinating varieties so as to avoid overset and the establishment of a biennial pattern of cropping.

Only if thinned well will the fruiting pollinating varieties produce abundant supplies of flowers and pollen on a consistent seasonal basis.

The orchard environment and its influence on pollen quality

Pollen quality is an important factor in successful pollination. Poor weather conditions, especially frost, can damage the pollen during its development and result in a large proportion of it being non-viable (dead). Such damage often occurs at the early stages of budburst when frost damage is not visible and is, therefore, often overlooked by the apple grower.

Careful monitoring of frost is essential from bud burst onwards.
Where frost damage to flowers and their pollen is anticipated, growers or advisors can check on the viability of the pollen using a simple test.

Avoid frost damage to pollinators by good site selection and, where possible, use of frost protection measures.
In the event of frosts after green cluster, check the pollen viability using simple pollen germination tests.

The number of trees of the pollinating variety planted in relation to the numbers of the main variety

It is essential to plant sufficient numbers of trees of a pollinating variety within an orchard, to provide adequate quantities of viable pollen.

There is no single ideal ratio of pollinating to main variety trees and no strong scientific evidence on which to make objective assessments.
For the average situation ratios of 1:9 were traditionally recommended in the UK.
Lower ratios of pollinator to main variety are recommended (e.g. 1 pollinating variety to 10 or more main variety trees) where:

Pollinating varieties with high flower abundance and high pollen production (e.g. Golden Delicious) are chosen.
The pollinating varieties are fully compatible with the main variety (see ‘Pollen compatibility’ section).
The pollinating varieties flower at the same time as the main variety in most years.
Orchard environmental conditions are favourable (shelter, aspect) for bee activity, pollen germination and rapid growth down the style.
The type of tree grown and/or the site lead to the production of well-controlled growth.
Where several of these criteria are not met the ratios should increase to 1:4 in the worst situations.

Bees tend to work down rows of trees rather than across them.

Wherever possible the pollinating varieties should be positioned in every tree row rather than in one in two or one in three rows.
This is less important in bed systems where trees in 2, 3 or more rows are in close proximity.

Another consideration is the maximum distance between the pollinating variety and the main variety. There is evidence to suggest that distances above 15 to 20 m are likely to result in reduced pollination efficiency (Mišić, 1994; Free and Spencer-Botth, 1964).

More recent trials in the former Yugoslavia indicate that, whilst distance from the pollinating variety was important for Golden Delicious (fruit set and yield better at 5m than at >20m), for Jonathan there was no similar effect (Milutinović, et al., 1996).
Trials undertaken in the Czech Republic showed that efficient pollination of the variety Golden Delicious was achieved with pollinators planted as far as 40 m away (Blažek, 1996).
It is suggested that this is explained by honey bees making extended foraging trips on some occasions (Free and Durrant, 1966).
The Czech author also suggests that better pollen mixing occurs when bee hives are located outside the orchard blocks rather than deeply within them.
Trials in Hungary suggest that for high density systems trained to slender spindles on M.9 rootstock 10-15 metres is the maximum distance between trees of the main and pollinating varieties (Soltész, 1997).

Take advice from your local advisor before choosing a ratio of pollinator to main variety for planting in a new orchard. This ratio will be influenced by:

How favourable the orchard location is, in terms of temperatures and shelter from winds.
The populations of bees, either wild or introduced in the orchard.
The pollen producing abilities of the pollinating varieties chosen.
The propensity of the main commercial scion variety to set abundantly or lightly.
The compatibility (full or partial) of the chosen pollinating varieties with the main variety.

The management of the pollinating variety in the orchard

If pollinating varieties, either standard fruit varieties or ornamental Malus, are to flower consistently, abundantly and produce good pollen, it is essential that they are well-managed within the main orchard. Unfortunately, their management is often neglected or forgotten.

It is essential that their tree size is maintained within the space allotted to them and excessive fruit set is avoided if biennial bearing is not to be a problem.
A trial conducted at Long Ashton Research Station in 1979 examined the influence on return bloom of either fruit thinning or summer pruning of pollinating varieties.
Fruitlet thinning produced most significant benefits with Golden Delicious.
Although stripping all the fruits off most of the other varieties produced increases in flowering, it is questionable whether these increases warranted the time and effort involved.
It is possible, of course, that earlier thinning might have given more benefits and that pollen quality (not measured here) may have been improved by the thinning treatments.
Thinning fruits is probably only warranted with pollinating varieties which regularly set excessive numbers of fruits with high seed numbers, or with varieties known to have biennial bearing tendencies.
Summer pruning, which is sometimes necessary to control the size of pollinating trees, severely reduced return bloom on Egremont Russet, and Golden Delicious, but effects on the other pollinating varieties were insufficient to cause concern.

Flower bud numbers per tree (1980) as influenced by fruitlet thinning and summer pruning treatments

Variety	Fruitlet thinning treatments			Summer pruned
Variety	None	To 1 fruit/cluster	All fruit removed	Summer pruned
Egremont Russet	207	248	349	77
Discovery	252	217	330	180
G. Delicious	137	257	643	12
James Grieve	261	233	368	212
L. Lambourne	286	276	303	238
Worcester Pearmain	140	116	123	129
M. Aldenhamensis	337	429	521	316
M. Golden Hornet	689	955	1059	724
M. Hillieri	739	883	1189	841
M. Winter Gold	1428	1480	1611	754

(from Church, 1981 see Further reading [hyperlink])

Prune and train pollinating varieties so as to stimulate renewal growth and adequate production of quality flowers.
Apply water and nutrients to pollinating varieties so as to sustain their growth and flowering.

Creating suitable habitat for wild insects suitable for pollinating

All types of bee and most other pollinating insects visit apple flowers to collect pollen and occasionally nectar.

It is generally believed that honey bees play an important role in pollinating apple trees (Free, 1970).
However, others have suggested that, as good crops of apples can often be obtained in conditions very unfavourable to the activity of hive bees, other vectors of pollen must also be involved or the trees must be more self-fertile than expected.
It has been suggested that wild bees are also important pollinators although others contend that there are rarely sufficient wild bees to do this efficiently.

If bees and other insects are to perform adequately as pollinators, it is essential to create the appropriate climatic conditions within the orchard.

Wild bees can be encouraged into the orchards by leaving grassy banks on headlands and by planting mixed deciduous species in windbreaks and various wild flower species.
Flowering plants, such as corn marigold, are attractive to beneficial insects and may also prove attractive to insects involved in pollination.
Work by the Oxford Bee Company (Gettinby, 2001) has resulted in the development of commercial nesting systems for Osmia rufa, the red mason bee, a UK native.
Research conducted in several European centres indicates that red mason bees are much more efficient than honey bees in pollinating apples and other tree fruit species.
They can also fly within a broader temperature range than honey bees and the dense brush of hairs on the underside of the females facilitates improved transfer of loose pollen between flowers.
American research even reports improved fruit shape and texture, as well as improved yields as a result of pollination by red mason bees.
The bees make nests in the holes made by beetles in dead wood and hollow plant stems.

Studies by the Oxford Bee Company have shown that one female red mason bee can be as effective in pollination as 120-160 honey bees.

It is estimated that only 500 red mason bees are needed to pollinate 1 hectare of apples, compared to 60,000 to 80,000 honey bees.
Fortunately, the red mason bee is not susceptible to the varroa mite which affects other native bees and is estimated to have caused a 45% drop in the number of commercial bee keepers.
Artificial nests for these bees can now be obtained, which can prevent the bees being attacked by a wasp parasite.
These should be located in sunny, sheltered south facing positions in the orchard in early spring.
Further information is available on www.oxbeeco.com.

Attractiveness to honey bees

Unfortunately, apple flowers are not as attractive to honey bees as several other crops. Pear flowers and the flowers of oil seed rape and several weed species prove much more attractive.

Apple varieties may also differ in their attractiveness to bees. Work at Long Ashton Research Station showed that flowers of Golden Delicious were much more attractive to honey bees than those of Cox’s Orange Pippin (Jefferies, et al., 1980). It was suggested that the Golden Delicious flowers had more perfume and that this may have accounted for them being preferred. However, they would almost certainly have also offered more pollen per flower and more flowers per tree than the Cox.

When introducing honey bees to an orchard, it is important not to do this too early.
If the introduction is before a significant number of flowers have opened, the bees will forage for better pollen and nectar sources and become habituated (exhibit what is called ‘flower constancy’) to another more attractive crop nearby. It can then prove difficult to encourage the bees back into the orchard.
Often bees show constancy to flowers of one colour; the reasons for this are not understood.
It can occasionally have implications for growers using ornamental Malus (Crab apples) as pollinators in orchards.
Several of these have dark pink flowers and bees may focus either on these or on the white apple flowers, so failing to move pollen between the two efficiently.
Czech studies have indicated that honey bees tend to prefer trees with abundant blossoms.
This has serious consequences for varieties which frequently produce only sparse flowering, either as a consistent and inherent trait, or as a result of bienniality.
In extreme cases, bees could choose to concentrate their foraging on the variety with abundant flowers and rarely visit the one with low flower abundance.

Introduce hive or bumble bees to orchards only when 15 to 20% of the flowers are open.

Introduction earlier may lead to the bees seeking food supplies on other crops growing nearby.
Once habituated to another crop it is often very difficult to attract the bees back into the apple orchard.
Remove (by mowing or use of herbicides) weeds or other species that are flowering in the orchard at the same time as the apples.
These may prove more attractive to the bees than the apple flowers.

