2017 ASHS Annual Conference

Blossom thinning can increase apple fruit size and promote annual bearing when compared to later application timings. In this study, the following blossom thinning strategies were evaluated on mature ‘Golden Delicious’ apple trees: 1) unthinned (control), 2) hand-thinned at bloom (HT), 3) 2% (v:v) liquid lime sulfur + 2% (v:v) stylet oil (LS + SO), 4) 2% (v:v) ammonium thiosulfate (ATS), 5) 1.5 mL∙L^-1 endothal (ET), and 6) 50 mg∙L^-1 naphthaleneacetamide (NAD). HT was conducted during bloom to manually adjust crop load to 6 blossoms per cm² limb cross-sectional area. All chemical treatments were applied twice during bloom, and a predictive model was used to determine application timing based on estimated ‘Golden Delicious’ pollen tube growth rates. Blossom thinner effects on pollen tube growth in vivo, leaf phytotoxicity, crop density (fruit per cm² trunk cross-sectional area; TCSA), yield, and fruit quality were determined. Compared to the control, LS + SO and ATS reduced the number of pollen tubes that entered the style by 75% and 63%, respectively. While ET did not influence pollen tube growth, increased damage to spur leaf tissue was observed. LS + SO and ET reduced crop density (7.5 fruit per cm² TCSA) when compared to the control (12.2 fruit per cm² TCSA), while HT had the lowest crop density (3.9 fruit per cm² TCSA). HT, LS + SO, and ET increased fruit weight when compared to the control. ATS and NAD did not affect crop density or fruit weight. Fruit russet and seed number was not influenced by any treatment. Of the products evaluated, LS + SO provided the best overall thinning response in this study.