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 20oC.
- Rates of penetration were much lower in the deionised water if applied at only 55% RH and 10oC.
- 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.
- 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 18oC 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.