Hand thinning of apple flowers

Hand thinning of flower clusters is rarely if ever carried out in mature commercial orchards. Hand thinning is too labour intensive and hence too expensive.

However, experiments conducted in Germany (Link, 1998) have shown the clear benefits of early hand thinning.  Experiments conducted in Switzerland on Elstar trees, which were exhibiting pronounced biennial bearing, also showed the advantages of hand thinning of blossoms (Bertschinger et al.,).

• The work demonstrated that biennial bearing of Elstar could best be avoided by removal of 2/3 of the flower clusters on the entire tree in the first year of treatment coupled with some further fruitlet thinning (hand or use of NAA) after June Drop in subsequent years.
• Also effective, was removal of all the flower clusters from half the tree in the first year of treatment, again coupled with subsequent hand thinning of fruitlets.
• This latter option is best for organic Elstar production, as it involves no use of chemical thinners.
• Approximately 8‑10 minutes were needed to remove all the clusters on 50% of a tree (250 hours/ha if 1500 trees/ha) in the first year of treatments.
• The subsequent treatments took on average 3 minutes per tree.
• The fruits from the trees on which clusters were removed from 50% of the tree canopy proved to be the firmest.

The biennial bearing variety Boskoop also benefits from very early hand thinning in the ‘on’ year.

• For good return bloom in the subsequent season flower thinning had to be completed in the first two weeks following flowering (Tromp, 2000).

Hand thinning of blossoms is occasionally practised on newly planted trees so as to prevent fruit set and encourage shoot growth to fill the allotted canopy space.

• 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.

Mechanical thinning of flowers

In recent years machines for mechanical thinning of apple flowers have been developed in Germany, Switzerland and the USA.

• The crudest types of machine use a series of knotted ropes, which are dragged through the trees to knock off a proportion of the flowers.
• The ropes are attached at regular spacings to a curved beam which, using the tractor PTO, is lifted to be just above the tree canopy.
• The tractor is then slowly driven down the alley pulling the ropes through the trees and knocking off many of the flowers.
• The crudest machines, which drag knotted ropes through trees to knock off blossoms, cause considerable damage to the trees and cannot currently be recommended in the UK.

Trials conducted in Switzerland (Bertschinger et al., 1998) have focused on using what is known as the ‘wire-machine’ (designed by H. Gessler, Friedrichshafen-Hirschblatt, Germany).

• The machine is powered by and pulled by a conventional orchard tractor (Schröder, 1996).
• It comprises a vertically turning axle with nylon wires of approximately 40 cm long attached.
• The tractor is driven at approximately 4 km/h and the wires on the axle rotate at 1500 revolutions/min.
• Further details of this machine can be gleaned from Stadler et al., (1996) and from the UK importer N P Seymour.
• Use of the machine at the balloon stage of flowering reduced fruit set in orchards of Jonica, Golden Delicious, Maigold and Idared significantly (35%-65%).
• Thinning earlier at bud swell caused significant damage to the trees and thinning later at full bloom caused some deformations to the fruits remaining; thinning between green cluster and balloon seems the best timing for use of the machine.

Thinning efficacy was not adequate on pyramidal shaped trees with branches longer than 70 cm and this was especially true on the tree tops and in the centres, close to the central leader.

• Some increases in pests (aphids) and diseases were noted on the machine-thinned trees.
• The authors concluded that for machines of the type described to be effective, tree architecture must be adapted significantly.
• Slender cylindrical trees with short branches would be ideal.
• Further research is warranted on the development of orchard canopy systems and suitable machines for mechanised flower and/or fruitlet thinning in UK apple orchards.

Chemical methods of thinning apple flowers

Most chemicals that have been tested and found effective in preventing apple flowers from 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). 

Other blossom thinners that have shown promise in trials both in the UK and abroad are:

• Elgetol (DNOC)
• Endothallic acid
• Lime sulphur
• Pelargonic acid
• Wilthin 
• Armothin
• Ethephon
• Urea 
• Zinc
• Vinegar
• Common salt (sodium chloride)

Other chemicals applied at the time of flowering for flower thinning may be effective through a different action to those listed above.

• Ethephon releases the gas ethylene and stimulates the drop of flowers and young fruitlets.
• Part of the thinning reaction induced by high rate urea sprays is undoubtedly attributable, indirectly, to the damage the sprays cause to the primary leaves surrounding the flowering spurs.
• The mode of action of zinc in causing thinning is not understood.
• The auxin naphthylaceticacid (NAA– Planofix) has also been applied close to full bloom as an apple thinner.
• This is thought to trigger the early drop of flowers and fruitlets by causing a temporary check to the production and movement of foodstuffs (carbohydrates) to the flowers. 

Ammonium thiosulphate (ATS) for blossom thinning

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 from setting fruits.

When using ATS as a blossom thinner on apples it is important to consider:

• Spray timings
• The ideal weather conditions
• Spray concentrations and volumes
• Use of adjuvants
• Variations associated with specific varieties

Spray timings

Trials conducted at East Malling and in Denmark have shown that flowers which are at the 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, despite being damaged by the ATS sprays.
• This is because the fruit receptacles and ovaries are not damaged by the sprays and once pollen tubes have penetrated the ovary they will not be influenced by the sprays.
• Nevertheless, in certain climatic conditions, such as very slow drying, the skin finish of these fruits may be slightly damaged.

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.

• This is because a high proportion of the total number of flowers will be at a vulnerable stage on the day of the spray treatment (70% full bloom).

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.

• A single spray in such seasons is rarely sufficient to effect adequate thinning.

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.
• To thin them ATS sprays must be delayed until early petal fall on spur blossoms.
• Care must be taken with this late treatment, however, as trials in Denmark have shown that in their climatic conditions (high humidity and slow drying) sprays applied to thin axillary blossoms may also damage and cause abscission of fruitlets already set on the spur blossoms.
• Although this is contrary to the results noted at East Malling, it does indicate that there is a potential problem with this strategy in some climatic conditions.

