The following experiments used 10-year-old slender spindle trees of 'Summerred', grafted on M9 rootstocks, growing in Norway. The blossom clusters on some trees were untreated, while on others they were hand-thinned in late June to leave fruit about 10-15cm apart.
Yet other trees were sprayed with ethephon either at 250, 375 or 500mg per litre when the king flowers opened or with 500, 625 or 750mg per litre when the average fruitlet diameter had reached 10mm.
All sprays were applied to run-off on whole trees using a small portable sprayer and no wetting agents were used. Spraying was done on days when the maximum temperature was between 15 degsC and 20 degsC and rain was not expected.
Three treatments gave satisfactory yields of good-quality fruit and the trees subsequently produced an acceptable number of flowers in the following year. The satisfactory treatments were hand-thinning and applying either 250mg of ethephon per litre when the king flowers were open or 625mg per litre when the fruitlets were 10mm in diameter.
Ethephon as a Blossom and Fruitlet Thinner Affects Crop Load, Fruit Weight, Fruit Quality and Return Bloom of 'Summered'
Apple (Malus xdomestica) Borkh. by Meland, Sekse and Kaiser (2011). HortScience 46 (3): 432-438. ISHS members can view HortScience from the website www.ishs.org.