Introduction of hive bees to aid natural pollination

When considering the introduction of hive bees (honey-bees) to apple orchards there are several important considerations:

Are hive bees necessary for pollination of apples?
Timing of the introduction of bees to the orchard.
Avoiding competition with other flowering crops.
‘Strength’ of the bee colonies.
Provision of shelter and favourable habitat for bee activity.
Aids to transfer pollen between bees.

Are hives of honey bees necessary for pollination of apples?

There is some dispute amongst experts as to whether bees, especially honey bees, are important for pollination of apple orchards. Whilst UK research (Free, 1970) shows their importance, research elsewhere has often questioned this.

It can be argued that the number of pollen grains delivered by pollinating insects from the pollinating variety (pollinizer) to the main variety is a measure of the pollination efficiency of those insects.
Most researchers have endeavoured to compare the quality of different insect species as pollinators, by assessing their performance in isolation i.e. without any interaction with other potential insect species.
When different insect species visit flowers, however, interactions are possible making it hard to determine their relative efficiency in pollination.

It is argued that effective pollinating insects should:

collect copious amounts of pollen from the pollinizer variety,
deliver sufficient amounts to the stigmas of the main variety,
make multiple visits to flowers depositing viable pollen at each visit and,
be attracted to flowers of both the pollen donor and pollen receptor varieties.

Preliminary studies in New York State, USA (Goodell and Thompson, 1997) compared the efficiency of honey bees (Apis) with bumblebees (Bombus) in pollen removal and deposition within orchards of apple varieties.

The studies indicated that both species appeared to be equally effective in the collection of pollen from the donor pollinating variety.
Honey bees seeking nectar may differ from bumblebees in their efficiency of pollen deposition, however.
This is because with certain varieties of apple with specific floral anatomy (e.g. the ‘Delicious’ variety) the honey bees ‘sidework’ the flowers and fail to deposit pollen on the stigmas.
Honey bees also carry far less pollen than bumblebees.
However, if no ‘sideworking’ takes place then honey bees can make more visits and be just as effective if not more effective than bumblebees.
Further research is needed to extend this USA study.

Recent experiments in Hungary have shown that the intensity of bee visits to an orchard can have a significant influence on fruit numbers set and the seed numbers in the fruits (Benedek and Nyéki, 1997).

This suggests that on sites where weather conditions are expected to be consistently unfavourable for bee activity, it will be essential to maximise the number of strong bee colonies per acre, if good fruit set and retention is to be achieved.

It has been estimated that approximately 50 pollen grains are transferred to a stigma by insect pollination. This contrasts with hand pollination, which results in the transfer of many hundreds of pollen grains to each stigma (Stosser et al., 1996).

Research conducted in Canada in the late 1920s showed that a strong colony of bees (e.g. >15,000) would, given favourable weather conditions, visit approximately 21 million flowers in a single day.

At the tree spacings and sizes in vogue at that time, this equated with the flowers open on 20 acres of fruit.
However, later studies suggested that if only 25% of a similar bee colony carried viable pollen, then they would pollinate all the flowers on 2.7 acres in a day.
More recent studies suggest that 60,000 to 80,000 bees are needed to pollinate one hectare of apples grown in the UK at commercial spacings (Gettingby, 2001).
A study in the 1960s went on to suggest that very few bees were needed for effective pollination (Horticultural Education Association, 1962).
To pollinate 5% of the flowers on an acre of mature apple trees, a population of only 37 insects was needed for 5 hours.

It has been shown that cropping of fruit trees diminishes the further they are located from suitable pollinators, even when bees are present (Free, 1962).

The explanation for this is that bees have limited foraging areas, they are attracted to other nearby plant species and they tend to work down one row of trees rather than across rows.

It has been suggested that wind borne pollen may also have a role in apple pollination (Burchill, 1963) and experiments have shown that wind borne pollen can collect on apple stigmas (Fulford, 1965).

However, research conducted by Free (1964) suggested that the importance of wind pollination in apple trees was negligible.
Free’s conclusion can be questioned, however, as the insect-proof cages used in this work also inhibited wind movement and the flowers studied were emasculated, which is known to reduce potential fruit set significantly.
It is possible, therefore, that in certain favourable circumstances, wind pollination may play a contributory role in apple pollination.

Some wild bee species, including bumble bees are known to work in less favourable weather conditions than honey bees and may also be useful pollen vectors in orchards.

The importance of bees in the pollination of apple orchards is still disputed by different authorities.
In ideal conditions, one strong hive of bees (>15,000) should be capable of pollinating all the flowers open on a hectare of apples in one day.
In less than ideal conditions, two to three hives per hectare, 60,000 to 80,000 bees, may be necessary.
Wind pollination is likely to have only a small contribution to apple fruit set.
Bumble bees and some other wild bees forage at slightly lower temperatures than honey bees and may prove better in pollen transfer between flowers in poor weather conditions.
However, bumble bees are expensive (currently more than £100 for a ‘triple’ with three queens and a total of 210 bees). For £50 a strong hive with many thousands of honey bees should be available for hire.

Timing of the introduction of bees to the orchard

It is vital that hives of honey bees are not introduced to the apple orchard too soon.
Work conducted at Long Ashton Research Station in the 1970s indicated that the optimum time was when 20% of the blossoms were open (Williams, 1976 see Further reading).
Do not introduce bees to the orchard until 20% of the blossoms have opened.

Avoiding competition with other flowering crops

It is essential that all potential pollen vectors, whether natural or introduced to the orchard, are encouraged to focus on transferring pollen within the apple trees.

The presence of flowers of other crops or weed species in close proximity or within the orchard may detract from this necessary focus.
Hive bees tend to be constant to a particular species on an individual trip from the hive when foraging for pollen and nectar (Free, 1963).
This is called ‘flower constancy’ and is an important factor in the success of insect pollination of apples, as hive or honey bees have been shown to form the majority of pollinating insects in European fruit plantations (Free, 1970).

Studies on the behaviour of hive bees undertaken in Hungary (Benedek and Nagy, 1996) show that flower constancy for pears was very high but was much smaller for apples.

This was supported by the findings also showing that bees working pear orchards carried only 2‑11% of pollen other than pear, whilst for apple the values were 30%-43%.
Bees are attracted away from apple flowers by rape, plum (if still in flower) and weed species such as dandelion and groundsel (Benedek and Nagy, 1996).

Mow the orchard grass and remove flowering weeds from the orchard, as some of these could prove more attractive to the bees than the apple flowers.
Bees prefer the flowers of rape, plum, dandelion and groundsel to those of apple.

‘Strength’ of the bee colonies

There is no benefit in paying for the introduction of honey bees to an orchard unless the hives contain strong colonies (e.g. 15,000 or more bees).

Pre-pollination feeding of hives with sugar may be necessary if colony strength is to be optimised and it may be necessary for growers to help pay for this.
Ensure that the hives of bees hired are strong, well-fed colonies with ideally, 15,000 or more bees.

Provision of shelter and favourable habitat for bee activity

Observations have shown that honey bee activity is limited when cloud cover is seven tenths or more, when wind speeds reach 10 mph or more and/or when the temperature is less than 15^oC (Brittain, 1933).

Anything that can be done in the orchard to lower wind speeds and increase temperatures must aid the pollinating activity of bees.
Endeavour to create sufficient shelter within the orchard to encourage bee foraging.
The ideal is wind speed no higher than 10 mph and temperatures of 15⁰C or more.

Aids to transfer of pollen between bees: Use of novel aids to pollen dispersal

One of the potential problems with using honey bees as pollen vectors within apple orchards is also often quoted as one of their strengths; namely flower constancy.

Not only do honey bees focus their foraging for nectar and pollen on a single species, they frequently tend to work only in one variety or down one row of trees (Van den Eijnde and Van der Velde, 1996).
If we accept that bees do aid pollination, then there must be some exchange of pollen between bees within the hive.
The extent that this occurs is at present not known.

Pollen dispensers attached to bee hives

Research by Williams and colleagues at Long Ashton Research Station in the 1970s first showed the value of ‘hive inserts’, which dispensed pollen onto bees as they left their hives.

Bees need to be preconditioned to use these hive inserts. The pollen in these inserts was diluted with pollen of Alnus (Alder) species.
The results of some of the earliest trials were variable with the dispensers improving yields slightly on some sites but having little effect on others (Williams et al., 1978).
Further work by the Long Ashton team in 1979 led to disappointing results with the pollen dispensers.
Only 7% of the seeds in Cox fruits were attributable to pollen delivered by the dispenser, even when no other pollinator was present (Jefferies et al., 1980).
When suitable pollinating varieties were planted nearby the contribution of the pollen dispenser dropped to only 2.5%.
These disappointing results caused research on this strategy in the UK to be abandoned at that time.

Research conducted in the 1990s began to investigate whether the use of soft bristles placed at the entrance to the hive could facilitate bee to bee exchange of pollen (Free et al., 1991; Hatjina et al., 1993).

