FACTORS INFLUENCING PREHARVEST DROP OF APPLES

Preharvest drop on apple trees appears to be a somewhat random event. A study was initiated to gain a better understanding of the process of preharvest drop in apples and try to more clearly relate fruit factors that are reported to be closely related to preharvest drop. Five paired trees of mature ‘McIntosh’/M.7 were selected in two different years and five paired trees of ‘Delicious’/M.26 were selected one year. Dropped fruit were counted and removed at 8 am and 3 pm each day under one of the paired trees and fruit weight, percent red color, seed number, starch index and internal ethylene concentration (only in one year) were measured on individual dropped fruit. At the same time 10-apple samples were randomly harvested from the other paired tree and these apples were evaluated similar to the dropped fruit. Sampling continued until most of the fruit had fallen. Fruit weight had no consistent influence on drop. Dropped fruit on ‘McIntosh’ had more red color and fewer seeds than those harvested from the tree in one of the two years. Dropped fruit of both cultivars had higher starch index (lower starch content) and higher internal ethylene concentration. Early preharvest drop appears to be linked to fruit ripening thus the key to determining the cause of preharvest drop appears to reside with factors that initiate advanced ripening of fruit.