Introduction

 

Apple trees frequently set and retain excessive numbers of fruits in relation to tree size and leaf area. The result of this is the production of fruits at harvest which are smaller than required by the markets and which often show poor storage potential.

• Reduce the fruit numbers on the trees so that only those that can be sized adequately are retained through to harvest.
• Remember that on a small tree it is better to have 80 apples which average 68 mm in diameter than 120 apples averaging 60 mm.
• Recent research shows that fruits from thinned trees have higher concentrations of polyphenols which are important to human health and nutrition.

 

 

 

• Larger fruits are:

                                 – Easier and cheaper to pick

                                 – Cheaper to grade

This essential crop load adjustment can be achieved using several strategies, which are discussed in the sections below.

Optimum levels of fruit set for UK apple varieties

Introduction

The best way to assess the optimum fruit set is to use at least three years historical cropping and grading information to calculate the average number of fruits per kilogram and from the yield data then calculate the average numbers of fruits per tree.

• By adding a number for fruit dropped at harvest (approximately 5%) the number of fruits left after thinning can be arrived at.
• In modern intensive orchards on M9 there is rarely a significant June drop but this will have to be taken into account on old or vigorous Cox orchards with a history of variable cropping.
• Thin to specified numbers of fruits per tree.
• Thin to specified numbers of fruits per flower cluster.

 

Summary of best practices for achieving optimum fruit numbers

 

Deciding whether to reduce the crop load (by thinning or other methods)

It is important to maximise yields of apples in the grades most desired by the markets in order to sustain reasonable profitability.

• Maximising fruit set and total yields is rarely the best strategy, as this usually results in the production of too many fruits of small size and poor storage potential.
• In contrast, to maximise the yield per tree of top quality fruits it is important not to thin trees more than is necessary.
• Decisions on whether to thin or not are, therefore, not always easy to make.

For some crops, decision support models are being developed to aid fruit growers in making these decisions. One example is a model focused on crop loading of kiwi fruit vines developed in New Zealand (Mills and Atkins, 1992).

• No sophisticated crop loading models exist currently for apples grown in the UK and the grower must use experience of the orchard, knowledge of weather conditions and, often, natural intuition in making decisions on the need to thin.

The main factors which determine the need or not to thin an orchard are:

• The variety
• The rootstock
• The market requirements for the fruits
• The current season’s weather
• The condition of the tree

The variety

Small-fruited varieties, such as Gala and Cox and their various clones, are more likely to require regular thinning than larger fruited varieties such as Jonagold, Braeburn or Bramley.

• If the small-fruited variety is also prone to set heavily, as Gala is, then the need for seasonal and early thinning becomes even more acute.

The rootstock

Trees grown on very dwarfing rootstocks, such as M.27, need thinning in most years if adequate fruit size is to be achieved. This is particularly important when growing varieties such as Gala or Cox on this rootstock.

Average fruit size on the popular rootstock M.9 (EMLA, Pajam 1, Pajam 2, etc.) is generally better than on most other rootstocks and thinning severity will not need to be as great as for trees on M.27.

• However, where M.9 is planted on soils of lower than average fertility or where temporary drought conditions prevail the shallow roots of trees on M.9 will make them vulnerable to producing small fruit sizes unless irrigation and/or judicious thinning is carried out.

Trees on the rootstocks MM.106 and P.22 are also prone to overset and small fruit size on many sites and careful attention should be paid to adequate thinning.

The market requirements for the fruit

Most dessert apple varieties are grown for the fresh markets.  Multiple retailers will now take a range of sizes and colour to satisfy their demand for consumer choice 

Where fruits are marketed through alternative channels (farmers’ markets, farm shops etc.) different standards of background colour and fruit firmness may be acceptable.

• Care must be taken not to over-thin large-fruited varieties such as Jonagold, or fruit too large for the UK consumer markets may be produced.
• Bramley fruit, especially those grown for the fresh market, are usually required to be in the range 90-110mm. This requires careful crop load management and pruning
• Slightly smaller fruits and hence higher crop loads can be tolerated where the Bramley are focused on the processing markets. 

The current season’s weather 

This is often the most important factor to take account of when deciding whether to thin or not.

