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 15oC result in poor efficacy of thinning; thinning increases linearly as temperatures increase from 12-24oC (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 >15oC, 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.