Paul Mawdsley, Jean Dodson Peterson,* and Federico Casassa  
*Wine and Viticulture Department, Cal Poly San Luis Obispo, 1 Grand avenue, San Luis Obispo, CA 93407, San Luis Obispo, CA 93407 (jdodsonp@calpoly.edu)

Cluster thinning is commonly practiced in Cabernet Sauvignon and Chardonnay, with little work done in Pinot noir from the Edna Valley in the Central Coast of California. In this three-year study, 50% crop reduction was applied at bloom (B), four weeks postbloom (B+4), eight weeks postbloom (B+8), and 12 weeks post-bloom (B+12) in Pinot noir grapes (clone 115) to evaluate the effect of the timing of cluster thinning on fruit and wine quality between treatments and against a control with no crop reduction. The vineyard experiment followed a complete randomized design (n = 5, ~100 vines/replicate) with three replicates made into wine. In 2016, cluster count was significantly reduced by 37% on average in the thinned treatments. Cluster weight was 43% lower (p < 0.05) in all four thinned treatments relative to the control, and, kg/vine decreased from 2.29 kg/vine in the unthinned control to 0.63 kg/vine in the B+8 treatment, giving an overall 66% yield reduction in the thinned treatments. There was no treatment effect on Brix and pH of the fruit at harvest, but berry weight decreased 18% in the B+8 treatment relative to the un-thinned control. Wine basic and phenolic chemistry showed marginal differences between treatments. Phenolics in wines (tannins, anthocyanins, and polymeric anthocyanins) showed no treatment effect; only catechin was marginally higher in B+4 and B+12 wines. Likewise, Cie-Lab color parameters (L*, C*, a*, b*, and H) and wine color (AU at 420+520 nm) showed no treatment effect. Overall, this preliminary study indicates no significant differences both inter-treatment and between treatments and the control in terms of grape and wine chemistry, in spite of a severe crop reduction in the thinned treatments, indicating a lack of effect of cluster thinning and the timing of cluster thinning.

Funding Support: Summer Undergraduate Research Program – Cal Poly