• Frost or cold, windy or wet weather during the flowering period usually equates with poor fruit set.
• When these unfavourable conditions prevail, growers are, understandably, reluctant to contemplate early (blossom) thinning.
• In these situations decisions on thinning are usually delayed until after fruit set, when the potential crop load can be assessed.
• Whilst this decision was sound in the past, as later thinning with Thinsec (carbaryl) was always an option, the withdrawal of carbaryl for use as a thinning agent, means that until BA is approved, hand thinning will remain the only option.
• Whilst declining to blossom thin will always give the best assurance of adequate fruit set should a frost occur in late bloom, it will be associated with a high cost for hand thinning if fruit set turns out to be bigger than required.

Temperatures during the cell division period (full bloom through to six weeks later) will influence fruit size.

• Where this period experiences higher than average temperatures, fruit size is more likely to be adequate.
• The exact relationship is not fully understood and fruit numbers per tree will override the temperature effect.

The condition of the tree

Trees in healthy condition with minimal shoot growth, dark leaves and many fruitlets that are clearly set should be thinned because such trees are likely to retain excessive numbers of fruitlets through to harvest.

• However, such healthy trees are also able to support and size up more fruits than less healthy trees. 

Trees in less healthy condition or those lacking nitrogen, as often found in organic plantings, also need thinning.

• Often this needs to be more severe thinning, as the weakened trees are able to size up fewer fruits per unit tree size than healthy or well fertilised trees.

 

Thinning to specific numbers of fruits per tree

Mature and well-managed apple trees develop approximately similar numbers of spur and extension leaves in each season. These leaves are vital for producing, by photosynthesis, the food stuffs (sugars, carbohydrates) that power the growth of the fruits.

It is reasonable to suggest, therefore, that optimum fruit numbers that can be developed to full size by the tree, are related closely to:

• Tree size, especially canopy volume, and
• Light interception.

If light interception and temperatures were similar in most seasons, then the numbers of fruits that the mature tree could grow to full size would also be similar in most seasons.

• Climatic conditions differ slightly in most seasons, however, and some adjustments to optimum fruit numbers need to be made in the light of these seasonal variations.
• Thinning to ideal numbers of fruit per tree, is more logical and gives less variable effects than the often used strategy of thinning to specific numbers of fruit (1 or 2) per floral bud.

Trials conducted in France on six-year-old Royal Gala/M.9 trees at a spacing of 4 x 2 m showed that cropping loads of 120, 180 and 240 fruits per tree were equivalent to 5, 7 or 9 fruits per square centimetre of trunk cross sectional area.

• In these trials, in which the crop loads were established by thinning 20 days and 50 days after flowering, the two lighter crop loads produced 90% of their fruits in the top grades.
• The highest crop loading produced only 70% in the top grades.
• Large differences in ripening were noted with 58% or fruits ready for harvest at the first picking date on the lightest cropping trees, but only 27% on the heaviest cropping trees.

 

Thinning to specific numbers of fruits per blossom cluster

 

The traditional practice in fruit thinning is to thin to either one or two fruits per flower cluster.

• On sites which regularly produce heavy crops of smallish fruits, thinning to single fruits per cluster is usually recommended.
• On more fertile sites, or with larger fruited varieties, thinning to two fruits per cluster is often preferred.
• Usually, this cluster thinning is combined with removal of all or most fruitlets on axillary wood.

Whilst this strategy of thinning is easy to explain to farm staff, it is rarely the best one for orchards.

• The abundance of flowering in trees varies from year to year and thinning to specific numbers of fruits/cluster may, therefore, result in very different crop loads per tree.
• Also, work in New Zealand has shown that it is better to leave two or even three fruits per cluster on ‘strong’ clusters and completely remove all fruits from other ‘weaker’ clusters.
• Similarly, strong floral buds on the one-year-old wood of varieties such as Gala can, on fertile sites with good tree growth, produce good fruit size and quality.

Ideally, growers should acquire an intuitive understanding of the type and size of crop which trees in particular orchards are capable of carrying and then thin accordingly. Unfortunately, this intuitive understanding is difficult to pass on to the casual labour often used for fruit thinning.

• Where possible growers should aim to thin trees to specified fruit numbers/tree (see above) rather than to specific numbers of fruits/flower cluster.

Optimum fruit numbers needed per tree in UK orchards

Trials and observations conducted by FAST in the UK have demonstrated the value of using thinning guidelines based on target numbers of fruits per tree at harvest.

• These target fruit numbers are adjusted to take account of the tree spacings within the rows or between rows in multi-row beds.
• Use the table below as a guide to fruit numbers per tree
• Better still using accurate grading records calculate the actual numbers per tree over the last three seasons and relate these to optimum yield and fruit size expectations.

