New strategies for the chemical thinning of apple (Malus domestica Borkh.)cultivars Queen Cox and Royal Gala

High volume sprays of ammonium thiosulphate (ATS), endothal, sulfcarbamide and pelargonic acid, applied at full bloom, were compared as flower thinner s for the apple cultivars Queen Cox and Royal Gala grown on M.9 rootstocks in the south-east of England. Sprays of ATS (10,000 or 15,000 mg l(-1)) or endothal (500 to 200 mg l(-1) reduced initial and final fruit set per 100 f loral buds in each of two consecutive years when applied to the same set of trees. Sulfcarbamide reduced fruit set only when applied at concentrations higher than those recommended (4000 mg l(-1)), and sprays of pelargonic ac id had no significant effect on the final set of these two cultivars. The A TS and endothal treatments increased fruit size in the first year of applic ation, but effects in the subsequent year were inconsistent. Sprays in the first year increased floral density in the second year. In both years of tr eatment, sprays of benzyladenine (BA), applied when fruitlets were approxim ately 12 mm in diameter, increased the percentage of fruitlets abscinding. As with the ATS and endothal sprays, the effects of the BA spray treatments on fruit size at harvest were variable. Possible reasons for the inconsist encies in fruit size recorded in the thinning trials are discussed.