The cluster-thinning practice called green thinning is commonly performed to improve uniformity of ripeness. It is based upon an underlying assumption that shoots on the same grapevine are phenologically autonomous. There is no empirical evidence to support this assumption. The objective of this study was to test this assumption. A field trial was established comparing early pruning (EP) versus late pruning (LP) of spurs on separate vines (SV), opposite cordons on the same vine (OC), and alternate spurs on the same vine (AS). Phenological stages were followed on shoots from one vine per replicate for each treatment combination using an updated Eichhorn-Lorenz method from budbreak to the end of veraison. There was a significant difference in median phenological stage between EP and LP spurs for all vine-level treatments throughout the year. Median bloom dates and median veraison dates were delayed to the same degree by late pruning applied either to separate vines (SV), opposite cordons (OC), or alternate spurs (AS). Bloom was delayed 15 days with late pruning and veraison was delayed six days regardless of whether or not it was done on separate vines or the same vine. There was no significant interaction between pruning date and vine treatment on phenology throughout the season, which supports the hypothesis of shoot autonomy. Fruit maturity sampling after veraison and at maturity also showed a significantly lower Brix in LP vines for all vine treatments and no interaction between pruning date and vine level. There was, however, a significant interaction of pruning and vine level on pH at maturity. The results provide evidence for phenological shoot autonomy in grapevines.

Funding Support: Treasury Wine Estates