Responses of Fruit Uniformity to Deficit Irrigation and Cluster Thinning in Commercial Winegrape Production

Fruit uniformity is generally regarded as an essential quality parameter in winegrape production. Deficit irrigation and cluster thinning are popular techniques to improve fruit quality, but there is little empirical information on how these practices influence fruit uniformity in the field. Deficit irrigation and targeted cluster thinning treatments were imposed in a commercial Cabernet Sauvignon vineyard in Dunnigan Hills, California, for three consecutive years. Fruit uniformity measures at three levels (blocks, vines, and clusters) were evaluated using variance components analysis and ANOVA on absolute residuals (Levene’s test). Regardless of the irrigation treatment, cluster-to-cluster variation within vines was the primary source of fruit variability each season. In general, postveraison irrigation deficits decreased cluster-to-cluster uniformity for Brix and fruit anthocyanin content. While the decrease in anthocyanin content uniformity was due to unevenness of fruit anthocyanin synthesis/loss, the decrease in Brix uniformity was due to unevenness in fruit shriveling. Cluster thinning increased Brix uniformity at veraison, but there was no effect at harvest on fruit composition and berry size uniformity. Brix uniformity was not correlated to crop load, pruning weight, or the pruning weight/crop load ratio, but an increase in Brix uniformity did occur as fruit matured.