More recent trials conducted in the Netherlands (Van den Eijnde and Van der Velde, 1996), compared different types of bristles used at hive entrances.
Paintbrushes made with pig’s hair and a paint roller covered with cotton wool (‘soft angora roller’) were compared.
A perspex and wood device was erected at the hive entrance, which forced the bees to crawl towards the light between two paintbrushes or paint rollers.
Bees emerging from hives on which the bristle/roller devices were placed carried pollen of significantly more plant species than hives with no attachment.
By inference, one would expect the same to be true when comparing pollen of different varieties, although this was not checked in these experiments.
The rollers and brushes both achieved the same objective but the rollers were easier to assemble.
Following inconsistent and sometimes disappointing results using hive inserts to aid pollen dispensing, trials at Long Ashton were abandoned and the technique never gained favour in the UK.
More recent trials conducted in the Netherlands may warrant a reconsideration of this strategy.

The influence of spray chemicals on pollen germination and growth

Studies conducted many years ago (MacDaniels and Hildebrand, 1940) showed that use of fungicides could inhibit the germination of apple pollen on the stigma.

More recent research has shown that sprays of certain fungicides, when sprayed onto apple trees during flower opening (anthesis) can have a toxic effect on the reproductive organs in the flowers (Bonomo and Tiezzi, 1986; Fedke, 1982). It has been suggested that some of the fungicides have an adverse effect on the cytoskeletal apparatus and mitosis within the pollen tube and the maternal tissues (Battistini et al., 1990).

Recent studies have tested sprays of benomyl, dodine and penconazole at various concentrations applied to the flowers of Golden and Red Delicious.

As shown previously, benomyl had a negative influence on pollen germination (70% reduction).
At recommended dose rates dodine also had a severe effect on pollen germination (80% reduction). Penconazole had negligible effects on Red Delicious but negatively affected pollen germination of Golden Delicious (75% reduction at recommended rates) even when applied at one eighth the recommended dose rates.

Surprisingly, and in contrast to the above results, researchers working at Long Ashton Research Station some years ago noted increased cropping of Cox’s Orange Pippin following sprays of benomyl at or around blossom time (Byrde et al., 1976).

This was attributable to the fungicide reducing stylar infections with various fungi (Williams et al., 1971).
Similar increases in yield were reported in South Africa and the UK following sprays of triadimefon (Strydom and Honeyborne, 1981; Church et al., 1984).
Work at Long Ashton also showed that in some years sprays of sulphur at green cluster or pink bud could also increase fruit set (Church and Williams, 1983).
Sulphur may occasionally damage the primary leaves of some apple varieties and it has been suggested that the increased fruit set recorded following its use may be attributable to sulphur’s effect in checking growth of bourse shoots.
This in turn would diminish unwanted competition between these shoots and the fruits for assimilates (the plant’s food stuffs) (Church and Williams, 1983).
This needs further research in the light of increased use of sulphur for organic growing systems.

Research therefore indicates that sprays of certain pesticides could negatively affect the germination and growth of pollen, if they were applied to open flowers.

The results showed that sprays of Captan, and penconazole (Topas) could all reduce the germination of pollen and influence fruit set on some apple varieties.
The above results were often obtained using in vitro tests and are likely to exaggerate slightly the negative influences of fungicides on pollen germination in comparison with field experiences.
Nevertheless, it is advisable not to spray these fungicides during flowering wherever possible.
Most new fungicides are now tested for their effects on pollen germination and growth and the manufacturers should supply information on the results of these tests.

Fungicidal sprays are likely to have no measurable effect on fruit set in years when pollen is transferred in abundance by insects and weather conditions are favourable for its growth.

However, in years unfavourable to pollen transfer and growth, they could have severely deleterious effects on fruit set (Church et al., 1976).
The mechanisms of how these effects are brought about are still not understood.

The influence on pollination and fruit set of the growth regulators paclobutrazol (Cultar) and CPPU (a chemical occasionally used abroad which has cytokinin action) was also evaluated in trials.

The effects of paclobutrazol were variable.
At high rates the sprays had no effect on pollen germination whilst at the recommended rates the pollen germination of Golden Delicious was slightly stimulated and that of Red Delicious slightly depressed.
Sprays of the growth regulator paclobutrazol (Cultar) therefore appear to have no negative effects on pollen germination.
CPPU had no significant effect on pollen germination. Research at Long Ashton showed that sprays of paclobutrazol (1.0 ml l-1) applied to Cox trees between green cluster and petal fall reduced initial and final fruit set (Church et al., 1984).
However, this may have been due to a negative effect of the sprays on the hormone balance within the flower, rather than any direct effect on the pollen.

Insecticides are known to have a very negative effect on bees and other insects (Johansen, 1977) and it is recommended that sprays of most insecticides are suspended during blossoming.

However, their effects upon bee visits (i.e. the attractiveness of the flowers) and pollen germination and growth are less clear.
Trials at Long Ashton Research Station showed that chorpyriphos (Lorsban, Equity) applied during flowering of Cox trees reduced fruit set (Church et al., 1984).

Sprays of Captan, penconazole (e.g. Topas) and chlorpyriphos (e.g. Equity and Lorsban) have been shown to reduce fruit set in some seasons if applied during flowering.

Their effect may be due to their inhibition of pollen germination and growth, as shown by in vitro tests on some of the pesticides.
Alternatively it may be a direct effect on the pollinating insects.
It is suggested that, where possible, they are not applied during flowering time.

Applying sprays of macro- and micronutrients to aid pollen germination, pollen tube growth and fruit set

Sprays of fertilisers, either major or trace elements, have often been tried in attempts to improve fruit set.

Spring applications of nitrogenous fertilisers should be avoided where possible, as these can stimulate shoot growth, which in turn reduces fruit set (Hill-Cottingham and Williams, 1967).

Most treatments are focused on improving flower quality, however a few treatments, such as those with either boron or calcium, are aimed at directly stimulating the processes of flower fertilisation.

Studies conducted mainly on bulbous and herbaceous crops have shown that boron, and to a more limited extent calcium can aid the germination of pollen.

Boron is always added to the sugar solutions or the Agar medium used to test pollen germination in the laboratory.
Boron has been shown to stimulate pollen germination and also to speed up the growth rate of pollen tubes down the style (Brewbaker and Majumdar, 1961).

Many attempts have been made to replicate these boron effects on the flowers of apple trees growing in the orchard. Sprays of boron (e.g. boric acid) have been applied either prior to leaf fall in the autumn or in spring just before flowering. The aim in both cases is to increase the boron concentrations within the floral parts. The results of these experiments have proved very variable.

In trials conducted at East Malling in 1973, three fortnightly sprays, commencing at 80% petal fall, of ‘Solubor’ (Na2B8O13.4H2O), were applied at 0.15% for the first and 0.25% for the second two sprays.
The treatments increased the initial set and the set measured after June Drop of Cox’s Orange Pippin but by harvest the effect was not statistically significant (Yogaratnam and Greenham, 1982).
In the subsequent year, initial set was increased but not final set.
In neither year was crop weight per tree increased and effects on Discovery were also negative.
A small but non significant increase in Cox fruit set was observed in one year of the same trials, following sprays of 0.5% urea at pink bud and petal fall.
The conclusion must be that, although occasional benefits in terms of improved fruit set can be achieved using sprays containing boron, the effects are very inconsistent.

Sprays containing boron, applied at various timings between autumn and late spring have given very inconsistent effects on fruit set and retention of apples and cannot be recommended for this use on current evidence.

Installation of a frost protection system

Damage from radiation frosts may be reduced by applying orchard heating, by sprinkler irrigation, by use of wind machines or by use of fogging systems during the frosts.

The objective of all of these measures is to alter what is called the thermal regime of the air layer near the ground and to reduce the loss of long wave radiation from the ground and from the trees.
Scientists at Pennsylvania State University in the USA have attempted to develop a Decision Support System for frost protection.

Orchard heating

In the late 1960s and during most of the 1970s frost protection of UK orchards using heating systems was quite popular, but their popularity declined as fuel prices rose. Three types of orchard heater were used: paraffin wax candles (the most popular), propane gas burners and ‘stack heaters’.

Usually approximately 35 ‘stack heaters’ per acre were recommended, whilst wax candles were positioned between trees in the row at about 8-10m spacings.
They were able to provide temperature lifts of 1 to 2oC on nights experiencing radiation type frosts. The life of the propane gas and stack heaters was estimated to be 5-10 years.
The heaters provided radiant and/or convection heat which increases the temperature of the air and the crop surfaces.
Although these systems operated best under low wind conditions, they were one of the few methods of frost protection that will help under wind frost conditions. This is explained by the radiant energy they produced.
A current problem concerning their use is the pollution they cause and in many areas their use is proscribed.

Research has shown that combinations of heaters with wind machines provide more benefits than would be expected from their additive effects. Experiments combining orchard heaters with porous covers have also been carried out with the aim of providing some protection under wind frost conditions but the benefits were not as great as expected.

Orchard heating during frosty nights can provide some reduction in damage from radiation frosts, by raising temperatures 1-2⁰C.
However, the fuel costs for such systems are high and the pollution of the aerial environment caused by the candles, or propane/paraffin burners is usually unacceptable.

Tractor-mounted warm air blowers can provide frost protection but need to be driven round the orchard constantly during a frost event and work better when the air is still.

The number of hectares that can be protected in this way depends on the orchard layout, the degree of frost, local topography and wind movement.

Sprinkler irrigation

Sprinklers used for frost protection in apple orchards may be high level (overhead) or low level (under tree) in type.

Under tree sprinklers

Under tree micro-sprinklers or spray jets are the most common form of frost protection used in California and Florida, in the USA.

