Response of Royal Gala apples to multiple applications of chemical thinners and the dynamics of fruitlet drop

Abstract

Apple trees bear an abundance of flowers that produce a surplus of fruit that the tree is unable to support. A self-regulatory mechanism enhances the abscission of immature fruitlets, leading to a reduced fruit load, but this is often insufficient to achieve fruit of marketable size. Supplementary fruit thinning is usually required to optimize economic fruit load and annual bearing. This 2 yr study investigated the response of Royal Gala apple trees to primary single and combination sprays of 6-benzyladenine (6-BA) and carbaryl during fruit set, followed by secondary sprays of either treatment 7 d later. The objective was to determine the effect of a second application of chemical thinner on fruitlet abscission, which compounds are most efficacious, and to characterize the pattern of fruitlet drop. The combinations of chemical thinners varied in their effectiveness at reducing crop density. About 12–14 d were required from the time of the first application of chemical thinner to observed fruitlet beneath trees. A single application of both thinners applied at 8 mm advanced fruit drop by 7 d in most instances. The tank-mix of 6-BA and carbaryl applied at 8–10 mm followed by a secondary spray of carbaryl at 15 mm was the most efficacious thinning combination. Secondary sprays of 6-BA at 15 mm were ineffective at inducing additional fruit drop. Combination sprays of 6-BA and carbaryl were more effective than repeat sprays of the same product. This study enhances our understanding of the dynamics of fruitlet abscission and the benefits of multiple applications of post-bloom chemical thinners.