Trials at East Malling (Webster and Spencer, 1999; Webster and Spencer, 2000), on the varieties Royal Gala and Queen Cox, showed that, although single sprays with ATS usually thinned very effectively in the first year of treatment, repeating the same sprays on the same trees in the subsequent season could produce disappointing results.

• The reason for this was the very abundant blossom numbers induced in the second year by the treatments in the first year.
• Although the treatments reduced set per 100 flower buds similarly in both seasons, the thinning was insufficient in the second season to compensate for the big increase in flower numbers.
• Where this is likely to be a problem, growers should consider increasing spray volumes, or ATS concentrations or even applying a second spray during the flowering period.

The ideal weather conditions

Research conducted in controlled environment chambers in Holland has shown that as temperatures increase the degree of thinning achieved with ATS sprays also increases.

• So, when temperatures are 15^oC or higher thinning should be very efficient. In contrast, when temperatures are very cool <10^oC the sprays are likely to prove much less effective.
• Temperatures are influential, not just at the time of spraying but for several hours after the treatment is applied.

Thinning is also influenced by the humidity at the time of spraying. 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.

Research in Canada has indicated that when sprays of ATS are applied at low volumes the spray concentration needs to be increased as much as 6 to 12 times (Sanders and Looney, 1993).

• Such concentrations can, however be very phytotoxic (Irving et al., 1989) and further work is needed before similar recommendations can be made for UK conditions.
• However, in contrast to the above findings, recent work conducted in Poland indicates that ATS thins best if applied at higher concentrations and low spray volumes.
• Differences in climatic conditions between Canada and Poland may help explain these anomalies.

Armothin

Armothin is a surfactant manufactured by the Azco-Nobel Co Ltd, which has showed useful activity as a blossom thinner on stone fruits. It is a 98% fatty amine polymer.

• In trials at East Malling, high volume sprays (1000 l/ha) of Armothin at 0.5% active ingredient were tested at three timings, balloon blossom, 5 days after full bloom on the two-year-old spurs or 10 days after full bloom (Webster and Spencer, 2000).
• The trees used for the test were eight-year-old Royal Gala on M.9 rootstock.
• Concern was expressed by the chemical manufacturers about possible fruit russeting.
• To counteract this a programme of Regulex (GA₄₊₇) was applied to some of the Armothin treated trees.

A second trial on Royal Gala also included sprays of Armothin, applied 10 days after full bloom. The results of the two trials were inconsistent.

• In one trial no significant thinning was achieved, whilst in the other, mean fruit size and grade out was improved in comparison with unsprayed controls.
• No significant phytotoxic damage was recorded, irrespective of whether the treatments were supplemented by Regulex sprays or not.

A third trial conducted on the variety Jonagold was also disappointing with only minimal thinning following the Armothin sprays.

On this limited preliminary evidence, Armothin appears to hold only minimal promise as a potential blossom thinner for apples under UK conditions.

• ‘Armothin’ has proved an effective thinner for stone fruits in Italian trials.
• In UK trials on apples it gave inconsistent results as a flower thinner.
• ‘Armothin’ is not approved currently for use as a flower thinner in UK orchards.

 

Endothallic acid (Endothall)

 European and USA researchers devoted much R&D effort in the 1990s to testing the efficacy of endothallic acid (TD 2337-2 or ‘Endothall’) as a blossom thinner for apples.

• The chemical is a 5% (w/w) active ingredient formulation of dipotassium 7-oxobicyclo (2,2,1) heptane-2,3,-dicarboxylate and it is approved for use in some countries as an aquatic weedkiller.

In trials at East Malling, it was tested as a blossom thinner for Royal Gala and Queen Cox at rates of 500, 1000 and 2000 ppm in high volume (1000//ha) sprays (Webster and Spencer, 1999; Webster and Spencer, 2000).

• Most of the treatments had some thinning effect and the thinning severity was related to spray concentration.
• In the first year of treatment, the sprays increased grade-outs of large size fruits.
• However, although treatment of the same trees in the second year also reduced fruit set, no improvements in final fruit sizes were achieved.
• This was explained by the large differences in flower abundance on the trees at the start of the second year.
• Treatments in the first year increased flower density in the second year very significantly, whilst overcropping of the unthinned controls in the first year reduced flower density in the subsequent season.
• Although the sprays reduced percentage fruit set in both years, this was insufficient in the second year to compensate for the large increase in flowering on the previously treated trees.
• In these situations two or more sprays of a chemical thinner may prove necessary in seasons or situations of high flower abundance.

No phytotoxicity to leaves or fruits was observed in UK trials. However, phytotoxicity and variable responses were recorded in USA trials with this product (Andrews and Collier, 1995).

• Although proven quite effective as a blossom thinner in European trials, work on endothallic acid has now ceased.
• It was believed that gaining clearance for its use as a blossom thinner would prove too difficult.
• Endothallic acid (Endothall) is not approved currently for use as a flower thinner in UK orchards.

 

Ethephon

Ethephon (Ethrel C) has been tried on many occasions as a blossom thinner for apples (Irving et al., 1989).

• However, the sensitivity of apple flowers varies greatly with the stage of flower development.
• Apple flowers show high sensitivity at the pink bud stage but almost no sensitivity by the time of petal fall (Veinbrants and Hutchinson, 1976).
• Also, the thinning action of ethephon is highly dependent upon temperature at the time of spraying and during the subsequent 24 hours.
• Temperatures lower than 15^oC result in poor efficacy of thinning; thinning increases linearly as temperatures increase from 12-24^oC (Jones and Koen, 1985).
• For the ethylene (which stimulates the thinning response) to be released efficiently by ethephon it is also essential that the spray solution is alkaline.