The aim is to transfer the heat contained in the irrigation water to the surrounding air, whilst maintaining the soil temperature at around 0 degrees C by release of fusion heat when water and ice co-exist on the soil surface.
Under tree sprinklers are most appropriate where only small lifts in temperature are required and the system is flexible in that it can also be used for irrigation and nutrition at other times of the year.
Sprinklers applying approximately 2mm/hour can raise temperatures at 2m above the surface significantly, if conditions are calm with no wind.
The treatment is most effective if applied in humid conditions to soils, which are already moist.
It is most useful with tree crops that are in full leaf at the time of application, but may also be of value with deciduous trees such as apples with less canopy cover.
As with over tree sprinkling, ‘pulsing’ of the under tree micro-sprinklers has also been investigated, as a means of saving on water use.
One particular advantage of the technique is that limb breakage, occasionally a problem with over tree sprinklers, is not a problem with these low-level systems.
When using over tree sprinkling this is often a problem due to excessive ice build up on nights of extended freezing.
Low level, under tree micro sprinklers can reduce frost damage.
In calm (no wind) conditions applications of 2mm water/hour to compact and previously moist soils can raise orchard temperatures by 1 or 2 degrees 2 m above the soil surface.
Micro sprinklers cause no limb breakage, which is common following extended use of over-the-tree sprinkler systems.

Over-the-tree sprinklers

This technique works by the release of the latent heat of fusion as the water lands on the tree and is turned into ice.

It is essential that this is a continuous process and that the surface of the ice layer is not allowed to freeze completely.
The tree tissues are, in efficient systems, maintained at approximately 0 degees C.
There is an additional advantage in that some of the heat in the water droplets applied also serves to lift the temperature of the air in the tree canopy slightly.
Over-the-tree sprinkler systems, using impact type nozzles applying 2-3mm of water per hour during frosts, can provide useful protection to the flowers.

The advantages of overhead sprinkler systems are that:

Once installed the running costs are low.
The heat release is directly to the plant surface and not indirect via the canopy air.
They cause no air pollution.
The equipment can also be utilised for other management operations in the orchard, such as irrigation application and in some parts of the world it is also used for inducing bloom delay.

The disadvantages of over-the-tree sprinkler systems are:

The high installation costs
The potential for limb breakage with excessive ice build up
The possibility for excessive evaporative cooling and hence enhanced frost damage.
The fact that it is essential that sufficient water is applied during the frost event; if too little is applied then frost damage may be worse than if none at all is applied.
In addition, if the frost persists for many hours very large quantities of water are applied.

Sufficient water supplies must be available to meet this demand and the soils beneath the trees must be capable of draining the applied water away.

The rates of water application needed vary to some extent in relation to wind speeds and to dew points.
Uniformity of water distribution is of paramount importance for success with these systems.

Most conventional systems utilise medium pressure impact-drive sprinklers.

Average application rates of 3.8 or 3.0mm/hour are recommended to achieve minimum application rates of 3 or 2mm/hour respectively.
New Zealand research has shown that the very high water volumes used can be reduced by approximately 30% by using a targeted mini sprinkler system suspended above the trees.
This saves water by not spraying between the rows and the headlands.
Also, it is argued that when water application rates are inadequate to account for a severe frost, the damage resulting will be spread more evenly throughout the orchard using this targeted approach in comparison with the damage in an orchard protected with the medium pressure impact sprinklers.

Another method of saving on water used is to ‘pulse’ the sprinkler applications (Hamer, 1980) and 18-70% savings can be made using this technique.

This uses a temperature sensor in the orchard, which is designed to respond the same way as a flower or bud.
It causes the system to switch on when the temperature of the sensor falls to below -1 degree C. Nevertheless, this pulsed system does have its problems and advice should be sought from experts before adopting it.

Wind machines

In the most commonly experienced radiation frosts, ground based temperature inversions occur. This means that the temperatures above the tree canopy are higher than the temperatures below and within the canopy of the orchard.

The aim of the wind machine is to mix these two air layers and to pull some of the higher and warmer air down into the orchard.
The machines comprise large two-bladed fans (5m in diameter) mounted on towers approximately 10m in height.
The fan should rotate around the top of the tower once every 4 to 5 minutes.
The fan blades are mounted with a small downward tilt to facilitate the vertical movements of air.
Although expensive to install, the wind machines have a 10-20 year life and cause almost no gaseous pollution (compared with orchard heaters) although noise pollution can be a serious problem.
Nevertheless, they can be expensive to run and are only effective in conditions where strong inversions occur.
Where nocturnal winds are more frequent their effectiveness will be less.
Given strong inversion conditions a modern machine of approximately 75 kW can provide a 1 degree C lift in temperature over an area of 3.5ha.
Use of wind machines for frost protection in UK orchards is not popular.
The reasons are the high costs of installation and the unproven effectiveness of such machines in UK frost conditions.

Fogging systems

Clouds and fog can modify night temperatures because water droplets of a specific size (10‑20μ) can intercept a proportion of the long wave radiation loss from the soil and trees and bounce it back towards the surface. The fog also precipitates on the crop surface and, as it freezes, releases latent heat of fusion.

Successful systems, which generate fog, have been used on vines in France and citrus crops in the USA.
Fog generating lines placed 6-8 metres above the crop canopies produced a 1-3 m layer of fog, which spread over 300 to 500m in the slight wind.
A line of 100m length gave some protection to 1-3ha.
It is essential for success that wind speeds are no more than 0.6‑1.0m per second.
The fog generating lines must deliver at least 25 grams/metre/second for any chance of success.

A mobile fog generating system has also been developed: the Gill saturated vapour gun.

The system relies on a jet burner using diesel fuel at a rate of 150 litres/hour to generate a high-speed jet of very hot gases.
Water is injected into this jet of gases via nozzles at a rate of 2 cubic metres per hour.
This water is partly vapourised and part is dispersed as small droplets.
The mixture of hot gases, water droplets and water vapour is then cooled with compressed air and by the ambient air.
The gun is capable of generating a wide range of droplet sizes and also a range of vapour to droplet ratios, so modifying the type and buoyancy of the fog.
The 60m jet on the gun creates a fog over approximately 20ha.

Fogging systems, used for frost protection of grapes in some parts of the world, have not been tested in apple orchards in the UK.

Decision Support System for frost protection

In the early 1990s, scientists in the USA produced a prototype decision support computer programme for the protection of crops from frost (Heinemann et al., 1992). The aim was to assist managers with the development, implementation and management of frost protection systems.

Use of chemical sprays designed to provide some protection from frost and winds

Sprays of plant growth regulators, antitranspirants and other compounds

Although sprays of plant growth regulators have been reported to influence the winter hardiness of trees, these treatments have very little direct effect on the frost tolerance of apple flowers.

In research conducted in Washington State and Australia, the dodecyl ether of polyethylene glycol (DEPEG) has been shown to have cryoprotectant activity on the flowers of both apples and blackcurrants.

The chemical was tested at a range of concentrations (0.5-8% w/v).
Another product extensively tested on blackcurrants as a cryoprotectant is Teric 12A23B (ethylene oxide condensate).

Damage from frost occurs when the water within the floral tissues forms crystals as it freezes. This is enhanced if the water contains ice nucleation proteins.

A number of bacteria, such as Pseudomonas syringae, P. fluorescens, Erwinia herbicola and Xanthomonas campestirs pv translucens, which are often common on plant tissues in large populations, are known to produce these ice nucleation proteins.
Attempts have been made to reduce these populations on plant tissues in order to reduce frost injury.
However, to date these attempts have met with little success.

None of the other chemicals or bacteria species listed above are currently approved for use in UK orchards.

Applying sprays to alleviate frost damage, to induce parthenocarpic fruit set and to aid fruitlet retention

Sprays of gibberellins, auxins and cytokinins

In the 1970s and 1980s, research conducted by scientists at Wye College in Kent on Cox and its clones showed that sprays of a mixture of gibberellic acid, an auxin and a compound exhibiting cytokinin-type activity could induce parthenocarpic fruit set and increase yields of apples (Kotob and Schwabe, 1971; Goldwin, 1978; Goldwin, 1981).

The Effective Pollination Period or EPP

The most common measurement of ‘flower quality’ is the ‘Effective Pollination Period’ or EPP (Williams, 1970). According to Williams the EPP is the difference between the number of days that the pollen tube requires to reach the ovules and effect fertilisation of the embryo sac and the number of days the ovules remain viable and receptive after anthesis (flower opening). In ‘laymen’s terms’ this means the number of days after the flower opens during which time it can receive pollen and still set a fruit.

Trials conducted in the 1970s at Long Ashton Research Station (Stott et al., 1973) showed that short EPPs were a common cause of poor fruit set on the early fruiting variety Discovery. Although 9% of flowers set fruits if pollinated on the day of opening (anthesis) only 1% set fruits if pollinated two days after opening.

The EPP is measured by protecting flowers from external uncontrolled pollination by enclosing them in large paper or other bags at the balloon stage of development. After removing the bags (temporarily) some of the flowers are pollinated on the day they open (anthesis). Further flowers are pollinated one, two, three, four or more days later. After each controlled pollination, the bags are replaced until after fruit set.

