Calcium Polysulfides-Thiosulfate, Ammonium Thiosulfate, and NC-99 Blossom Thinners Impact Fruit Set of Apples, Peaches, and Plums

Crop load adjustment, using blossom and/or post-bloom thinners is a crucial practice to ensure production of commercially acceptable fruit size and yield efficiency in apples and stone fruit. In a study with two apple, one peach and one plum cultivars, application of calcium polysulfides and thiosulfate mixture (lime sulfur), ammonium thiosulfate (ATS), and NC-99 (Natural-cal), applied during bloom, reduced the fruit set of apples and/or certain stone fruit. In most cases, higher concentrations or double applications of a bloom thinner resulted in more blossom thinning which led into lower fruit set. Double applications of ATS at a rate of 15 mL.L-1 or a single application of this chemical at a rate of 25 mL.L-1 resulted in adequate levels of thinning in Fuji apple. Double applications of ATS at a rate of 25 mL.L-1 resulted in excess thinning on Fuji apple. In Gala apple, a single application of NC-99 at 3% or 6%, with and without refined oil, or double applications of lime sulfur, each time at 5%, significantly reduced fruit set in 2013. In Jonathan apple, applications of ATS at 25 mL.L-1 significantly reduced fruit set and increased fruit size. Application of lime sulfur at 6% with or without refined oil significantly reduced fruit set in Elberta peach. In plums, application of lime sulfur at 6% with 1% refined oil significantly reduced fruit set, while application of this mixture at 3% lime sulfur (3% lime sulfur plus 1% oil) did not affect fruit set.