Fruitlet size and position within the cluster influence apple fruitlet susceptibility to chemical thinners

Six experiments were performed to determine the effect of fruitlet diameter on the day of thinner application, fruitlet position within a fruiting cluster, and chemical thinners on retention of individual apple (Malus domestica Bork.) fruitlets, days of fruitlet growth, and days to fruitlet abscission following a thinner treatment. Fruitlet diameter at thinning time was used as a covariate in an analysis of covariance to account for differences in fruitlet diameter between king and side fruitlets within a fruit cluster. Fruitlet diameter, but not fruitlet position, consistently influenced the percentage of fruitlet retention. Carbaryl reduced fruitlet retention in four of six experiments and was most effective on fruitlets smaller than 14.0 mm in diameter. Accel was used in one experiment and was effective on small side fruitlets. Ethephon was used in three experiments and consistently reduced fruitlet retention regardless of fruitlet size (8.0 to 31.0 mm diameter) or position. Days from treatment to fruitlet growth cessation, ut not days from treatment to fruitlet abscission or days from growth cessation to abscission, was positively related to fruitlet diameter on the day of treatment, but none of these response variables were consistently affected by fruitlet position. Ethephon was the only thinner that consistently reduced days from treatment to fruitlet growth cessation. In general, the number of days from treatment to growth cessation was 13-18 d on untreated trees and carbaryl-treated trees, and 8-9 d on ethephon-treated trees. The number of days from treatment to fruitlet abscission was 14-23 d on untreated and carbaryl-treated trees and 10-12 d on ethephon-treated trees.