Flowers that set fruits only when pollinated on the day of opening, or not at all, have low EPPs and are classed as poor quality.
In contrast, flowers which can be pollinated 4 or more days after opening and still set and retain fruits have long EPPs and are classed as higher quality.
In UK conditions, good quality apple flowers should, under orchard conditions, have EPPs of three to five days.

The reasons for these differences in EPPs and flower quality are not fully understood. Most theories relating to apple flower quality focus on differences in the ovules of the flowers.

Poor quality flowers are thought to have ovules which either have a short life or which develop out of synchrony with the rest of the flower.
An example of this might be flowers where the ovule becomes receptive to the pollen tube in advance of flower opening but which has begun to degenerate by the time that pollen eventually reaches it.

Flowers that are initiated late in the previous summer, such as those formed as axillaries on one-year-old wood, develop only partially by the autumn and are usually, but not always, of poor quality, with short EPPs in the subsequent spring.

In this instance incomplete or shortened flower development may explain the quality differences.
This means that the window of time for effective pollination and fruit set with axillary flowers is much shorter and the risks of poor fruit set much higher than with spur or terminal flowers.

Climatic conditions have a strong influence on apple flower quality. and this is discussed more fully by Williams (1970) and Abbott (1984).

Early work by Williams (1965) showed significant differences between varieties and seasons in terms of their EPPs.

Whilst a variety such as Scarlet Pimpernel exhibited long EPPs (nine days) in each of the two seasons measured, Cox’s Orange Pippin flowers showed five days in one season and only two days in another.

For most diploid varieties of apples grown in reasonable temperate climatic conditions EPPs of three to five days are normal.

The variety Cox’s Orange Pippin (and its various ‘sports’) has been shown to have a short EPP (Williams, 1970), as has the variety Discovery (Stott et al., 1973).

Pollinations made after the EPP has expired still result in pollen germination and growth of the pollen tube into the ovary. Failure to set fruits in these cases must, therefore, be associated with ovule degeneration and death.

The aim of the apple grower must be to encourage the rapid production of high quality flowers on the spurs and short terminal shoots of young trees.

With many of the newer varieties, such as Gala, Braeburn, Kanzi, Jazz and Rubens, axillary flowers can be relied upon to contribute significantly to cropping on young trees.
These axillary flowers are more likely to set and produce good size fruit if they are produced on strong wood of approximately 10 mm diameter.

Growers should strive to produce flowers of high quality with Effective Pollination Periods of three days or more. Flower quality is improved by:

Minimising the spring and summer use of nitrogenous fertilisers, which encourage strong vigorous shoot growth and extended growth into the late summer and autumn.
Limiting strong shoot growth by branch bending and brutting techniques especially in Cox and Bramley.
Improving light penetration into the tree canopy.
Optimising crop loading by appropriate flower or fruitlet thinning techniques.
Not delaying fruit harvesting too late in the season.

Computer models for predicting fruit set

In the early 1980s scientists working at Long Ashton Research Station, developed mathematical models for use in investigations of the many variables involved in fruit set and fruit drop (Brain and Landsberg, 1981).

The authors argued that as the number of fruits harvested per tree accounted for 70% of the yield variation for apples (Landsberg, 1980), the processes involved in determining fruit set and retention were critical in determining apple yields.
Although much research was conducted in the late 1960s and 1970s which pointed to the importance of planting suitable pollinating varieties and of introducing bees for pollen transfer between cross-compatible varieties, the efficacy of these measures in improving yields was considered difficult to estimate.
This was because of the many variables involved in pollination and fruit set and the difficulties of controlling them.
To help solve this problem Brain and Landsberg developed mathematical models for pollination and fruit drop, which they attempted to use to analyse the consequences of variable ovule fertility, effective pollination period (EPP) and insect activity.
The authors constructed a model, which takes account of the EPP, the ovule fertility, the insect visiting rate and the probability that insects will be carrying viable pollen.

Using the model, Brain and Landsberg suggested that, although variability in the length of EPPs for flowers may have an effect on pollination success, it is not an important factor determining yields.

They hypothesised that the most important factor was insect visiting rate.
They went on to suggest that placing high numbers of hives in orchards was much more important than either planting high densities of pollinating varieties or ensuring that bees were carrying compatible pollen (hive inserts etc.).
The authors discussed the feasibility of using the model to predict fruit set but believed that two important components of the model, namely the insect visiting rate and the probability that a bee visit will result in successful fertilisation of an ovule, would prove very difficult to quantify.

More recently, researchers in the USA developed a computer model for predicting fruit set on ‘Delicious’ apple trees (Degrandi-Hoffman et al., 1987). This is available in a PC compatible version, which makes predictions for Golden Delicious and Granny Smith, as well as for Delicious (Degrandi-Hoffman et al., 1995).

It is based on the assumption that the primary vectors of pollen in apple orchards are honey bees (Free, 1970; Crane and Walker, 1984).
Based on weather conditions in the orchard, the state of the flowers and the size of the potential honey bee population, it then computes a prediction of the numbers of bees foraging and the potential amounts of pollen transferred.
Weather conditions after pollen transfer and the age of the flowers then determine how many flowers will be fertilised successfully (see Dennis, 1979).
This model has the ability to run simulations with varieties not included in the programme (i.e. Golden Delicious, Delicious and Granny Smith) by entering their bloom parameter values through menus. It also has a day degree calculator.

Supplementing pollen supply in the orchard using floral bouquets

In some seasons flowering of the pollinating varieties in an apple orchard may be insufficient to guarantee set of an optimum crop. This may be a consistent seasonal problem, possibly due to there being inadequate numbers of pollinators planted in the orchard.

More often, the problem only occurs in one season and is caused by the pollinators failing to flower adequately or in synchrony with the main variety.
Biennial bearing is a frequent cause of poor flowering on some varieties used as pollinators.
One temporary solution to this problem is to cut flowering branches from another orchard containing suitable pollinating varieties and introduce these to the affected orchard as ‘bouquets’.
The branches are placed in large buckets of water between the trees and in the tree rows.

Supplementing pollen supply in the orchard by grafting

Where the problem of inadequate pollination is a more permanent one, whole or part trees of the main variety can be grafted over to a pollinating variety using frame working or top working grafting techniques (see Garner, 1988).

Work conducted at Long Ashton Research Station showed that Malus species could perform well when grafted as branches into trees of the cropping variety (Williams, 1976).
If the trees of the main cropping apple variety are suspected as being virus infected, grafts of Golden Hornet or Winter Gold should not be used, as they are very virus sensitive.
In these situations, it is possible to use the less sensitive Malus aldenhamensis and Malus Hillierii.
When grafting crab apple pollinators into trees of the main commercial variety, it is essential to control the vigour of the latter in the first few years, so as to allow the graft to develop without too strong competition.
Grafts should be placed in a dominant position on the tree.
Once established the grafted crabs can compete with the main variety quite effectively.
Indeed, M. aldenhamensis and M. Hillieri frequently prove to be very strong growers as grafts.
Grafting other fruiting apple varieties into trees of the main variety (e.g. James Grieve grafted into Cox) can also prove effective in pollination.
However, there is seldom sufficient time to pick the pollinating branch and failure to do this can depress flowering in the subsequent year.
Also, unless great care is exercised it is very easy to remove the branch during winter pruning (Williams, 1966).

Pollen compatibility

Most diploid apple varieties are cross compatible (i.e. self-incompatible). This means that they will not set fruit with their own pollen in most environmental conditions.

They require the viable pollen of another variety to be transferred to their receptive stigmas and the subsequent germination and growth of this pollen down the style into the ovary.
Once there, it should be able to grow, albeit relatively slowly, into the micropyle of an ovule and effect fertilisation.

However, some apple varieties are wholly or partially cross incompatible with each other.

This has been attributed to a multi-allelic gametophytic locus S.
In instances of cross incompatibility, the pollen grains germinate but their growth is arrested in the top one third of the style; this is known as gametophytic incompatibility.

Early work (Kobel et al. 1939) showed that by making various possible cross pollinations and observing the speed of pollen tube growth down the style it was possible to distinguish between compatible, semi-compatible (partially compatible) and incompatible crosses. On the basis of this work, S alleles were assigned to some 20 different cultivars.

Further research (Broohaerts et al., 1995; Janssens et al., 1995) isolated cDNAs responsible for the stylar ribonucleases associated with some of the S alleles first mentioned by Kobel et al.

This led to the development of a molecular diagnostic technique for the identification of these alleles based on allele-specific PCR amplification and restriction digestion.

Recent research at East Malling (Bošković and Tobutt; 1999) has extended this work and characterised the S alleles in a range (56) of diploid and triploid apple varieties based on their stylar ribonucleases.

Diploid varieties with the same pair of S alleles are in most instances incompatible with each other.
Where the two varieties share one S allele in common, then only 50% of the pollen produced will be capable of germination and growth down the style to fertilise the ovule.
This is known as partial compatibility or semi compatibility.

Self fertile clones of the apple varieties Cox’s Orange Pippin and Queen Cox have been produced using irradiation techniques of fruit breeding.

These clones produce less viable pollen than their self-incompatible parents but are capable of setting fruits with this pollen and with no requirement for the planting of other pollinating varieties in the orchard.
Work by Petrapoulu (1985) showed increased stylar receptivity in two of these self-fertile clones of Cox and the author concluded that this may be due to mutations at loci controlling stylar receptivity rather than at the S incompatibility locus.