Although ethephon has fallen from favour as a blossom thinner since the 1980s, more recent work in Australia shows that with appropriate timing it can perform well with the variety Fuji (Jones et al., 1990).

• With this variety thinning can be achieved with sprays applied either at full bloom or 14 days later (Jones et al., 1989).
• However, the authors also showed that the concentrations necessary for effective thinning varied with the rootstock.
• On the strong growing seedling rootstocks only 25-50 ppm was required, whilst on MM.106 rootstock 100‑200 ppm was necessary.
• Variability of response is also influenced by the spray volume used (Koen et al., 1986).

Research conducted at East Malling in the 1980s (Knight et al., 1987) showed that ethephon at 500 ppm could be an effective thinner for the variety Spartan.

More recent evidence from Norway indicates that sprays of 300 ppm at early bloom can be effective in thinning the early ripening variety Summered.

The new variety Delblush (Tentation) has also been effectively thinned in Dutch trials using sprays of ethephon applied when 20% of the flowers are open.

Taking all the evidence into account ethephon produces a very variable thinning response, ranging from almost no thinning to total inhibition of fruit set, even in climatic conditions much more stable than those experienced in the UK.

• It is unlikely, therefore, that ethephon can be used with any reliability as a flower thinner in the UK.
• In regions of the world where temperatures during blossom time are >15^oC, sprays of ethephon in water with a slightly alkaline pH often give consistent beneficial effects in apple flower thinning.
• Where temperatures are lower, the thinning response to ethephon is usually variable and often very poor.
• Ethephon is cleared for use as a thinner in several European countries but is not approved currently for use as a flower thinner in UK orchards.

 

Pelargonic acid

Pelagonic acid (60% a.i v/v) is registered in the USA as a blossom thinner under the trade name of ‘Thinex’.

• Trials in the USA have shown it to have some useful activity as a blossom thinner (Williams, 1994).
• However, in trials conducted at East Malling and by researchers in Holland the product performed poorly as a blossom thinner and caused significant phytotoxicity to spur leaves (Webster, and Spencer, 1999; Webster and Spencer, 2000).
• In the East Malling trials, rates of 750-3000 ppm were compared in high volume sprays to the varieties Royal Gala and Queen Cox. Phytotoxicity and variable responses have also been observed in USA trials with this product (Andrews and Collier, 1995).
• Pelargonic acid seems to offer little promise as a flower thinner in UK climatic conditions.
• Pelargonic acid is not approved currently for use as a flower thinner in UK orchards.

Urea 

Urea has been tested and used as a blossom thinner for apples in Germany for many years.

• It is recommended at 3% to 4% high volume sprays and is especially useful with varieties prone to biennial bearing, such as Elstar (Graf, 1997).
• The product seems to bring about its thinning action by scorching the spur leaves.
• Where no spur leaf damage is noted, thinning is often poor. One of the concerns with this urea strategy to thinning is that early loss of spur leaves may well reduce calcium uptake and increase subsequent problems with bitter pit (Volz et al., 1994).
• Growers using urea as a blossom thinner should also be aware that it may lead to poorer fruit colour at harvest, fruit russeting and reduced flowering in the subsequent season (Wertheim, 1997).
• Urea at concentrations of 3-4% has given good, albeit inconsistent, effects as a flower thinner on apples in German and Danish trials.
• However urea causes considerable damage to the spur leaves and for this reason appears to have little potential as a thinner in UK conditions.
• Urea is not approved currently for use as a flower thinner in UK orchards.

 

Wilthin

A 79% active ingredient formulation of monocarbamide-dihydrogen sulphate (MCDS or sulfcarbamide), which is marketed in the USA as ‘Wilthin’, has shown promise in USA trials as an apple thinner (Williams, 1994).

• Trials using the product in high volume sprays at 1000‑4000 ppm on Queen Cox and Royal Gala at East Malling proved less successful (Webster and Spencer 1999; Webster and Spencer, 2000).
• Only at the highest dose rate tested, 4000 ppm, (which was above that recommended on the product label) was effective thinning achieved.
• Also, some phytotoxicity on spur leaves was noted. Phytotoxicity and variable responses have also been observed in USA trials with this product (Andrews and Collier, 1995).
• Although effective in USA trials on apples, ‘Wilthin’ gave disappointing results as a thinner in UK trials.
• ‘Wilthin’ is not approved currently for use as a flower thinner in UK orchard.

 

Zinc

Sprays containing the trace element zinc are often recommended for improving fruit quality of harvested apples.

• Trials conducted at East Malling in the 1980s showed that zinc sprays applied during flowering reduced fruit set but, as applications in the previous year also increased flower bud abundance, this negative effect was cancelled out (Yogaratnam and Greenham, 1982).

More recent trials, conducted at East Malling (Hipps, personal communication), indicate that sprays applied during the stage of active cell division in fruitlets (i.e. in the few weeks following petal fall) can, on occasions, stimulate thinning.

• Sprays of 1.0 g l^-1 zinc (as 4.4 g ZnSO₄.7H₂O l^-1 mixed with a proprietry wetting agent), applied in 1998 at high volume to semi-mature Bramley’s Seedling apple trees on M.9 rootstock, reduced initial fruit set by 17%.
• However, similar sprays in 1999 had no effect on fruit set. Further research is needed to understand this variable effect of zinc on fruit abscission
• The thinning potential of sprays of zinc products needs further investigation.

 

Vinegar

• In HDC project TF 148 (Thinning of apple flowers using chemicals sensitive to the environment) trials conducted at East Malling showed that undiluted vinegar applied at full bloom of the spur buds was an effective thinner but significantly increased skin russet.

