The table grape cultivar ‘Concord Seedless’ is adapted to the northeastern U.S., but small berry size, poorly filled clusters, shot berries, and low marketable yields has hindered its’ commercial potential. Gibberellic acid (GA3) is a plant growth regulator widely used in viticulture to reduce fruit set and to increase cluster length, cluster weight, and berry weight, but effects vary depending on cultivar, application timing and rate. We evaluated the effects of GA3 on cluster quality in ‘Concord Seedless’, a cold-hardy table grape cultivar, grown using two training systems: Munson and Vertical Shoot Positioning (VSP). Field experiments were conducted in 2023 and 2024 at the UNH Horticultural Research Farm in Durham, NH. Treatments included four concentrations of GA3 applied pre-bloom (0, 21, 42, and 63 mg/L) and one concentration post-bloom (0 and 50 mg/L), in a 4 × 2 factorial arrangement. A split-plot design with four blocks was used, with training system as the main plot and GA3 treatments as subplots. The experimental unit for sub plot was two adjacent shoots; all treatments were applied on a single vine. We measured cluster weight, frequency of shot berries and thinned berries, and percentage of clusters exhibiting curling. We hypothesized that pre-bloom GA3 application would result in cluster elongation and that post-bloom GA3 application would increase cluster and berry weight. We also hypothesized that GA3 application at either time would reduce the incidence of shot berries. Results showed significant year × GA3 interactions. In both years, cluster weight was higher in Munson than in VSP-trained vines, but there was a significant interaction between training systems and GA3 treatments in 2023. VSP-trained vines had a higher frequency of shot berries and thinned berries in both years, but the effect was not significantly different. In both years, pre-bloom GA3 applications reduced cluster weight and induced cluster curling and excessive berry thinning, which made clusters unmarketable. In 2023, post-bloom application (50 mg/L) increased cluster weight, and did not cause berry thinning or cluster curling in either training system. Our findings suggest that a single application of post-bloom GA3 when berries are 6 to 9 mm in diameter can improve cluster quality in ‘Concord Seedless’ under cold-climate conditions.
