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

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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.