Thinning Response of Gala and Honeycrisp apple trees to lime sulphur and ATS blossom thinners in combination with carbaryl as a fruitlet thinner

Early crop load management of apple trees (Malus domestica Borkh.) is important to enhance fruit quality and reduce biennial bearing. Orchard experiments were conducted from 2020-23 to evaluate the thinning efficacy of blossom sprays of 2.5% lime sulphur (LS) combined with 2% mineral oil, and 2% ammonium thiosulphate (ATS), each applied alone or in combination with post-bloom fruitlet thinning with 1500 mg.L-1 carbaryl on Gala and Honeycrisp trees. Blossom applications were timed using the pollen tube growth model. Treatments were compared with untreated control trees and hand thinned controls. Averaged over the four years and two cultivars, ATS and LS reduced fruit set by 9% and 29%, and crop load by 13% and 21%, respectively, relative to untreated controls. The carbaryl fruitlet treatment provided greater reductions in fruit set and crop load, but the integration of blossom and fruitlet thinner provided superior performance. In one year when natural crop loads were high, both blossom thinners significantly enhanced return bloom of Honeycrisp. Overall, blossom thinners provided only modest and inconsistent reductions in fruit set, but their use in sequential thinning programs may benefit biennial-prone cultivars by enhancing return bloom under heavy fruiting conditions. The benefits of early crop load reduction using LS and ATS blossom thinners need to be balanced with product costs, thinning efficacy, risk of natural thinning by frost, leaf phytotoxicity, and potential fruit russeting. Future research should focus on developing cost-effective blossom thinners with the aim of providing more consistent early reductions in crop load.