The Influence of Cluster Thinning at Various Stages of Fruit Development on Flame Seedless Table Grapes

¹ Dept. of Viticulture and Enology, University of California, Davis, and the University of California Kearney Agricultural Center, 9240 S. Riverbend, Parlier, CA 93648
² University of California Cooperative Extension, Fresno, CA.

A study was conducted in 1991 and 1992 to determine the influence of cluster thinning at various stages of fruit development on Flame Seedless table grapes grown in the San Joaquin Valley of California. Grapevines adjusted to 45 clusters prior to bloom had one-third of their clusters removed at one of the following stages of fruit development: prebloom (PB), berry set (BS), berry set + 2 weeks (BS + 2), berry set + 4 weeks (BS + 4), and berry set + 6 weeks (BS + 6). A group of unthinned vines, carrying 45 clusters throughout the season, were included in the study. The leaf area:fruit weight ratios for vines thinned at BS and unthinned vines, respectively, were 11.9 and 10.2 cm² g^-1 in 1991 and 15.5 and 11.4 cm² g^-1 in 1992. Little difference in berry fresh or dry weight was observed among the treatments prior to fruit softening. After fruit softening, vines thinned between PB and BS + 4 generally exhibited greater berry fresh and dry weights than vines thinned at BS + 6 and unthinned vines. Berry diameter did not differ among the treatments at harvest in either season. Berry color was the compositional parameter most sensitive to date of cluster thinning. Vines thinned between PB and BS + 4 accumulated color more rapidly than vines thinned at BS + 6 and unthinned vines. Fruit soluble solids at harvest were greater for cluster thinned vines compared to unthinned vines. Total and cull yield were greatest for unthinned vines in both seasons. In 1991, vines thinned at BS + 6 had the lowest packable yields in the experiment. In 1992, the packable yields of vines thinned at BS + 6 and unthinned vines were similar and significantly lower than those of the remaining treatments. The results indicate that mid-season ripening Flame Seedless table grapes may be cluster-thinned between PB and BS + 4 with similar effects on fruit growth, fruit composition, and packable yield. Due to reduced packable yields, as well as variable effects on fruit composition, cluster thinning after berry softening (BS + 6) is not recommended.

Key words: Vitis vinifera L., crop load, fruit growth, fruit composition

Submitted on August 15, 1994

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