Impact of Cluster Thinning on 'Pinot Noir' Fruit Quality Across a Large-scale, Multi-site Study

Premium wine grape production regions have long-held traditions of maintaining low yields to achieve high fruit and wine quality. A large scale, multi-site study was designed to determine the impacts of reducing yield on vine performance and fruit composition of Vitis vinifera ‘Pinot noir’ winegrapes in Oregon. The trial was conducted annually in more than ten commercial vineyards from 2012 to 2014. Each site had two or more yield levels implemented at lag phase of berry development using cluster thinning down to a pre-determined number of clusters per shoot. The majority of vineyards implemented 1 cluster per shoot, 2 clusters per shoot and/or full crop (non-thinned). All thinning treatments were applied to whole rows in a randomized complete block design with three replicates across blocks that ranged 0.4 to 1.2 hectares. Given the differences in baseline yield over the three years, cluster thinning resulted in an average yield reduction of 0.53, 0.45, and 0.77 kg·m^-1 in 2012, 2013 and 2014, respectively. Fruit composition at harvest, including basic ripening parameters, yeast assimilable nitrogen, anthocyanin and phenolics, did not differ by cluster thinning within 30% of sites each year. For the remaining 70% of sites, there were few consistent differences in these parameters over the three years. However, the most consistent effect was found for anthocyanin concentration. A negative linear regression for yield and polymeric anthocyanin was found in 20%, 29% and 42% of vineyards in the study during 2012, 2013 and 2014, respectively. Significant regressions were found for total anthocyanin at only 10% of sites in 2012, the lowest yield year, and 30% of sites in both 2013 and 2014. The mean difference in anthocyanin concentration across years and sites was 2 mg·L^-1 higher polymeric anthocyanin and 162 mg·L^-1 higher total anthocyanin in fruit-thinned vines compared to those with full crop. There were no differences in dormant pruning weight for any site, even after three years of vines carrying a full crop level, suggesting that vines were not over-cropped. These data suggest that 'Pinot noir' vines had sufficient vine size to support fruit ripening; however, cluster thinning increased anthocyanin in some sites which may increase wine color intensity, an important quality parameter for red wines.