Multi-year study of the effects of cluster thinning on vine performance, fruit and wine composition of Pinot noir (clone 115) in California’s Edna Valley AVA (USA)

Abstract

A three-year study was conducted at a commercial vineyard site in California’s Edna Valley AVA to evaluate the physiological and agronomical effects of the timing of cluster thinning on Pinot noir (clone 115) grapevines. Vines were thinned to one cluster per shoot at three selected time-points during the growing season (bloom + 4 weeks, bloom + 8 weeks and bloom + 12 weeks), and fruit from each treatment was harvested and made into wine. Across all growing seasons, yield decreased 43% in thinned vines relative to non-thinned control vines. No effect of cluster thinning or interaction with growing season was found in vine shoot diameter, internode length, fruit zone light level, or cluster weight. The growing season significantly affected more fruit and wine parameters than did cluster thinning treatment, with interactions between treatment and growing season found in fruit Brix, titratable acidity, and anthocyanins, as well as wine anthocyanins and wine b* (yellow component). For example, bloom + 8 and bloom + 12 thinning treatments advanced Brix in 2017 but had no effect in 2018. Cluster thinning treatments respectively increased berry anthocyanins and total phenolics by 43% and 87% in 2017 and by 103% and 140% in 2018 relative to the non-thinned control, with no significant differences found between the different thinning treatments. However, the resulting wines from cluster thinning treatments showed no differences in the levels of anthocyanins and total phenolics. The fact that different cluster thinning treatments resulted in nil or minor effects on fruit and wine suggests that the vines tested were at or below a balanced crop load prior to the application of cluster thinning. Edna Valley AVA Pinot noir grapes could likely support crop loads (expressed as the Ravaz index) higher than 3.2 without negatively impacting fruit or wine composition and reducing crop load below that level is unlikely to increase fruit or wine quality.