 

Target numbers of fruits/tree at harvest to obtain optimum grade out

 

Tree spacing	Target number of fruits/tree
	Cox & Discovery	Gala	Jonagored	Katy	Bramley
1-1.25m	80	96	95	70	40
1.5m	95	115	120	80	50
1.75-2m	110	125	130	100	60
2.5m	215	230	240	200	100
3.5-4.0m	420	460	475	400	200

 

• The above numbers of fruits are those required at the time of harvest and, if thinning is carried out prior to ‘June Drop’, extra fruit numbers will need to be left on the trees to compensate for this.
• For early thinning, approximately 10% to 20% more fruitlets should be left on the trees than shown in the above table.

 

Modern weight/size grading equipment will provide accurate weights of fruits and therefore fruits per kg for each size band.  Where these are not available use the figures below as a guide

 

Dessert Fruit		Bramley
Size	Kg	Size	Kg
<55 mm	0.06	<70 mm	0.13
55-60 mm	0.07	70-80 mm	0.16
60-65 mm	0.08	80-90 mm	0.22
65-70 mm	0.10	90-100 mm	0.27
70-75 mm	0.13	100-110 mm	0.37
75-80 mm	0.16	110-120 mm	0.45
>80 mm	0.20	>120 mm	0.50

 Methods of reducing crop load including flower/fruitlet thinning

First decide whether there is a need to thin the apple orchard and to what level and then determine which method of thinning is the most appropriate.

There are several possible strategies to optimise the crop loads on apple trees.  The potential numbers of fruits can be reduced by:

• The removal of sites of floral bud development in winter pruning
• Flower thinning and/or
• Fruitlet thinning. 

It is important to decide which of these methods are appropriate.

Do I need to thin and which method should I use?

Flower (blossom) thinning

If current guidelines on rates and timings are observed, over thinning with a chemical blossom thinner is unlikely.  The aim is to reduce the cost of hand thinning.

Before deciding on blossom thinning it is important to ask:

• Does historical data show consistently that thinning is necessary?
• Are the flower cluster numbers twice as high (or more) than necessary for a good set?
• Has there been little or no frost damage and none forecast? 

If the answer is yes to the above it is definitely appropriate to consider blossom thinning.

Fruitlet thinning 

When contemplating the need for thinning of fruitlets it is vital to ask:

• Are trees compact in growth?
• Are leaves small and dark?
• Are fruitlet numbers double or more than required?
• Are fruitlets clearly visible above foliage?

If the answer is yes to the above it is definitely appropriate to consider a fruitlet thinning treatment.

Hand thinning

When contemplating the need for hand thinning of fruitlets, it is vital to follow the following guidelines:

• Check fruit numbers on at least 10 representative trees in each orchard.
• Estimate whether fruit numbers more than 120% of those required at harvest?
• If thinning is warranted, hand thin to optimum fruitlet numbers (before fruitlets reach 25 mm in diameter).
• Remove small and shaded fruitlets and those on weak wood.
• Where fruitlet numbers are between 100% and 120% of optimum, wait and reassess 2‑3 weeks later.  If necessary, thin at this later stage removing poor quality fruit.

 

Removal of sites of floral bud development in winter pruning

 

Winter pruning can reduce the need for flower or fruitlet thinning on trees that regularly set too many fruits

• On trees which are making excessive numbers of flowers, too many of which set resulting in poor fruit size, spur reduction should be carried out as part of winter pruning.
• On trees where excessive flowering is accompanied by minimal new shoot growth, some renewal pruning of branches should be carried out to restore a better balance between shoot growth and flowering.
• This usually entails heading back some of the branches quite severely.

Flower (blossom) thinning

• Flower thinning involves the removal of a proportion of the flowers (hand or mechanical methods of flower thinning) or alternatively treating flowers in some way to prevent them setting fruits (chemical methods of flower thinning).
• Mechanical methods of flower thinning have been developed and have proved successful on close planted orchards on the continent. These mainly involve the use of revolving plastic filaments similar to that used in a strimmer.

Hand thinning of apple flowers

• Hand thinning of flowers has the advantages that it is environmentally sensitive (uses no chemicals) and allows competition between developing fruitlets to be reduced at the earliest opportunity.
• However, hand thinning of flower clusters is rarely if ever carried out in mature commercial orchards. It is too labour intensive and hence too expensive.
• Hand removal of flower clusters on newly planted trees may be appropriate in the first one or two seasons.

