Effect of Severity of Leaf and Crop Removal on Grape and Wine Composition of Merlot Vines in Hawke’s Bay Vineyards

Abstract

Manipulation of vine balance is widely practiced to enhance grape and wine quality. Reported benefits from crop thinning are ambiguous. Studies were undertaken over two growing seasons (2006–2008) in Hawke’s Bay, a cool-climate region of New Zealand, to investigate the influence of basal leaf and crop removal on fruit and wine composition and the indices of vine balance associated with optimum quality. Three levels of basal leaf removal in the fruiting zone were used. Crop removal achieved average reductions of 15% with moderate treatment and 35% with severe treatment below the average nonthinned yield of 17.5 t/ha. Treatments were undertaken at the preveraison growth stage on Merlot vines. Leaf removal had no effect on fruit composition, but levels of total anthocyanins and the flavonol quercetin-3-glucoside were enhanced in the wines. In contrast, crop removal increased sugar concentration and decreased titratable acidity of the must. While crop removal had no effect on wine anthocyanins, the proportions of anthocyanins as malvidin-3-glucosides and total phenolics in the wines were significantly increased. Relative to accepted indices of vine balance, the study vines were unbalanced with excessive shoot growth and dense canopies. A crop reduction of ~6.0 t/ha brought the capacity of the exposed leaf area into balance with that required to ripen fruit to optimum maturity and produce high-quality wines.