Autonomous Phenological Development in Different Merlot Grapevine Shoots

The common viticulture practice called “green thinning,” performed to improve uniformity of ripeness, is based on an underlying assumption that shoots (and their respective clusters) on the same grapevine are phenologically autonomous. There is no empirical evidence to support this assumption. The theory of phenological shoot autonomy in grapevines was tested over three seasons in three different Merlot vineyards located in Napa Valley. A trial was established comparing early pruning versus late pruning of spurs on separate vines or applied to the same vine in two forms: opposite cordons on the same vine or alternate spurs on the same vine. Phenological stages were followed on shoots from one vine per replicate for each treatment combination using an updated Eichhorn-Lorenz (E-L) scale from budbreak to end of veraison. The ordinal logistic regression analysis of the median phenological stages for all seasons revealed an effect of pruning date, but not pruning unit. Budbreak was delayed by 10 days the first season, by seven to nine days the second season, and between 10 and 13 days in 2020 with late pruning. Veraison was delayed five to eight days each season with late pruning on both separate vines and same vine treatments. The analysis of fruit composition three to five weeks after veraison was complete (E-L 41) also showed a lower Brix with late pruning on separate vines or the same vine, and that there was no interaction between pruning date and pruning unit in any vintages. Overall, the results support the phenological shoot autonomy theory in grapevines.