 

Common salt (sodium chloride)

In HDC project TF 148 (Thinning of apple flowers using chemicals sensitive to the environment) trials conducted at East Malling showed that common salt (sodium chloride) applied at full bloom of the spur buds was an effective thinner.

• Growers may be interested in applying common salt to small areas of orchard at a time when they would normally apply ATS.
• It is advisable to apply 12 g of common salt per litre of water applied at high volume.
• Although 16 g per litre of water is likely to be more effective, in view of the evidence from abroad that this may result in over-thinning, interested growers may like to compare the effects of the two rates in small areas of orchard.

 

Chemical thinning with benzyl adenine (BA: ‘Perlan’, ‘Accel’, ‘Paturyl’, ‘Expander’)

With the impending withdrawal of Carbaryl in Europe, scientists have been seeking alternative fruitlet thinners for apples.

• One product that has shown some promise in trials conducted mainly in the USA is benzyl adenine (BA). Approval has been gained in some European countries.
• Trials conducted at East Malling have produced variable results with BA, as shown below.

Thinning Royal Gala with BA

In trials conducted in 1995 and 1996 at HRI-East Malling and funded by APRC (Webster and Spencer, 2000), sprays of benzyl adenine (BA, Paturyl formulation) at concentrations ranging from 50 ppm to 200 ppm reduced percentage final set on Royal Gala trees.

• However, the sprays, which were applied at the 12 mm fruitlet diameter stage, had only small and insignificant effects on fruit size and grade out.
• On this preliminary evidence BA appeared not to hold much potential as a fruitlet thinner for Gala.
• However, trials in 1998, comparing sprays of three different formulations of BA (Paturyl, Accel and Perlan), showed that all three formulations of BA, applied at high volume (100 ppm), improved the proportion of Class I fruits in the >65 mm category without reducing total yields.

 

Effects of sprays of BA (Paturyl, Accel or Perlan) on the fruit set, yield and fruit size of Royal Gala (sprays at 12 mm stage –100 ppm in 1000litres/ha)

 

Treatment	Final set/100 floral buds	Total yield/tree (kg)	% Class I >65 mm	Mean fruit wt. (g)
Unthinned	146	24.5	11.5	94
Paturyl	140	25.2	39.7	111
Accel	132	26.5	30.8	101
Perlan	126	26.1	29.7	109

 

• Further research is needed on Gala clones to find strategies for reducing this inconsistent thinning response to BA.

Thinning Queen Cox with BA

Sprays of BA applied at the 12 mm fruitlet diameter stage to Queen Cox trees in 1995 reduced final fruit set/100 floral buds and increased mean fruit size and grade out of the larger Class I fruits (Webster and Spencer, 2000).

• The most effective spray concentration was 100 ppm applied in high volumes of water.
• The sprays caused no significant reduction in total yields/tree.

 

Effects of BA sprays on the fruit set and yields of Queen Cox in 1995

 

Treatment	Final set/100 floral buds	Total yield/tree (kg)	Mean fruit wt. (g)	Class I (%)
				>70 mm	65-70  mm
Unthinned	105	13.0	84.3	9.6	12.5
BA 50 ppm	75	11.8	105.0	17.0	25.3
BA 100 ppm	66	11.0	107.6	13.6	37.1
BA 200 ppm	68	11.4	116.2	38.3	29.9
Carbaryl 750 ppm	54	9.8	117.9	20.1	52.9

 

• Although the BA sprays reduced fruit numbers/100 floral buds, when applied for the second year (1996) to the same trees, this thinning was insufficient and failed to compensate for the much increased blossom abundance stimulated by the sprays applied in 1995.
• Consequently, fruit sizes were not improved in 1996.
• Further trials using BA as a fruitlet thinner for Queen Cox are warranted.

Thinning Jonagold with BA

• Sprays of BA (100 ppm) applied at the 12 mm stage to ‘Jonagored’ trees in 1998, improved fruit size and grade out, although the benefits were less than those achieved with hand thinning.

 

Effects of BA (Paturyl, Accel or Perlan) sprays on the fruit set, yield and fruit size of Jonagold in 1998 (sprays applied at 100 ppm in 1000 litres/ha at the 12 mm fruitlet diameter stage)

 

Treatment	Final set/100 floral buds	Total yield/tree (kg)	Mean fruit wt. (g)	% Class I >90 mm
Unthinned	62	39.2	186	0.1
Hand thinned	34	29.2	262	27.8
Paturyl	51	37.4	223	15.5
Accel	43	31.8	238	24.1
Perlan	52	37.2	219	11.3

 

Trials in continental Europe with BA

• In trials conducted in mainland Europe, sprays of 100 ppm proved ineffective in thinning the early ripening variety Summered in Norwegian trials.
• However, in Swiss trials, sprays of the same concentration increased the mean size of Boskoop fruits at harvest.
• Results on the variety Golden Delicious have been variable with no benefits reported in Switzerland but improved fruit size reported in Spain.
• In trials conducted in Slovenia, mixtures of BA and NAA have proved effective in thinning Gala.
• In German trials BA has also proved an effective thinner for the varieties Delbard Estivale and Pinova.
• In Italy, BA, which is known under the trade name of ‘Expander’, has proved effective in thinning Royal Gala when applied at 100 ppm at the 10 mm fruitlet diameter stage and trials on the variety Fuji have also shown good thinning responses from BA at 200 ppm.
• On the variety Pink Lady, trials in France show that BA (150 ppm) was an effective thinner applied at the 15 mm fruitlet diameter stage.
• Recent work conducted in Poland indicates that thinning with BA increases with increasing concentrations up to 200 ppm and that it is improved by use of high spray volumes.

BA has shown promise in thinning apple fruitlets in trials conducted in several countries.

• More work is needed on this thinner if the inconsistencies of response are to be understood and overcome.
• BA is not approved currently for use as a thinner in UK orchards.