Research in Switzerland (Kellerhals and Wirthner-Christinet, 1996) showed that hand pollination of Gala with its own pollen followed by cross pollination, using Spartan pollen, 24 hours later led to poor fruit set.

The initial selfing in some way inhibited fertilisation by the Spartan pollen.
It is not known whether this could have a role to play in poor fruit set observed with open pollination in the orchard.

Testing pollen compatibility

Growers wishing to test compatibility of varieties in the field can adopt the following simple method:

Collect flowers of the variety chosen to provide pollen 24 hours before the planned pollination.
Choose flowers at the balloon stage and remove petals before placing on trays at room temperature.
On the tree to be pollinated choose and tag representative branches of a horizontal or semi-horizontal and similar orientation and height above ground level.
Remove petals from the chosen flowers on the spur buds at the balloon stage; this makes them unattractive to pollinating insects.
Remove surplus flowers from 25 spaced clusters to leave 4 flowers/cluster; remove the king flower and any potentially late opening flowers.
Use only the pollen donor flowers that have begun to dehisce their pollen; each flower will usually be sufficient to pollinate four flowers on the variety to be pollinated.
Thoroughly brush the stigmas with the anthers of the pollinating flower.
Ideally, each flower is pollinated twice, once at the time of anthesis (flower opening) and again two days later.
The reason for pollinating twice is to take advantage of the phenomenon known as ‘pioneer pollen’.

The above technique does not entirely exclude the possibility of cross pollination but it can give a quick idea of pollen compatibility.

A modified strategy, used in the Netherlands, involves no removal of petals and only one pollination at the time of flower opening (anthesis) (Wertheim).
Cross pollination is prevented by putting a very small dab of vaseline on the stigmas after they have been hand pollinated.

Pioneer and mentor pollen effects

According to Dutch research (Visser and Verhaegh, 1980), the application of compatible pollen twice with a one to two day interval between applications, could stimulate the activity of the pollen applied on the second occasion.

This was thought to be due to some action by the first applied pollen, thereafter called the ‘pioneer pollen’.
This phenomenon has been used in attempts by plant breeders to overcome self-incompatibility by applying compatible pollen from another variety first, followed by self pollen one to two days later (Visser et al., 1983).

Research conducted in Russia many years ago showed that whilst self pollen generally grew poorly on apples, its addition to pollen from another compatible variety on the stigma, actually helped fruit set and the number of seeds in the fruits (Nesterov, 1956).

In further work conducted in the Netherlands (Visser and Oost, 1982), combinations of self-incompatible pollen with viable but sterile (irradiated) compatible pollen were shown to be effective in overcoming self-incompatibility and this phenomenon came to be known as the ‘Mentor Pollen’ effect.

However, more recent studies in Switzerland (Kellerhals and Wirthner-Christenet, 1996) have shown that double pollination of Gala or Elstar, first with self pollen and secondly with compatible pollen one day later, inhibited the growth of the compatible pollen.

It is possible that the contribution of self pollen to fruit set in apple is greater than previously thought.
If combined with warm weather conditions and some cross compatible pollen, the self pollen may play a significant contribution to fruit set and seed numbers in some situations.

Pollen quality (viability)

It is essential that the pollen grains moved by insect vectors, wind or artificially by man, between the pollen donor (pollinating variety) and the main commercial variety, remain viable and capable of germination and growth down the style into the ovary and micropyle of the ovule.

Pollen viability can be judged crudely by examining the grains under a low powered light microscope.
Small shrunken grains are not viable.
Many of these are found in the pollen collected from triploid varieties of apple.
The self fertile clones of Cox and Queen Cox also produce a significant proportion of pollen that is small, shrunken and not viable.

A more accurate assessment of pollen viability can be carried out using germination tests on artificial media, such as agar or sugar solutions. Sometimes staining procedures are also used to estimate pollen viability (Seilheimer and Stösser, 1982).

Growers wishing to store pollen for use in hand pollination in the subsequent season can achieve this by placing the pollen in a sealed desiccating jar in a refrigerator.
Very long-term storage (e.g. 10 years) is possible at -20⁰C and reduced pressure.
Aborted pollen grains are common in the self fertile clones of Cox’s Orange Pippin and Queen Cox but may also occur in other varieties in some seasons (Gagnieu, 1947).
This is due to faulty reduction division from the diploid to the haploid.
High temperatures (not common in the UK) can increase the amount of aborted pollen produced.

It should be noted that the anthers of some cider varieties are unable to release their pollen resulting in a form of male sterility.

Dormancy and synchrony of flowering times

For effective transfer of viable pollen its growth and the fertilisation of the ovule, it is essential that, where cross-pollination is required, the two varieties flower at approximately the same period.

At the very least there needs to be an overlap of several days in the flowering periods of the main and pollinating varieties.
Scion varieties of apple differ in their dormancy characteristics and this can, on occasions, influence the synchrony of flowering between the main variety and its pollinators.

Dormancy and chilling requirements

The timing of bud break and flower opening is influenced by the dormancy characteristics of the scion varieties and the climatic conditions in late autumn, winter and early spring.

For effective bud break and flowering, flower buds of apples require a period of chilling to break dormancy, followed by a period of higher temperatures (‘forcing’ temperatures) to enable the flowers to complete their development and open.
In areas where winters are normally too warm, as in the tropics, apples can only be grown at higher altitudes (e.g. above 1,000 metres).
For most varieties, 1,000 hours of temperatures of 7^oC-8^oC or less are required to fulfil their chilling requirement (Kronenberg, 1979 see Further reading [hyperlink]).
If this requirement is fully satisfied, the buds will develop and flower normally, if given suitable ‘forcing’ temperatures.
Where this requirement is only partially met, then flowering may prove erratic and flower quality be lessened.
The figure of 1,000 hours at temperatures of less than 7^oC is somewhat arbitrary and some geographic locations where apples grow quite successfully (Napoli, Roma, Pisa and Barcelona) often fail to record this total of chilling units.
In most parts of the UK, the chilling requirements should be satisfied usually by mid January in and around the south east of England.

Global warming in the future could influence the date at which the chilling requirement is satisfied for apples grown in the UK. Further studies are warranted on this possibility and its implications to UK apple yields.

Synchrony of flowering necessary for effective fruit set

Apple scion varieties have been classified, rather crudely, on the basis of their average flowering dates recorded over several seasons. Varieties are often grouped into those flowering early, mid-season or late (Dalbro, 1966; Way, 1978).

One of the problems affecting growers wishing to choose suitable pollinating varieties is that varieties have been grouped according to their average date of flowering.
Even when the full range of flowering dates are given, as in the information recorded in the fruit trials at Brogdale, Kent, these ranges do not represent the actual length of flowering periods in every year.
This means that, whilst sufficient overlap of flowering dates of two varieties may occur in many years, it does not necessarily occur in every year, especially when varieties from early and mid-season, or mid-season and late groupings are planted with the aim of mutual pollination.

Research in Denmark (Grauslund, 1996) has shown that the 10 year average length of flowering periods for 11 apple varieties was 16 days.

Advice in the Netherlands (Kemp and Wertheim, 1992) is that the average flowering ranges of any two varieties should overlap by a minimum of six days.
However, the Danish research showed that even when the overlap was 11 days it was not always ideal in every year.
This is because many of the first flowers to open on a variety are those with the best setting and fruit growth potential.
If, for instance, a late flowering variety has been planted as a pollinator in a block of a mid-season flowering variety, it is unlikely that many of the flowers opening first on the mid-season variety will receive adequate pollination in sufficient time to set fruits.
If the mid-season variety also produces flowers with short EPPs (like Cox), the problem is exacerbated.

Models for predicting apple flowering dates

Attempts in the Netherlands to develop a model for predicting flowering dates of apple were only partially successful (Kronenberg, 1983; Kronenberg, 1985) and further work is needed under UK conditions.

Research at Long Ashton Research Station also endeavoured to develop a model for apple fruit bud development and hence time of flowering (Landsberg, 1974).

The stigmatic surface and style

The stigma is the organ in the flower on which pollen must germinate to achieve subsequent fertilisation.

The surface of the stigma consists of tiny papillae, which are turgid at anthesis (i.e. when the flower first opens).
A surface secretion develops as the flower opens and this is when the flower is considered to be receptive. Apple stigmas are considered to be ‘wet’ in type.
The papillae on the stigmatic surface desiccate rapidly and collapse soon afterwards.
This process is very rapid in apple flowers taking only 2 days from flower opening (Braun and Stösser, 1985).
However, experiments by the same authors in Germany have shown that apple pollen grains are able to germinate on stigmas after they have turned brown.
As late as 10 days after anthesis pollen grains can germinate on the stigma, even though the surface papillae are totally collapsed by this stage.
It would seem that the condition of the stigmatic surface is not a significant constraint on pollen germination or the effective pollination period (EPP).
Poor fruit set as a result of short effective pollination periods (EPPs) is more related to ovule longevity and possibly also growth of the pollen tube down the style, than limitations on pollen germination.

The style is the conducting stem, which connects the stigma with the ovary, within which are the female egg cells or ovules.

The style centre is made up of special conducting or transmitting tissues; this connects with the papillae on the stigma surface.
The cells of the transmitting tissue are elongated and have large spaces between them filled with pectic substances.
The pollen tubes grow down the style.