Chemical methods of thinning apple flowers

Most chemicals that have been tested and found effective in preventing apple flowers setting fruits work by desiccating the flower organs and preventing pollination and/or fertilisation (fruit set).

• There are a number of chemicals that work in this way but the only one currently available to UK growers is the nutrient ammonium thiosulphate (ATS)

Ammonium thiosulphate (ATS)

 

The foliar nutrient ATS has a very useful side effect as a blossom thinner on apples and other crops.

• If applied at flowering time, ATS works by desiccating and, therefore, damaging the stigmas and styles of apple flowers, so preventing them setting fruits.

When using ATS as a blossom thinner on apples it is important to consider the best timings for the sprays, the ideal weather conditions, the optimum spray concentrations and any variations in treatment associated with different apple scion varieties.

 

Spray timings when using ATS

 

• Thinning using ATS is most efficient if the sprays are applied between 20% and 50% full bloom.
• Flowers which are at balloon stage through to those that have been open for 2 days are the most sensitive to the sprays.
• Although the petals of flowers at the pink bud stage are damaged by ATS sprays, the flowers still remain capable of setting fruits.
• Flowers that have been open for more than two days and have been pollinated by bees will often still set fruits, although damaged by the ATS sprays.

 

• In seasons when flowering is concentrated over just a few days, then a single treatment with ATS will often be sufficient to thin the trees effectively.
• In years when the blossoming period is extended two sprays may be required; the first when 25% of the blossoms have opened and a second when most of the spur flowers have opened.
• With varieties that are prone to set large numbers of fruits on one-year-old wood (axillary blossoms) growers often endeavour to selectively prevent this fruit set using sprays of ATS applied after full bloom.
• Axillary blossoms, which often give rise to smaller than average fruits at harvest, flower several days after spur blossoms and to thin them ATS sprays must be delayed until early petal fall on spur blossoms. 
• Care must be taken with this late treatment,  however, if damage to fruitlets set earlier is to be avoided

The ideal weather conditions

• As temperatures increase above 15^oC, the efficiency of thinning when using ATS is increased
• Slow drying conditions (high humidities) improve thinning slightly, but may also cause phytotoxicity on the spur leaves.
• Spraying high concentrations of ATS in slow drying conditions is not recommended on account of the potential problems of phytotoxicity to the spur leaves.
• Spur leaves are essential to the early growth of the persisting fruitlets.

Spray concentrations and volumes 

Throughout the world the most effective ATS concentrations for thinning apples have ranged from 0.5% to 2.0%.

• In the UK concentrations of 1.0% or 1.5% have generally proved most effective in the trials conducted.
• The required concentration and also spray volume for effective flower thinning is influenced by both the temperature and the relative humidity.
• Research in Canada has indicated that when sprays of ATS are applied at low volumes the spray concentration needs to be increased, in comparison with sprays applied at high volumes.
• Research in Poland indicates that sprays of ATS applied at low volumes (at appropriate concentrations) are more effective than sprays applied at high volumes.

 

The table below gives some guidance on concentrations and spray volumes as influenced by the temperatures and humidities at the time of spraying

 Slow Drying

Relative Humidity (%)	Temperature 0C
	10-15		15-20		20+
	ATS %	Litres/ha	ATS %	Litres/ha	ATS %	Litres/ha
>95	1.0	200-400	1.0	300-500	1.0	300-500
85	1.0	300-500	1.5	400-600	1.5	400-600
<80	1.0	400-600	1.5	600-800	1.5	800-1000

                                                                                                            Quick Drying

• It may be necessary to increase the concentrations of ATS by 0.5 % on very heavy setting varieties or alternatively, apply a second spray.
• Most proprietary brands of ATS contain 85% or more of ATS with smaller amounts of impurities.
• Trials at East Malling have shown that two of the ATS brands available in the UK, Thiosul and F.3000, are approximately similar in their thinning efficacy and differ very little in the small amount of phytotoxicity induced on spur leaves.
• The brand ‘Blossom Plus’ is similar in its efficacy.
• 1.176 litres of products containing 85% active ingredient ATS in 100 litres of water will give a spray concentration of 1.0%.