Other chemicals trialled for fruitlet thinning

Chemical thinning with NAA and NAAm (NAD)

NAA, or its amide NAD (NAAm), has long been used in continental Europe for thinning apples.

• Although approved for use in the UK up until the 1970s, these auxins failed to prove consistently effective when used on Cox’s Orange Pippin and they were withdrawn from use.

NAA

Thinning results using NAA are very variable from season to season, variety to variety and site to site.

• In recent years, it has proved consistently effective only in Polish trials.
• NAA is either sprayed a little later than NAAm, (up to 14 days after petal fall), or during flowering.
• Concentrations of up to 20 ppm are used and many different wetters and surfactants have been tested in attempts (largely unsuccessful) to improve the consistency of response.
• However, recent work in Germany has provided some explanation of the variability of response and made suggestions for future use of more appropriate wetters (Schönherr et al., 2000).
• Recent trials in Norway have shown NAA to be ineffective in thinning the early ripening variety Summered, when applied at the 10 mm fruitlet diameter stage

NAAm (NAD)

trials on several varieties grown in central Germany (Link, 1998) showed that the NAAm thinned Cox in some situations but did not result in any increase in monetary value when the results of trials over an eight year period were analysed.

• In contrast, hand thinning either by removing whole blossom clusters at the time of flowering or of fruitlets after completion of ‘June Drop’, usually produced more economically worthwhile results.

 

Effects of thinning treatments on the yields and grade-outs of Cox grown in Hohenheim, Germany (mean of years 1982-1990)

 

	Unthinned	NAD (NAAm)	Hand thinning of bloom clusters	Hand thinning of fruitlets post June drop
Total yield/tree (kg)	23.2	21.5	21.4	20.6
Yield of large/medium sized fruits/tree (kg)	12.0	15.2	17.3	16.2
Monetary value of fruit/tree/year (£)	7.20	7.10	8.40	7.90

(from Link, 1998) 

• Recent trials in Germany have shown good thinning of the variety Delbard Estivale using a combination of NAAm and Ethrel applied at the late bloom stage.
• NAAm in liquid formulations is normally applied at a concentration of 40 ppm in high volume sprays of 1500 l/ha just after petal fall.
• However, concentrations of up to 100 ppm have been used in some situations. It is normally sprayed shortly after flowering.

In the absence of carbaryl or other potential fruitlet thinning chemicals NAAm is likely to remain a major thinner in many orchards in mainland Europe.

• NAA and NAAm are still approved for use as fruitlet thinners on apples in many European countries. However, they are not approved on apples.
• The thinning effects achieved are inconsistent and further work is needed on ensuring that the pH of the water used is optimal and that the best surfactants are used.

Chemical thinning with Ethephon (Ethrel C)

Ethephon (Ethrel C) applied at the 12 mm fruitlet diameter stage has been shown to thin many varieties of apple.

• Unfortunately, the results are extremely variable from season to season and between varieties.
• A major influence on the efficacy of Ethrel sprays in thinning is the temperatures during and shortly after treatment.
• Unless the day temperatures are 15^oC or above, the response to the sprays is likely to be poor.
• Another factor influencing the variability of response of Ethrel thinning sprays is the pH of the water used.
• Alkaline spray solutions are needed to maximise the release of ethylene by the product.

Knight (1981) showed that sprays at concentrations of 150 ppm were effective in thinning Discovery but that the growth rate of the persisting fruitlets was often reduced by the treatment.

• Similar effects on fruit growth rates have also been noted in Dutch trials.

Although still used in several European countries as a thinner, Ethrel is extremely variable under UK climatic conditions and in some situations may cause slight russeting and poor storage potential in fruits.

• The results achieved with Ethrel are often variable. Thinning efficacy is improved by temperatures higher than 15^oC and by use of alkaline water.
• Ethephon (Ethrel) is still approved as a fruitlet thinning chemical for apple in several European countries, but is not approved for this use in the UK.

 

Inducing thinning by shading trees

research conducted in the USA in the late 1980s showed that thinning of apple trees could be enhanced by shading the trees (Byers et al., 1991). Severe shading for brief periods during the development of apple fruitlets can induce fruit drop.

Attempts were made to verify the USA trials in further work conducted in Switzerland (Bertschinger, et al., 1998) where structures used to support hail netting were used to create shade within orchards.

• The shade was created using polypropylene sheeting and 50% and 100% shade levels were imposed for 5 days at either 14 or 28 days after full bloom to ten‑year-old trees of Arlet and Jonagold on M.9 rootstocks.
• The ideal level of fruit set was achieved using the 100% shade for 5 days commencing 14 days after full bloom.
• This fruit set was equivalent to that obtained by hand thinning after June drop.
• Fruit quality and yield productivity were acceptable with this treatment.
• Application of 100% shading 28 days after full bloom caused 100% fruit drop.

Although successful in the Swiss trial, shading as a means of fruitlet thinning does carry some inherent risks.

• It is known that shading reduces relative growth rates of fruitlets in the period of 1 to 5 weeks after full bloom.
• This is the phase of cell division and other studies have shown that reductions in growth in this period may result in reductions in yield (Lakso et al., 1989).
• Similarly, it has been shown in Italian studies that moderate shading (to 40% normal light) of extension shoots three weeks after full bloom suppresses the movement of carbohydrates to fruits.
• Further studies are needed before this non-chemical strategy for inducing fruit drop can be recommended.

 

Combined flower and fruitlet thinning treatments

Thinning treatments used with considerable success in North America for many years comprised a spray of Elgetol (DNOC) as a flower thinner, followed by sprays of carbaryl or NAAm.

• Recently, researchers at East Malling have investigated the use of other, more environmentally sensitive combinations of spray treatments to achieve effective apple thinning.