Pollen germination and pollen tube growth down the style

In favourable conditions, pollen grains deposited on the stigma become hydrated (take up water) and subsequently germinate.
The pollen tube emerges through one of the three germ pores and grows into the papilose epidermis on the stigma.
After it penetrates the stigmatic surface between the papillae, the pollen tube enters the connecting tissue within the style.
Starch in this transmitting tissue is used as a food source by the extending pollen tube.
The tubes usually take two to four days to grow to the base of the style, but this is very dependent upon ambient temperatures.
At temperatures of 5-10⁰C pollen tube growth is very slow.
Not all of the germinating pollen grains reach the base of the style.
If approximately 50 germinate and start to grow down the style only five or ten pollen tubes make it to the base.
At the base of the style the pollen tube must penetrate the pericarp before entering the ovule.
Although a very short distance, this penetration can be quite slow.
For example whilst pollen grains may take only two to three days to germinate and reach the base of the style, they may take a further six to eight days to transverse the pericarp and effect a union with the ovule.
Although the distances within the pericarp are less than down the style, the conducting tissues located in the latter are much more favourable for rapid growth (Stösser et al., 1996).
Once in the locule of the ovary, pollen tubes frequently show very little directed growth towards the micropyle of the ovule and may ramify for more than a day before locating and penetrating the micropyle (Stösser, 1986).

Factors influencing fertilisation

Pollen germination, its growth down the style into the ovary and the achievement of fertilisation, by the pollen’s entry into the micropyle of the ovule, are influenced by a number of factors. The most important of these are:

The apple scion variety
Pollen quality
The quantity of pollen on the stigma
Pollen compatibility
Climatic conditions

Apple scion variety

Apple scion varieties differ in the proportion of viable pollen produced, the average germination percentages achieved (over a number of seasons) and the climatic conditions under which the pollen will germinate.

The self fertile clones of Cox’s Orange Pippin (Cox Self-fertile Clone 8) and Queen Cox (Queen Cox Self-Fertile Clone 18) both produce low proportions of viable pollen compared with the standard clones of Cox or Queen Cox.

Much of the pollen produced by these self-fertile clones is aborted and infertile.
Whilst this has little consequence when the self-fertile clones are planted as monocultures (i.e. solid orchard plantings of the same clone), their use as pollinators in other orchards of self-sterile clones or varieties cannot be recommended.
Insufficient viable pollen is produced by these self-fertile varieties to be collected and transported efficiently by insects between flowers on different trees.

Pollen of some scion varieties germinates and grows down the receptor styles of flowers at quite low temperatures.

Although temperatures of 15-25^oC are required by most varieties for efficient pollen germination and growth, pollen of a few varieties, such as Falstaff and Redsleeves, is able to germinate and grow efficiently at much lower temperatures (Petrapoulu, 1985).
These varieties also exhibit what is known as high stylar receptivity.
These studies showed also that high stylar receptivity, high pollen germination at low temperatures, precocity and high yield efficiency seem to be associated in apples with a low chilling requirement.
In support of this, it has been shown that lack of winter chilling is associated with poor fertility of Cox flowers (Jackson et al., 1983).

Quantity of pollen on the stigma

There is evidence with many flowering plants that the quantity of pollen deposited on the stigma influences its subsequent germination (Brewbaker and Majumdar, 1961).

Increasing the number of grains deposited on the stigmatic surface is thought to have a mutually stimulatory effect on grain germination, possibly attributable to the increased calcium ions associated with increased pollen abundance (Brewbaker and Kwack, 1963).

Climatic conditions

The ideal climatic conditions for pollen germination and growth are temperatures between 15^oC and 25^oC, no rain, and minimal or no desiccating winds.
At lower temperatures pollen germination and growth will be slower.
Very little pollen germination occurs at temperatures less than 10^oC.

Fertilisation and fruit set

Once the pollen tube has located and penetrated the micropyle and fused with the embryo sac, fertilisation has taken place.
Pollen grains of apples contain two nuclei, which move with the cytoplasm in the pollen tube as it moves down the style.
At some point one of these nuclei (known as the vegetative nucleus) degenerates and the second one (the generative nucleus) divides into two sperm nuclei.
When a single pollen tube enters the micropyle it discharges the sperm nuclei into the embryo sac.
One of these nuclei fertilises the egg to form the zygote (embryo).
The other unites with the two polar nuclei in the ovule to form the triploid endosperm tissue.

Induction of fruit set and retention with plant growth regulating chemicals

Researchers have for more than 30 years sought to improve the fruit set and retention of apples, using sprays of various plant growth regulating chemicals. The principal ones tested have been

Auxins
Auxin transport inhibitors
Gibberellins
Gibberellin biosynthesis inhibitors
Cytokinins
Mixtures of auxins, gibberellins and cytokinins
Ethylene biosynthesis inhibitors
Polyamines

Auxins

Attempts to improve apple pollination and fruit set with sprays of auxins, such as IAA and NAA have proved ineffective.

However, when combined with gibberellins (GA₃ or GA₄₊₇), auxins such as NAA, NAAm and 2,4,5-TP have proved effective in increasing fruit set (see below).
Although auxins such as IAA are strongly implicated in the processes associated with fruit set and retention, sprays of IAA are largely ineffective in improving fruit set.
One of the contributory reasons for this may be the high instability of IAA, when dissolved in aqueous solutions and sprayed onto trees.

Auxin transport inhibitors

Attempts by researchers at Long Ashton Research Station to increase fruit set on several tip or spur bearing apple varieties, using sprays of 2,3,5-tri-iodobenzoic acid (TIBA) at 150 ppm, were unsuccessful.

The only effect of the sprays was to reduce fruit size at harvest.

Gibberellins

Gibberellins have been shown to be an essential hormone if parthenocarpic fruit set is to be induced in apple (Schwabe and Mills, 1981).

However, they are largely ineffective used alone on varieties such as Golden Delicious, Jonagold, Cox’s Orange Pippin and Boskoop (Wertheim, 1993; Varga, 1966).
Research in Germany over a period of 10 years showed that sprays of either GA₃ or GA₄₊₇ to frost-affected flowers had no significant effect on fruit set (Bangerth and Schröder, 1994).
Studies conducted in Canada (Looney et al., 1985) suggested that whilst GA₃ and GA₇ had inhibitory effects on apple flower initiation, GA₄ had a stimulatory effect.
These results could not be repeated, however, in German experiments (Bangerth and Schröder, 1994).

Gibberellin biosynthesis inhibitors

Trials with daminozide (SADH or Alar) in the early 1970s showed that, not only did the sprays increase flowering on trees of Discovery in the year after spraying, they also stimulated pollen tube growth in the styles of the flowers produced (Stott et al., 1973; Stott, et al., 1974).

Alar is no longer available for use on fruit trees.

Cytokinins

Treatments of apple flowers with cytokinins on their own do not induce fruit set.
However, in combination with gibberellins stable cytokinin-type compounds such as diphenylurea or CPPU can have beneficial effects on fruit set and yields.

Mixtures of auxins, gibberellins and cytokinins

Research conducted in the 1960s in the USA (Williams and Letham, 1969) first showed that combinations of gibberellins with cytokinins might have promise as setting agents for apples.

The ‘Wye Mixture’, developed by researchers based at Wye College in Kent, enjoyed some popularity in the 1970s and early 1980s as a fruit setting spray for use with apple varieties in the UK (Kotob and Schwabe, 1971; Goldwin, 1978; Goldwin, 1981).
Goldwin and colleagues demonstrated the feasibility of inducing parthenocarpic (seedless) fruit set in the apple variety Cox’s Orange Pippin. It had been known for many years that low concentration, high volume sprays of gibberellic acid GA₃could have a very beneficial effect on pear varieties such as Conference.
Applied following severe frost-damage to flowers, the sprays stimulated the production of a crop of parthenocarpic fruits.
Trials on apples using GA₃ sprays with no other hormones added were not successful, however, when applied after frost damage.

The work conducted at Wye College demonstrated that, if the GA₃ (200 ppm) was supplemented by an auxin and a ‘cytokinin’, the spray mixture was capable of inducing good fruit set.

Various auxins were tested including 1-naphthaleneacetic acid (NAA), 2‑naphthoxyacetic acid (NOXA at 50 ppm) and 2,4,5-trichlorophenoxy propionic acid (2,4,5‑TP at 10 ppm) and although there were small variations in response all seemed to work well with the other two components of the mixtures.
The ‘cytokinin’ used was not a true cytokinin, but a product known to have cytokinin type activity in plants, namely NN’‑diphenylurea (DPU at 300 ppm).

Trials in Germany, however, (Bangerth and Schröder, 1994) showed no consistent benefits when using this Wye Mixture on varieties such as Golden Delicious, Jonagold and Boskoop.

Trials in the USA (Greene, 1980) have also shown that applications of gibberellins plus 6‑benzylaminopurine (a cytokinin) caused reduced rather than improved fruit set and this was attributed to increased ethylene levels induced in the flowers by the sprays.

Blasco et al., (1982) showed that whilst sprays of the Wye Mixture increased fruit set of Cox trees on M.9 rootstock, no similar effects were recorded on trees on M.26, M.7 and MM.106 rootstocks.

