Effect on 'Thompson Seedless' Fruit of Gibberellic Acid Bloom Sprays and Double Girdling

Abstract

The thinning of Vitis vinifera L. `Thompson Seedless' fruit in response to gibberellic acid (GA) applied to vines during bloom was studied in Kern County, California. Also included was the response to GA treatment of fruit on double-girdled vines.

With crop level per vine held constant, commercially mature clusters were significantly looser on vines sprayed with 5, 15, and 25 ppm GA at 10, 60, and 95% bloom than on unsprayed vines. The main reason was fewer berries per cm on The basal four laterals. Clusters were loosest from treatments at 60% bloom. Density of berries on hand-thinned clusters was significantly greater for thinning before than after shatter. Fruit from vines sprayed at 1.0 and 60% bloom shattered more during postharvest handling than fruit from unsprayed vines. Shot berries were fewer with prebloom thinning than with GA sprays applied during bloom, with the percentages of the latter related directly to the GA concentration and time of treatment being greatest for 10% bloom and least for 95%. The percentages were greater on the first and second laterals than on the third and fourth.

Water berry formation was not conclusively affected by the concentration and timing of GA sprays. Thinning, whether done by hand before bloom or during bloom with GA sprays, increased the variability of water berry formation among the basal four laterals of the clusters.

The degree Balling and Balling-acid ratio of fruit were significantly higher for hand-thinned before bloom and GA-thin ned during bloom than for vines thinned after shatter. They were also significantly higher for GA-thinned fruit than for hand-thinned fruit. Concentration of GA and time of application had no clear effect on either the degree Balling or Balling-acid ratio.

Although berry size showed no clear relation to degree Balling in any treatment, it was directly related to Bailing-acid ratio in all but the unthinned treatment with very small berries (because the percentage of total acid decreased as berry size increased).

The degree Balling was significantly higher for vines double-girdled at shatter or single-girdled at shatter plus another girdling when the fruit was 8-10°B than for vines single-girdled at shatter or single-girdled at shatter and the girdle reopened when the fruit was 8-10°B. Within, each pair of treatments the difference was not significant.