Leaf Removal and Cluster Thinning Efficiencies Are Highly Modulated by Environmental Conditions in Cool Climate Viticulture

One of the several challenges of cool climate viticulture with a short growing season is consistently reaching a uniform and optimal fruit technological maturity at harvest before the first autumn frost. Weather conditions in Michigan from veraison to harvest are widely variable and unpredictable between years, constraining the pre-harvest assessment of fruit quality for grape growers and wineries. In these environmental conditions, the commonly adopted viticultural techniques to enhance fruit ripening are cluster thinning and leaf removal. Cluster thinning consists of a selective elimination of clusters, therefore optimizing the source/sink ratio of the vine. Cluster zone leaf removal induces changes of fruit micro-environmental conditions, involving solar radiation, temperature and aeration. In this work, we evaluated the effects of cluster thinning and cluster zone leaf removal, applied separately or combined at veraison on Cabernet Franc for two consecutive years (2011 and 2012). The two seasons had very distinct weather patterns from veraison to harvest. Fruit maturity was enhanced at 15–20 days after veraison in both years in response to the viticultural techniques, but with completely different dynamics. At harvest, the combination of leaf removal and cluster thinning led to higher uniformity of fruit and better chemical composition in 2011, a year characterized by low heat accumulation after veraison. Meanwhile in 2012, when heat accumulation and mean temperatures after veraison were higher than 2011, no differences were observed among treatments.