Maturity, Quality, and Production of ‘Thompson Seedless’ Grape as Affected by Frequency of Gibberellic Acid Sprays with and Without Naphthaleneacetic Acid

Effects of frequency and timing of gibberellic acid (GA) with and without naphthaleneacetic acid (NAA) on time of harvest, yield, berry maturity and quality of ‘Thompson Seedless’ grape (Vitis vinifera)under the desert conditions of southwest Arizona over two growing seasons were studied. Vines sprayed with GA at bloom and/or fruit set had smaller and more tender berries with higher soluble solids concentrations (SSC), color, and SSC/acid ratio, thus higher number of harvestable clusters and yield at the first harvest than those which received a higher number of GA sprays. Three GA sprays (at bloom, at fruit set, and a week later) showed overall satisfactory yield, berry maturity and quality, and thus are recommended for the conditions of this experiment. A fruit set addition of NAA to 5 frequent GA sprays tended to enlarge berries over control or one GA spray treatments and to increase SSCover 5 frequent or infrequent GA sprays alone. Five sprays of GA alone delayed berry maturity, without any yield or quality advantages over 3 GA sprays, and thus are not recommended.