However, initial fruit set was increased on trees on all rootstocks, irrespective of their vigour.
It is hypothesised that the sprays facilitate the retention of fruitlets, which have initially set with very few seeds.
Later in the season, when competition for assimilates with extension and bourse shoots increases, these fruits are shed from the trees on the more invigorating rootstocks.
On M.9, where shoot competition is less, they are retained.
Intuitively, if this hypothesis is correct, one would expect treatment with chemicals which reduce excessive shoot growth to aid the retention of hormone-stimulated fruitlets on trees grafted on invigorating rootstocks.
However, as most of the growth retardant chemicals currently available reduce natural gibberellins in the tree (e.g. Cultar and Regalis, they are likely to partially negate the value of the gibberellins in the setting mixtures.

According to Bangerth and Schröder (1994), the best effects are achieved with cytokinins which are biologically stable such as the phenyl urea type and natural cytokinins such as zeatin-riboside give poorer results.

In these trials, the best cytokinin type compound was N‑(2-chloro-4-pyridyl)-N-phenylurea (CPPU) at 20 ppm.
The best combinations were of GA₄ or GA₇ (100 ppm) together with CPPU; GA₃ and GA₅ gave poorer results.
It is hypothesised that the effects are brought about by the hormones’ direct action on the fruit ovary and not by any changes in fruit ‘sink strength’.
It is possible that a short hormone pulse, provided by the treatments, establishes hormone autonomy in the unfertilised ovary (Browning, 1989).
The addition of the cytokinin may enable this hormone autonomy to be sustained through until harvest.
It is interesting that fruits set using this mixture are not susceptible to abscission induced by seeded fruits or strong shoot growth close by.
In other crops such as kiwi fruits, CPPU causes a reduction in the natural levels of the inhibitor ABA.
Unfortunately, the GA + CPPU mixtures used in the trials described above reduced flowering significantly in the subsequent season.
Also, the CPPU in these mixtures causes russeting and flattening of the fruit shape. Because of this and because registration may prove difficult, it is unlikely that these gibberellin CPPU mixtures will become available to growers in the near future.

One recent suggestion is that neither the gibberellins nor the cytokinins have a direct inhibitory effect on flower initiation, but it is the increased fruit set induced by them that inhibits subsequent flowering (Bangerth and Schröder, 1994).

Ethylene biosynthesis inhibitors

Ethylene is involved in the abscission of flowers and young fruitlets and application of chemicals that inhibit ethylene production by the apple tree can sometimes reduce flower and fruitlet abscission.

Aminoethoxyvinylglycine (AVG) has been reported to inhibit both ethylene production (Baker et al., 1978) and its action (Beyer, 1976); AVG inhibits the enzyme system that converts S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid.
This same enzyme system is also influenced by the endogenous levels of auxin (IAA) in the fruits and spurs.

Sprays of 200 ppm AVG applied at bloom time to Red Delicious apple trees increased fruit set but reduced fruit size and increased the length diameter ratio of fruits at harvest time (Greene, 1980).

In further studies Greene (1983a) showed that sprays at 500 ppm pre harvest increased the initial but not the final set in the subsequent season of McIntosh, Spartan and Spencer apple trees grown in Massachusetts.
Two clones of the variety Red Delicious also responded similarly, but here the treatments increased final as well as initial set.
However, final fruit sizes were reduced by the treatments (10% to 20%) and the length diameter ratios of the fruits were increased.
Applications of fruit thinners were shown to be ineffective in reversing the negative influence of AVG on fruit sizes (Greene, 1983b).
It has been argued that the increase in fruit set achieved with the autumn AVG treatments is due to reduced post petal fall abscission of very young fruitlets (Williams, 1981).
Williams also showed that the AVG treatments suppressed ethylene production in the leaf buds and induced increases in bud break vegetative growth.

Trials conducted in the early 1980s at Long Ashton Research Station (Child et al., 1986) showed that sprays of AVG at 250 ppm applied at full bloom increased the initial and final fruit set of Cox’s Orange Pippin apple trees.

The AVG inhibited the ethylene produced following treatment with gibberellins but had no effect on the ethylene induced by treatments with the thinner NAA.

Other known inhibitors of ethylene production by the tree, such as aminooxyacetic acid and silver nitrate have generally proved ineffective in increasing apple fruit set (Dennis et al., 1983; Rahemi, et al., 1997).

Recent studies by Rahemi, et al., (1997) led the authors to suggest that differences in ethylene evolution are not responsible for differences in initial fruit set.

They argued that the effects of AVG in increasing set are independent of its effects on ethylene evolution.
In earlier work, UK scientists had suggested that the AVG sprays delayed the senescence of ovules and increased the setting capacity of older flowers (Child et al., 1982).

Polyamines

Sprays of the aliphatic polyamines, spermine, spermidine and putrescine, have been shown to increase the retention of very young fruits on apple trees (Costa and Bagni, 1983).

However trials conducted at HRI-East Malling gave diappointing results with these spray treatments (Knight, personal communication).

Branch orientation and its effect on fruit set and retention

Research conducted at HRI-East Malling in the 1980s (Robbie et al., 1993) showed that training branches of the variety Cox’s Orange Pippin to the horizontal during August, produced flowers that gave better fruit set in the subsequent year than flowers on branches trained to the vertical.

However, if branches trained to the vertical in August were subsequently trained to the horizontal in the following April, during flowering, then fruit set on these was also greatly improved.
This indicates that the horizontal orientation is important not just to flower development but to the success of pollination/fertilisation itself.
The failure of poor fruit set on vertically orientated shoots was not attributable to more vigorous shoot growth and competition between shoots and flowers for assimilates, minerals and water.
Flowers on vertical shoots with all the competing shoot growth removed still set poorly.
The studies also showed that the spurs and flowers on vertically and horizontally orientated branches did not differ in their mineral content or the ability of the primary leaves to acquire carbon and morphologically the flowers from the two branch types were similar.
In a way that is not understood, training branches to the horizontal appeared to increase the proportion of healthy ovules in the flowers at anthesis (flower opening) and later.
Increases in ovule fertility and associated increases in the Effective Pollination Periods (EPPs) of flowers from horizontally orientated flowers were evident, even in flowers on branches moved to the horizontal close to flowering time.

Vigour of shoot growth and its effect on fruit set and retention

Research at Long Ashton showed that management of apple trees that results in excessive growth of bourse or extension shoots can reduce fruit set (Abbott, 1960).

The reasons for this are probably primarily associated with competition between the shoots and the young fruitlets for assimilates and minerals, although hormones produced by the shoots may also play a role in this effect.
Management practices, which reduce the vigour of shoot growth, generally result in improved fruitlet retention.
Whilst initial bourse shoot growth may compete with fruitlets and cause fruitlet abscission, bourse shoots aid fruitlet growth later in the season, possibly by supplementing the supplies of available assimilates.

The effect of irrigation and water supply to the tree on fruit set and retention

Trees grown in droughty situations show significant reductions in shoot growth, leaf area and yields (Jones and Higgs, 1985; Jones et al., 1983; Lakso, 1983).

However, experiments in Germany showed that the numbers of flowers formed per inflorescence (cluster) or the fruit set per linear branch length were not reduced (Sritharan and Lenz).
The reduced yields must be mainly attributable to reductions in tree size or to reductions in fruit size.

Apple Best Practice Guide

Pollination and fruit set – additional information

Site altitude, aspect and slope

Provision of adequate shelter in the orchard

Frost damage

Suitable pollinating varieties

Self fertility

Pollination using ornamental crab apples or other Malus species

Production of adequate quantities of viable pollen

The number of trees of the pollinating variety planted in relation to the numbers of the main variety

The management of the pollinating variety in the orchard

Creating suitable habitat for wild insects suitable for pollinating

Attractiveness to honey bees

The influence of spray chemicals on pollen germination and growth

Applying sprays of macro- and micronutrients to aid pollen germination, pollen tube growth and fruit set

Installation of a frost protection system

Use of chemical sprays designed to provide some protection from frost and winds

The Effective Pollination Period or EPP

Supplementing pollen supply in the orchard using floral bouquets

Supplementing pollen supply in the orchard by grafting

Pollen compatibility

Pioneer and mentor pollen effects

Pollen quality (viability)

Dormancy and synchrony of flowering times

The stigmatic surface and style

Induction of fruit set and retention with plant growth regulating chemicals

Branch orientation and its effect on fruit set and retention

Vigour of shoot growth and its effect on fruit set and retention

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Apple Best Practice Guide

Pollination and fruit set – additional information

Site altitude, aspect and slope

Provision of adequate shelter in the orchard

Frost damage

Suitable pollinating varieties

Self fertility

Pollination using ornamental crab apples or other Malus species

Production of adequate quantities of viable pollen

The number of trees of the pollinating variety planted in relation to the numbers of the main variety

The management of the pollinating variety in the orchard

Creating suitable habitat for wild insects suitable for pollinating

Attractiveness to honey bees

The influence of spray chemicals on pollen germination and growth

Applying sprays of macro- and micronutrients to aid pollen germination, pollen tube growth and fruit set

Installation of a frost protection system

Use of chemical sprays designed to provide some protection from frost and winds

The Effective Pollination Period or EPP

Supplementing pollen supply in the orchard using floral bouquets

Supplementing pollen supply in the orchard by grafting

Pollen compatibility

Pioneer and mentor pollen effects

Pollen quality (viability)

Dormancy and synchrony of flowering times

The stigmatic surface and style

Induction of fruit set and retention with plant growth regulating chemicals

Branch orientation and its effect on fruit set and retention

Vigour of shoot growth and its effect on fruit set and retention

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