Berry Set Response of ‘Thompson Seedless’ Grapes to Prebloom and Bloom Gibberellic Acid Treatment1

Abstract In 1967, 10 ppm of gibberellic acid (GA) applied to ‘Thompson Seedless’ grape inflorescences 10 days before bloom reduced the set of normal berries by 20%. In 1968, either 10 or 30 ppm GA applied 10 days before bloom reduced berry set by 8.2% and similar treatments applied at the beginning of bloom (< 1% of calyptras had fallen) reduced berry set by 21%. GA applied at 50% or 70% bloom was ineffective as a thinning agent and increased shot-berries in proportion to concentration. Direct counts of berries set relative to number of flowers present are necessary to estimate fruit thinning responses.