 

The tables below show amounts of ‘Blossom Plus’ required to achieve different ATS concentrations in different volumes of water

 

Water vol. GPA	Pints of ‘Blossom Plus’					Water vol. L/ha	Litres of ‘Blossom Plus’
	ATS concentration %						ATS concentration %
	0.5	1.0	1.5	2.0			0.5	1.0	1.5	2.0
20	0.8	1.6	2.4	3.2		200	1.1	2.2	3.3	4.4
40	1.6	3.2	4.8	6.4		400	2.2	4.4	6.6	8.8
60	2.4	4.8	7.2	9.6		600	3.3	6.6	9.9	13.3
80	3.2	6.4	9.6	12.8		800	4.4	8.8	13.2	17.6
100	4.0	8.0	12.0	16.0		1000	5.5	11.1	16.6	22.2

Use of adjuvants

A low rate of a non ionic wetter will improve the action of ATS and can be used except in very slow drying conditions.

• Do not tank mix ATS with any other spray material or apply within 2 days of applying other sprays, or excess leaf damage may occur.

Variations associated with specific varieties

Varieties differ slightly in terms of their thinning requirements and hence in the ideal ATS treatment required for optimum thinning; brief recommendations are presented below:          

• Alkmene (Early Windsor/Ceeval) One application of ATS at 1.0-1.5% at full bloom is sufficient and no adjuvant is usually necessary.
• Braeburn Braeburn tends to have a very concentrated blossom period and ATS can thin a greater proportion of flowers in one spray than on other varieties. For this reason it should be used with caution especially on young trees.
• Bramley Two applications of 1.5-2.0% ATS, the first at full bloom on two-year-old wood and the second approximately 7 days later.
• Cox One application of ATS at 1.0-1.5% just after bloom on two-year-old wood. Do not include any adjuvant unless blossoming is very profuse.
• Discovery One application of ATS at 1.0-1.5% just after bloom on two-year-old wood. Do not include any adjuvant unless blossoming is very profuse.
• Egremont Russet Two applications of 1.5% ATS are usually necessary. The first is applied at 80%-90% full bloom on two-year-old wood and the second approximately 7 days later. Only include adjuvant if profuse amounts of bloom are present and fruit size is a known problem associated with the orchard.
• Gala Two applications of ATS at 1.0-1.5%; the first timed at 80% full bloom on the two-year-old wood and the second approximately 7 days later. The second spray is aimed to target axillary blossoms on one-year-old wood.
• Golden Delicious Trials in Europe have shown that sprays of 1% to 2% (depending upon blossom abundance and season) are effective in thinning this variety.
• Jonagored One application of ATS at 1.0-1.5% at full bloom is sufficient and no adjuvant is usually necessary. Only apply to Jonagored if the orchard has a history of oversetting with small fruit and poor tree growth.
• Spartan Two applications of ATS at 1.0-1.5% with the first timed at full bloom on two-year-old wood and the second approximately 7 days later. The second application should be omitted if weather conditions during bloom are not favourable for good fruit set.
• Summered Recent evidence from Norway, indicates that sprays of 1% ATS are effective in thinning this variety. 
• Worcester Pearmain Two applications of ATS at 1.0-1.5% with the first timed at full bloom on two-year-old wood and the second approximately 7 days later. The second application should be omitted if weather conditions during bloom are not favourable for good fruit set.

Other chemicals tested as flower thinners for apples

Other blossom thinners that have shown promise in trials both in the UK and abroad include Armothin, common salt (sodium chloride),  Endothallic acid,  Ethephon,  Lime sulphur, Pelargonic acid, Urea,   Vinegar, Wilthin and Zinc.

Lime sulphur

Lime sulphur was used many years ago as a thinner for Victoria plum. More recently, organic growers in parts of continental Europe have begun to show renewed interest in the product as a thinner for apples.

• Recent trials conducted in the Netherlands indicate that lime sulphur at 4% applied at full bloom can provide useful thinning of the variety Elstar.
• Lime sulphur is not currently approved for use as a flower thinner in the UK.

Urea

Sprays of 3-4% urea have occasionally produced good results when applied at full bloom in German and Danish trials.

• However, the product appears to thin only when it also causes significant damage to the spur leaves.
• Such damage will probably have negative effects on fruit development and calcium uptake into the persisting fruitlets.
• Urea is not approved for use as a flower thinner on apples in the UK.

 

Fruitlet thinning

In the absence of suitable approved chemicals for thinning fruitlets on apple trees they can only be thinned by hand. This is a very expensive procedure and, with pruning and harvesting, represents one of the major costs in apple production.

• Attempts have been made to thin fruitlets of some crops (e.g. plums) using mechanical aids, but little or no work has focused on this strategy, which relies on tree shaking or combing devices, for apple.
• Fruitlet drop may also be induced using other techniques. One of these, recently tested in the USA, is to induce thinning by shading trees.

