2019 ASHS Annual Conference

The profitability of apple fruit producers is mainly dependent upon the consistent, high annual yield of quality fruit. In commercial apple production systems, this cannot be achieved without effective crop load management strategies that make a balance between vegetative and fruiting buds, reduce fruit-to-fruit competition, allow fruit enough room to grow, expose fruit to adequate sunlight and enhance return bloom. It is already well-established that crop thinning during bloom produces the largest fruit, the highest return bloom and reduces biennial bearing. However, our knowledge of chemicals and conditions that achieve efficient thinning at bloom stage is still limited, especially in the Eastern US. The objective of the present study was to evaluate materials for chemical blooms thinning that can achieve proper thinning activity without having negative impacts on fruit quality. To this end, we assessed the thinning efficacy of five chemicals including ammonium thiosulfate (ATS), potassium thiosulfate (KTS), lime sulfur, Regalia and potassium bicarbonate. All materials were applied alone or in combination with JMS Stylet-Oil to mature ‘Honeycrisp’ and ‘Pink Lady’ trees in 2018 and 2019 growing seasons, using a backpack sprayer. The timing of thinning treatments was determined according to the pollen tube growth model (PTGM). The experiment was arranged in a completely randomized design (CRD). No post-bloom chemical thinning applications or hand thinning were applied to the examined blocks. Fruit set (%) data were recorded after two, four and six weeks of the first thinning application; and at harvest (for the 2018 season). Our results indicated the following: a) ATS and KTS at low concentrations (1 % and 0.5%, respectively) did not cause fruit russets, but their thinning efficiencies were too low, b) Potassium bicarbonate was a potent blossom thinner; however, lower concentrations should be considered to keep crop losses due to russet at a minimum, c) Potassium bicarbonate-oil mix caused severe damages to fruit and apple foliage, d) Low concentrations of lime sulfur (1%) did not cause fruit injury; however higher concentrations should be considered for better thinning results, e) Thinning applications combining Regalia and oil showed promising results, and f) thinning materials showed minimal impacts on the physical (fruit color, firmness, diameter and mass) and chemical (fruit starch index, titertable acidity, soluble solid content and pH) properties of the ‘Honeycrisp’ and ‘Pink Lady’ apples.