Royal Gala

• In a trial conducted in 1998, a spray of ATS (0.5 or 1.0%) at full bloom, followed by a spray of BA (Perlan 100 ppm), all at high volume (1000 litres/ha), showed some promise for thinning the variety Royal Gala.

 

Effects of combinations of ATS and BA on the fruit set, yield and fruit size of Royal Gala in 1998

 

Treatment	Final set/100 floral buds	Total yield/tree (kg)	Mean fruit wt. (g)	Yield/tree Class I (%)
				>70 mm	>65 mm
Unthinned	183	27.7	101	7	25
ATS (0.5%) + BA(100 ppm)	131	23.7	122	13	52
ATS (1.0%) + BA(100 ppm)	105	21.2	130	13	62
Hand thinned	66	14.2	138	11	78

 

Jonagold

• Combined treatments of 0.5% ATS, applied at full bloom, followed by 100 ppm BA, at the 10-12 mm fruitlet diameter stage, gave promising thinning results on Jonagored trees in 1998.
• Although the degree of thinning was much less than that achieved by hand, reductions in total yield were less.

 

Effects of ATS and BA spray treatments on the thinning of Jonagold in 1998

(all sprays applied at 1000 litres/ha)

 

Treatment	Final set/100 floral buds	Total yield/tree (kg)	Mean fruit wt. (g)	% Class I >90 mm
Unthinned	57	44	179	0.5
ATS (0.5%) + BA(100 ppm)	47	42	208	13.4
ATS (1.0%) + BA (100 ppm)	36	34	219	15.4
BA (100 ppm)	50	42	190	4.9
Hand thinned	26	29	260	32.4

 

• 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.

Combined chemical thinning strategies tested abroad

Trials conducted in mainland Europe have on occasions shown the benefits of using more than one chemical for thinning apples.

• Trials in the Netherlands have recently shown that a combination of the flower thinner lime sulphur and the fruitlet thinner BA can prove effective on the variety Elstar.
• In Spanish trials, mixtures of Ethrel (applied at 20% open flower) and BA (applied at the 10 mm stage) have thinned Fuji effectively, as have combinations of NAA and carbaryl.
• Similarly, Gala and Golden Delicious were very effectively thinned with mixtures of BA and NAA in Slovenia.
• The same combination of chemicals has proved effective in thinning the variety Elstar in Dutch trials.
• In Northern Italy sprays of NAAm or Ethrel, followed by later sprays of BA have proved very effective in thinning Royal Gala.
• However, in French trials this combination treatment overthinned the variety Pink Lady.

 

The natural abscission of flowers and fruitlets

According to Bangerth (2000) there are, currently, two hypotheses advanced to explain the abscission of young apple fruits:

• Abscission is caused by insufficient supply of assimilate to fruitlets as a result of limited assimilate production and/or allocation to the fruit.
• Abscission is caused by a regulatory hormonal mechanism by which the plant protects specific fruits from assimilate limited growth later in the season.

Although there is some support for the first hypothesis (Stopar, 1998), abscinding apples often have equal or higher amounts of carbohydrates in them compared with the persisting fruits (Abruzzese et al., 1995). 

In recent years, therefore, research has focused on understanding how hormones influence fruitlet abscission, although the causes of abscission are probably due to a combination of hormonal and assimilate supply factors.

The general hypothesis concerning abscission is that various factors influence the balance of auxins and ethylene around the abscission zone, and also influence the receptivity of the tissues in this zone to either auxin (prevents abscission) or ethylene (stimulates abscission).

• For instance, reductions in the supply of foodstuffs (assimilates produced by photosynthesis) to the fruit, will in turn reduce the amounts of auxins that are produced and transported from the seeds out of the fruitlets.
• This renders the abscission zone more sensitive to ethylene stimulated abscission.
• Low light levels in the orchard can reduce photosynthesis, as can applications of the synthetic auxins NAA or NAAm or inhibitors of photosynthesis such as terbacil.
• The processes involved are much more complicated than described here and new information continues to improve our knowledge.
• For a fuller account of the biochemical and molecular processes controlling fruitlet abscission the reader should consult Bangerth (2000) and Bonghi et al., (2000).

Where trees are carrying a very heavy crop, the most vulnerable fruitlets to abscission are those that set slightly later and are, therefore, slightly smaller (Bangerth, 1989).

• Problems can be experienced occasionally where most fruitlets are of similar size and ‘sink strength’.
• In this situation, when conditions reducing assimilate supply are experienced, fruit drop may be excessively severe.
• Where trees develop vigorous growth of extension or bourse shoots, natural fruit drop may also be severe.
• This is thought to be due to the strong competition between the shoots and the fruits for vital assimilates (Bangerth, 1986).
• Competition also seems to play an earlier role, in that when floral abundance is very high, the levels of fruit set (per 100 flowers) are invariably lower than when floral abundance is lower (Jones and Koen, 1986).
• Indeed, work at East Malling showed that diminishing competition between flowers by removal of some of them increased the set of the remaining flowers (Knight et al., (1987).

Research conducted at East Malling has shown that carbon assimilation and its partitioning either from dormant season reserves held in the woody tissues of the tree or from current leaf photosynthesis is influential in fruit set and development.

• Fruit set can be depressed by the removal of the first leaves to emerge on the tree in the spring (the spur leaves) (Ferree and Palmer, 1982).
• The same research showed that the early removal of the bourse shoot could also reduce initial fruit set but increased final yields.
• This apparent paradox is probably explained by the removal of bourse leaves initially reducing assimilate supply and inducing fruit drop but, later, lack of bourse shoots reduces potential competition for assimilates by the bourse shoot as the persisting fruits grow.

Whilst spur leaf removal may appear an attractive method of early thinning, this is not recommended as these leaves are vital for the uptake of calcium into the fruits, as shown by complementary work at East Malling (Jones and Samuelson, 1983).