 

Hand thinning

Hand thinning is the best way to achieve the correct crop load and ensures that the largest and best shaped fruits are retained. It also allows the best fruitlet distribution on the tree to be established by allowing fruits in poor positions on the branches or spurs to be removed.

• Even when hand chemical thinning has been carried out, growers should double check the fruit number and hand thin down to the correct crop load per tree.

Procedures for hand thinning:

Check the fruit numbers on a small sample of trees and compare with target numbers.

Calculate how many fruits per cluster or per foot of branch length need to remain.

Decide on a simple set of rules for the fruit thinners, e.g:

• Remove all fruits under branches.
• Remove all fruits on one-year-old wood.
• Leave one (or occasionally 2) fruit per cluster.
• Leave fruits to be spaced 10 cm (4 inches) apart.
• On short-stalked varieties (Cox, Discovery, Egremont Russet and Bramley) remove the ‘king’ fruit as this may be misshapen. Leave the next largest lateral fruit in the cluster.
• On long-stalked varieties (Gala and Jonagored) the king fruit is retained as it is the largest and is not misshapen.
• Check the crop load by counting or by using special binoculars provided by advisory specialists. This aid to assessing crop load has proved quick and very reliable.
• Begin thinning as soon as possible as this maximises the benefits to fruit size and texture.

Timing of hand thinning

In a research trial conducted at East Malling in 1997, semi-mature trees of Royal Gala on M.9 rootstock were thinned to single fruits per cluster at different timings and the effects on yields and grade outs recorded. The table below shows some of the results:

 

Effects of different timings of hand thinning Royal Gala trees in 1997

 

Treatment timing of hand thinning	No. fruits/tree	Weight of fruit harvested/tree (kg)
		Total	>65 mm diameter	>70 mm diameter	% of total 65 mm diameter
None (control)	235	18.6	1.4	0.1	8
Full bloom	52	7.8	6.1	2.6	78
Late initial set	80	10.9	8.0	4.4	73
12 mm diameter	100	12.6	5.9	2.7	43
18 mm diameter	84	9.5	2.8	0.2	29
24 mm diameter	83	9.1	3.3	0.8	36

 

• Trial results show that thinning at or before the 12 mm fruitlet diameter stage is essential.
• Thinning Gala at the time of initial set is particularly beneficial.
• Very early thinning of Cox may, however result in some problems in storage.

Chemical thinning of fruitlets

• Currently there are no approved products for thinning of fruitlets in the UK.

• Trials with benzyladenine (BA) appear promising and it is hoped that a registration for this may be approved in 2010 or 2011.
• Other chemicals trialled for thinning fruitlets include NAA/NAD and ethephon.

 

Combined flower and fruitlet thinning treatments

Trials and grower experience have shown the benefits of combining flower and fruitlet thinning treatments for maximum thinning effect.

• Consider supplementing any blossom thinning achieved using ATS with supplementary hand thinning at the 12 mm fruitlet diameter stage.
• Recently promising results have been achieved using combinations of ATS for blossom thinning and BA for fruitlet thinning.
• However, BA is not currently approved for use as a fruitlet thinner in the UK.

 

The physiology of flower and fruitlet drop in apples

The natural abscission (drop) of flowers and fruits

Fruitlets were traditionally thought to drop off in response to the death of their seeds (embryos) and the cessation of the supply of auxins from the live seeds across the abscission zone in the fruit stalk.

• Although embryo death possibly explains a significant proportion of natural fruitlet abscission, it does not explain it all.
• Fallen fruitlets can often be found which have live plump seeds within them.
• Also, research has failed to explain conclusively the causes of the embryo abortion itself.

Hormones occurring naturally within the tree that are associated with flower and fruitlet abscission

• Several natural plant hormones have been implicated in the tree processes leading to flower and/or fruitlet abscission (drop), particularly auxins and ethylene.

Application of plant growth regulators to induce flower and fruitlet abscission

• Various chemicals have been shown to have thinning action when sprayed onto apple trees including, auxins, carbaryl, ethylene releasing chemicals, photosynthesis inhibiting chemicals and cytokinins (benzyl adenine – BA).
• Recent studies have shown that jasmonic acid and n-propyl dihydrojasmonate (PDJ) may have potential as a fruitlet thinner in certain fruit crops (Fujisawa et al., 1997).