The recent release of interspecific hybrid grape cultivars that are hardy for the northern climates of the United States has led to a rapid expansion of the grape and wine industry in the Upper Midwest, as well as other cold-climate regions. These cultivars often exhibit high vegetative vigor and possess fruit quality concerns when their grapes are to be used in wine production. Adaptation in the viticultural practices used to produce the grapes may help to improve fruit quality for winemaking, which could promote increased sales and profitability of the grape and wine industry in the Upper Midwest.

This study examines the effectiveness of three canopy management practices to improve irradiance within the fruiting zone of the grapevine canopies of Frontenac, La Crescent, and Marquette grapevines, and their impact on fruit quality for winemaking. The labor required completing the management practices, irradiance, harvest variables, and fruit quality were measured.

All canopy management practices required additional labor and provided increased irradiance into the fruiting zone. However, the increased irradiance did not correspond to improved fruit quality. These effects varied across the different cultivars, and no overall relationship between increased labor and increased irradiance or between increased irradiance and improved fruit quality was established across the cultivars. While Frontenac vines required an average of 5.6 additional minutes of labor per vine for any canopy management practice, the practices increased irradiance by about 10.1%. La Crescent vines required an average of 4.4 additional minutes of labor per vine for any canopy management practice and the practices increased irradiance by 16.5% on average. Marquette vines required an average of 7.7 additional minutes of labor per vine for any of the canopy management practices, which led to a 19.5% averaged increase in solar irradiance. The effects of the increased irradiance on fruit quality showed minor potential for improvement in quality by lowering malic acid content with shoot thinning in La Crescent.