Evaluating Application Timing Strategies, Suitability, and Efficacy of Apple Blossom Thinning Chemicals for Commercial Use

• Main Author: Allen, William Chester G.
• Other Authors: Not found
• Format: Others
• Published: Virginia Tech 2020
• Subjects: Apple; Blossom; Thinning; Chemical; Timing; Suitability; Efficacy
• Online Access: http://hdl.handle.net/10919/96444

With post-bloom chemical fruit thinning responses being heavily influenced by tree carbohydrate reserves and weather conditions, there is a need for alternative thinning practices such as chemical blossom thinning in the Mid-Atlantic apple growing region. This project sought to 1) evaluate timing strategies for lime sulfur + stylet-oil blossom thinning sprays and 2) screen chemical agents for their suitability and efficacy as blossom thinners. In the first study, two 'Gala'/'M.9' blocks in different states (North Carolina and Virginia) were utilized in 2019 to compare between pollen tube growth model-guided blossom thinning sprays and those that are based on fixed time intervals between the initial application and subsequent thinning sprays. It was generally found that model-guided blossom thinning sprays and sprays applied at 20% open bloom and 48 hours after reduced fruit set, crop load, and improved fruit weight. In the second study, a 'Honeycrisp'/'B.9' and 'Cripps Pink'/'M.9' apple block in Virginia were used in 2018 and 2019 to evaluate multiple chemical agents with and without stylet-oil. Most of the treatments under-thinned compared to untreated control trees. However, it was determined that ammonium thiosulfate with and without stylet-oil was the most effective thinning agent. Potassium bicarbonate + stylet-oil was observed to cause excessive fruit russeting and phytotoxicity. This project demonstrated that optimum apple crop loads can be obtained if lime sulfur + stylet-oil blossom thinning sprays are applied at the proper time, and that multiple chemical agents offer potential use for chemical blossom thinning in the Mid-Atlantic region === Master of Science in Life Sciences === For apple trees to produce high quality fruit, a proportion of the flowers and/or fruit must be removed in a practice known as "thinning". Chemical blossom thinning is a relatively new method of thinning in the Mid-Atlantic apple growing region. This project sought to 1) evaluate spray timing of blossom thinning sprays and 2) evaluate different chemicals for their suitability in blossom thinning. In the first study, two 'Gala' apple orchards in two states (North Carolina and Virginia) were utilized in 2019 to compare a computer model-guided spray timing to structured spray timings based on the amount of time between the first and second sprays. It was found that the model-guided sprays, and sprays occurring once 20% of the blossoms had opened and reapplied 48 hours after, were the most effective in thinning the flowers. In the second part of the study, a 'Honeycrisp' orchard and a 'Cripps Pink' orchard in Virginia were used in 2018 and 2019 to evaluate different chemicals for their potential as blossom thinners. Unfortunately, most of the treatments did not achieve sufficient thinning results compared to untreated trees. However, it was determined that ammonium thiosulfate with and without stylet-oil was the most effective thinning chemical. Potassium bicarbonate with stylet-oil was found to cause excessive injury to the fruit and foliage. This project demonstrated that blossom thinning can be effective when the sprays are applied at the correct time and that multiple chemical agents offer potential for blossom thinning in the Mid-Atlantic.