Experience with Thinning: What Factors Contribute to Contradicting Results?

Abstract

POLLINATION There are two species of parasitic mites that essentially have eliminated wild honeybees in the mid-Atlantic region and have drastically reduced the number of hives available from beekeepers in Virginia. This raised some concerns about apple (Malus ×domestica Borkh.) fruit set and the activity of our chemical thinners on fruits with poor pollination. In 1996, we performed an experiment to try to determine if thinners were more effective on fruits with few seeds. During the pink stage, varying numbers of stigmas were removed from king flowers on ‘York Imperial’ trees. Some trees were sprayed with carbaryl when mean fruit diameter was ≈10 mm, and another set of trees were not sprayed with a chemical thinner to serve as a control. Fifty days after treatment, the number of treated fruits that remained on the tree was recorded and expressed as a percentage of the original number of treated flowers. Fig. 1 shows that fruit set was 30% for the control flowers with no stigmas removed and flowers with three stigmas removed set 22%. Removal of four stigmas reduced fruit set to 12%. Trees that were thinned with carbaryl had 8% to 14% fruit set regardless of the number of stigmas removed. Based on this preliminary experiment, it seems that poorly pollinated flowers set fewer fruits when not thinned, but the severity of drop following a thinning treatment may not be related to the degree of pollination. When pollination is considered inadequate, one may want to delay the application of thinners as long as possible to evaluate natural fruit drop. We have found that carbamates will provide fairly good thinning when mean fruit diameter is ≤14 mm and ethephon will provide thinning when fruit diameter is <30 mm.