Leaf removal and cluster thinning efficiencies are highly modulated by environmental conditions in cool climate viticulture

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