When ideal fruit growth occurs before and after time of thinning. Blossoms and young leaves are injured by frost before or soon after spray application. Low humidity causes rapid drying of the spray and decreased absorption occurs before and after spraying. Foliage is conditioned for increased chemical absorption by prolonged cool periods. Cool periods follow bloom, without any tree stress.
Rain occurs before or after spray application.
Endogenous ethylene production is low. Prolonged cloudy periods reduce photosynthesis before or after application of chemicals. 1 From Williams and Edgerton, 1981. Table lists the chemicals currently labeled for use as apple thinners. Fruit thinning chemicals for apple.