Comparative Evaluation of the Mode of Action and Efficacy of Blossom Thinning Materials In Apple

Present study summarizes the major results on blossom thinning experiments in apples obtained at the Ohio State University. Five sets of lab and field trials were established to study blossom thinners/thinning programs from both physiological and practical viewpoints. In Trial 1, a comparison of potential thinning materials with physical (petroleum oil, PO – 2%; kaolin clay, KC – 12.5%; and carnauba wax emulsion, CW – 20%) and chemical mode of action (ammonium thiosulfate, ATS – 1.5%; lime sulfur, LS – 2%; and sodium chloride, SC – 2%) were evaluated in ‘Fuji’ and ‘Golden Delicious’ cultivars. Although the efficiency of thinners with chemical mode of action in reducing fruit set was significantly higher than the physical group in both cultivars, they had considerable phytotoxic effects on young leaves (decreased relative chlorophyll content, reduced photosynthesis). Scanning electronmicroscopy imaging of surface morphology of apple stigmas revealed considerable differences among the materials applied, however, no visual differences of anthers were observed. Pollen germination was not significantly reduced by chemicals with physical mode of action but by caustic materials. The timing experiment (Trial 2) in ‘Golden Delicious’ with CW (20%) or ATS (1.5%) showed that single applications at 50% full bloom were the most effective, compared to the applications at 0, 20 or 80% full bloom. Multiple sprays at 20%+50%, 50%+80% or 20%+50%+80% were equally effective. Trial 3 was set up on the comparison of thinner concentrations in ‘Golden Delicious’; 10, 20 or 40% CW and 0.5, 1.5 or 4% ATS were sprayed at full bloom. Single applications of any of these concentrations caused reduction in fruit set but they were not significantly different from each other. 4% ATS caused severe leaf damage and fruit russetting. In Trial 4 (cultivar comparison), no significant differences were found in fruit set among cultivars tested, however ‘Golden Delicious’ and ‘GoldRush’ seemed to produce more russetted fruit after 2% LS+PO application at full bloom than ‘Fuji’, ‘Honeycrisp’ or ‘Gala’. Trial 5 in ‘Golden Delicious’ cultivar with 2% LS+PO applied at full bloom on the comparison of different growing sites (Berlin Heights, Wooster and Carroll) pointed out that no significant difference exists in the efficiency of blossom thinners among geographical locations within the state of Ohio.