Cluster thinning effects on yield and grape composition in different grapevine cultivars

Vines of cultivars Sangiovese, Merlot and Cabernet Sauvignon grown in vineyards located in the Umbria region, Central Italy, were thinned at three levels (0%, 20% and 40%) for three consecutive years (1995, 1996 and 1997). For each cultivar, fifty 8-year-old vines on Kober 5BB rootstock, trained to simple curtain with 30 bud load per vine were used. The cluster thinning was done just before veraison tend of July) eliminating the distal clusters. In all cultivars, the cluster thinning significantly reduced the yield only at the intensity of 40% and in two of the three year studied. The reduction of yield observed is not proportional to the cluster thinning intensity due to significant increases of berry and cluster weights. Decreasing yields by cluster thinning, above all in 1995 and 1996, caused earlier ripening (about one week) and increased the pH and the contents of soluble solids, anthocyanins, polyphenols and total nitrogen in the must. At the same time, the juice titratable acidity decreased and the bud fertility was unaffected. In 1996 the percentage of clusters affected with more 5% incidence of Botrytis cinera was reduced above all in 40% cluster-thinned vines. The results show that the regulation of yield level through cluster thinning is strictly dependent on year and that, in general, this management practice can be used to improve grape quality. This assumes even greater importance in years characterised by unfavourable environmental conditions (i.e. 1995 and 1996 with low degree-days and high rainfall during the two months before harvest) a well as in very productive vineyards due to cultivar (i.e. Sangiovese) or high soil fertility. The increase of anthocyanins, that resulted positively correlated with total soluble solids, and polyphenols and total nitrogen observed in 20% and above all 40% cluster-thinned vines assumes great importance for producing high quality red wine, above all when it is destined to aging.