• The most severe reductions in fruit set on Cox were brought about by removal of spur leaves in the period between full bloom and two weeks later (Proctor and Palmer, 1981).
• Leaf removal in these trials also reduced fruit calcium levels at harvest and removal of bourse shoots reduced return bloom in the subsequent season.

 

 

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).

• Most of the literature cites ethylene as the main promoter of flower, fruit or leaf abscission in trees, including the apple.
• Applications of chemicals, such as ethephon, that result in the production of ethylene, have been shown to increase abscission, whilst applications of chemicals which inhibit production of ethylene, such as silver, 2,5‑norbornadiene or aminoethoxyvinylglycine (AVG or Retain) have all been shown to reduce fruitlet abscission.
• However, more recent literature reviews suggest that abscission may be better understood as controlled by a balance of ethylene and auxin levels at the zone of abscission (Sexton, 1997).
• Also of importance is the responsiveness of the tissues associated with the abscission zone to either auxins or ethylene. Auxins move from the seeds or other tissues of fruits back into the fruit stalk and spur; this is referred to as basipetal movement.

During the opening of apple flowers the unfertilised ovules show very little growth and the production and export of auxins is low (Gruber and Bangerth, 1990).

• The flowers are, therefore, vulnerable to ethylene-induced abscission during this phase (Wertheim, 1997).
• Pollination, pollen tube growth and fertilisation of the ovules and subsequent seed growth all lead to a significant increase in hormone activity in the ovary.
• The auxins diffusing from the ovary help prevent abscission.
• Thereafter, the vulnerability of fruits to abscission changes as the balance of auxins and ethylene changes.

Applications of plant growth regulators to induce flower and fruitlet abscission

Several types of chemicals have been shown to have thinning action when sprayed onto apple trees.

• The influence of most of these is increased if they are applied in low light conditions (Byers et al., 1990a; 1990b) because the competition between the various growing sinks within the tree (shoots, roots flower buds, fruits) is stronger in such conditions.
• Use of such chemicals should, therefore be adjusted to take account of the prevailing light conditions in the orchard at the time of spraying (Lehman, 1987).

The main plant growth regulating chemicals applied to apple trees to thin the flowers or fruits are:

• Auxins
• Carbaryl
• Ethylene releasing chemicals
• Photosynthesis inhibiting chemicals
• Cytokinins (benzyl adenine – BA)
• New chemicals showing some potential as thinners

Auxins

Although the natural auxins produced in the seeds of apple fruits, such as indole acetic acid (IAA), are important in preventing fruit drop, other synthetic auxins, such as NAA, can have the reverse effect when applied to trees.

• The auxins NAA (1-naphthylacetic acid) and its amide NAAm (NAD) have proved effective fruit thinners for apples in many experiments.
• Their mode of action is subject to some debate. One suggestion is that they lower the export of diffusible IAA (a natural auxin which inhibits abscission) from fruit ovaries and across the abscission zone on the fruit stalk (peduncle) (Crowe, 1965).
• Another idea is that they restrict the supply of assimilates (foodstuffs) to the developing fruitlets either directly or indirectly by causing a temporary reduction in photosynthesis (Schneider, 1978a; 1978b).

Variable weather conditions during or after application of NAA affects their uptake and may also influence its transport and breakdown within the apple tree.

• Uptake through apple leaves increases with increasing temperatures (Black et al., 1995) and is also increased in low light conditions (Flore et al., 1990).
• The effect of humidity on the efficacy of NAA thinning is often important (Jones et al., 1988) but is also inconsistent.
• However, spraying NAA after cool moist weather may increase rather than decrease fruit set and retention.
• NAA and NAAm sprays also induce unwanted ‘pygmy’ fruits, especially in varieties such as Elstar.

More recent studies in Germany (Schönherr et al., 2000) have demonstrated that penetration of NAA through leaf cuticles is approximately 40% in eight hours, if applied in deionised water.

• However, applications in water with a pH >7 almost eliminated any cuticular penetration, even at high humidities (100%) and moderate temperatures 20^oC.
• Rates of penetration were much lower in the deionised water if applied at only 55% RH and 10^oC.
• The addition of Tween 20 helped uptake but not significantly. Most of these problems of uptake were reduced if the NAA was applied in solutions buffered to pH 4 with DL –lactic acid and the accelerator adjuvant Genapol C-100 (0.2-2.0gl^-1) was added.
• This combination improved uptake at low temperatures and in hard water or at low humidities.
• NAA is destroyed by UV light and sprays are best applied in the late afternoon or early evening.

NAAm is slightly less dependent upon ideal weather conditions than NAA; the latter can only be used with confidence where stable climatic conditions during blossoming prevail.

• However, even when using NAAm seasonal variability in response is common and varieties differ in their sensitivity to the sprays.
• When applied late, NAAm often reduces the speed of growth of the persisting apple fruitlets and may have no beneficial effects on fruit size at harvest.
• NAAm is usually applied soon after flowering, whilst NAA is applied a little later, although it will also thin if used during flowering (Jones et al., 1989).
• Concentrations of NAA are usually up to 20 ppm whilst those for NAAm are up to 100 ppm.
• The liquid formulation of NAAm is more active than the powder formulation.

Carbaryl

Little is known concerning the mode of action of carbaryl in thinning fruitlets.

• However, one suggestion is that the sprays inhibit or slow down seed development in the fruit and that this in turn reduces the production and diffusion of auxins from the fruits.
• The fact that carbaryl thins more severely if applied during low light conditions may support this hypothesis.
• The product is a weak thinner but applications at the time of flowering may stimulate mild skin russeting in some situations.
• The thinning action of carbaryl increases with spray concentration up to approximately 750 ppm and this is the dose used in many countries.
• Overdosing and overthinning is unlikely to occur when using carbaryl.
• The product is very toxic to bees and sprays to flowers that are being worked by bees should be avoided (Helson et al., 1994), although formulations providing more safety to bees (e.g. Sevin XLR) have been used (Nichols, 1996).
• Note that carbaryl is not approved for use on apples in the UK and that it is likely to be withdrawn in Europe.

