516 Predicting Fruit Set and Evaluating the Effects of Chemical Thinners on Apples

The lack of pollination and the effects of blossom thinners were simulated by enclosing selected `McIntosh' apple spurs in super-light insect barrier netting at the pink stage of flower development. Fruit set was recorded and fruit size measured at 2- to 3-day intervals from petal fall until initial set. The effects of lack of pollination or the use of blossom thinners on initial set could not be determined with any degree of accuracy until at least 8 days after petal fall. NAA was applied at 8 ppm when fruit were 8.5 mm in diameter. Fruit set and fruit size were taken at 2- to 3-day intervals until the end of June drop. Fruit set on NAA-treated trees was greater than that on check trees for 2 weeks following application. Although NAA ultimately did cause significant thinning, it was not until 3 to 3.5 weeks after application that it was possible to determine with accuracy the thinning response to NAA. However, the thinning response to NAA could be predicted within a week after application, since growth of fruitlets that ultimately abscised slowed 4 to 7 days after the application of NAA. A working model to predict effective pollination and the response to chemical thinners in apples will be discussed.