Chemical thinning of grape clusters (<i>Vitis vinifera</i> L.)

Abstract

Twelve thinning compounds were tested at 0.1 to 4,000 ppm on 'Carignane' and 'Thompson Seedless' clusters at 4 developmental stages (prebloom, bloom, fruit-set, and 2 weeks after fruit-set). Clusters were usually least sensitive to the compounds ut post fruit-set and, in general, 'Thompson Seedless' was more sensitive than 'Carignane'. On 'Thompson Seedless' the morphactin IT 5733 had a wider activity range than IT 5732 or IT 4433. Potassium iodide reduced set when applied at prebloom or bloom. Ethephon was usually effective at some concentration at all growth stages. Ascorbic acid, iso-ascorbic acid, and Pennthin usually were not significantly effective in thinning either cultivar.
The 3-CPA had a wide range of activity on both cultivars at most stages of growth, and this compound at 10 ppm reduced number of berries at bloom and fruit set for 'Carignane' and 'Thompson Seedless', respectively. Iodoacetic acid at 1,000 ppm was effective in reducing number of berries in both cultivars at all growth stages. The 4-CPA at 100 ppm was effective at prebloom stage en 'Carignane', and .'.lt bloom on 'Thompson Seedless'. Salicylic acid at 1,000 ppm effectively reduced number of berries of each cultivar when applied at fruit-set stage.