Ethylene releasing chemicals

Treatments with ethephon, a chemical that releases ethylene when sprayed onto plants, are known to stimulate abscission of flowers and fruitlets. Ethephon (Ethrel C) influences the production and transport of the auxins (which normally inhibit abscission) and also increase the evolution of ethylene gas by the fruits and leaves.

• The problem with the use of ethephon as a flower or fruitlet thinner is the extreme variability of response recorded in many experiments.
• Extensive research in Tasmania has shown that both spray concentration and timing of the sprays were an important cause of variation in response (Jones et al., 1983; Koen and Jones, 1985) see Further reading [hyperlink]).
• The same team also showed that the thinning response to ethephon sprays was very closely associated with increasing temperatures (Jones and Koen, 1985).

Photosynthesis inhibiting chemicals

Research in the USA has shown that sprays at low concentrations of the herbicide terbacil (a photosynthetic inhibitor) have had beneficial thinning effects (Byers et al., 1990a; 1990b).

• The theory is that the temporary check to photosynthesis triggers a chain of events leading to fruitlet abscission.
• Although proved effective in USA trials, this strategy is a high risk one.
• Extensive leaf damage and abscission has occurred in some trials testing terbacil and other similar products.

Cytokinins (benzyl adenine – BA)

Sprays of the synthetic cytokinin benzyl adenine at or around the 7-12 mm fruitlet diameter stage have been shown to induce fruitlet abscission and/or increase final size of apple fruits at harvest (Bound et al, 1991;Bound et al., 1993; Greene and Autio, 1989; Greene and Autio, 1994).

• In Australian trials, and when applied at the correct timing, BA has proved more consistent in thinning than carbaryl.
• Moreover, unlike carbaryl BA is not toxic to important predatory mites species used for biological control and the toxicity of BA to mammalian and arthropod species is low (Thistlewood and Elfving, 1992).
• However, the responses are temperature dependent and 18^oC or more is needed if thinning is to be consistent (Greene and Autio, 1994; Bound et al., 1997).

The thinning mode of action of the BA sprays is not understood.

• One theory was that the sprays promoted the growth of bourse shoots, which in turn competed with the adjacent fruitlets for minerals and/or assimilates, causing the fruitlets to drop off.
• However, studies at East Malling showed no significant stimulation of bourse shoot growth following BA sprays.
• Trials at East Malling did suggest that BA sprays thinned fruitlets quite effectively if applied following a spray of a blossom thinner such as ATS.
• One possibility is that the blossom thinner reduces the seed numbers in the fruitlets, making them more vulnerable to the later BA sprays.
• Further research is needed to understand how BA brings about its thinning action and how environmental and other factors influence its uptake.
• This understanding will be necessary if consistent thinning responses to the chemical are to be achieved in the future in UK orchards.

In several trials, BA treatments have improved the size of harvested fruits but caused no reduction in final fruit numbers per tree (Greene et al., 1992; McArtney et al., 1992).

• It has been suggested that this is due to a direct effect of the sprays on fruitlet cell division.
• USA trials showed BA sprays to induce cortical cell division and to increase cell layer formation in ‘Empire’ apples (Wismer et al., 1995).
• However, further work is needed to support this theory and explain why the effect is not consistent from season to season

Other compounds with cytokinin activity may also prove useful as fruitlet thinners for apples.

• Both forchlorfenuron (CPPU) (Greene, 1993) and thidiazuron (THI) (Greene, 1995) have shown some thinning activity in trials.

 

Modelling crop loading and aids to decision making in thinning

It is recognised throughout the world that one of the most difficult and often most expensive decisions for commercial apple growers is whether or not to thin. Many factors must influence the decision including tree and variety characteristics, previous cropping history and the weather conditions before, during and after flowering.

Researchers in the USA have developed decision support systems (computer models) that take account of a multitude of climatic, cultural and application conditions to make recommendations on the use of chemical thinners.

• One such system, THIN-IT, developed in Washington State, produces a range of recommendations and is mainly appropriate for the dry arid climatic conditions of the Pacific North West of the USA (Williams and Wright, 1991).
• A slightly more sophisticated model has been developed in Pennsylvania and is incorporated into a larger suite of models distributed as the Penn State Apple Orchard Consultant (Crasweller et al., 1992).
• Unfortunately, although this system is more appropriate for maritime climates similar to the UK, it has little immediate value to the UK grower, as it focuses on use of products either not approved in the UK (NAA and NAD) or on products about to be withdrawn (Carbaryl). Another USA computer programme, focused on thinning apples has been developed for growers in New York State (Stover, 1992).

Researchers in Tasmania, Australia (Jones et al., 1987) have also developed computer models to aid growers in making decisions on apple thinning.

• The basis of the model is a mathematical model of optimum crop loading.
• Data from experiments over a twenty-year period have been used in this model.
• Factors, such as the scion variety, the rootstock, the thinning chemicals to be used, the age and size of tree and the fruit size required, are all taken into account.
• In addition, the model takes account of what are known as modifying factors; these are last year’s crop, rate of shoot growth and pruning severity.
• The aim is to deliver a best practice for Tasmanian orchardist.
• Unfortunately, this model, like those developed in the USA is not appropriate for UK use on account of the climatic differences and the reliance on use of ethephon as a thinning chemical.
• A full explanation of the Tasmanian model is given in a recent review paper (Jones et al., 2000).
• No models have yet been developed suited to UK conditions and utilising thinning strategies available to the UK grower.