Excessive fruit set can reduce the quality of 'Crimson Seedless' table grapes (Vitis vinifera L.), resulting in compact, tightly filled clusters with small berries that are prone to bunch rot at harvest. Two separate studies were conducted to examine the effects of application timing and rate on the efficacy of GA3 for berry thinning. In the first experiment, vines were treated with 2 g·ha-1 GA3 at one of the following stages of bloom: 1% to 5%; 20% to 30%; 50% to 60%; 80% to 90%; or 100% capfall (falling of the calyptra). The second experiment compared treatments of 0, 2.5, 6.25, 12.5, 18.75, or 25 g·ha-1 GA3 applied at ≈80% bloom. Fruit set (number of berries/cm shoulder length) was similar among vines treated between 1% to 5% and 100% bloom with 2 g·ha-1 GA3, although berry set was reduced relative to the control. However, applications made during the later stages of bloom significantly increased berry length and weight. Fruit set generally declined, and the number of shot berries per cluster increased, as the amount of GA3 applied at bloom increased. Applications ≥6.25 g·ha-1 GA3 resulted in commercially unacceptable levels of shot berries, as well as significant reductions in cluster weight and packable yield per vine. The results indicate that a single application of 2.5 g·ha-1 GA3 near full bloom may be used to reduce fruit set and increase berry size of this cultivar without detrimental effects on packable yield or cluster number per vine the subsequent year. Chemical name used: gibberellic acid (GA3).
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