Sensitivity of cold-climate wine grape cultivars to copper, sulfur, and difenoconazole fungicides

The development of wine grape cultivars that can withstand temperatures as low as −40 °C, hereafter referred to as cold-climate cultivars, has been critical to the establishment and growth of the wine industry in the northern USA. While some grape cultivars are susceptible to leaf injury following application of copper, sulfur, and difenoconazole fungicides, the sensitivity of most cold-climate cultivars to these fungicides is not known. In field trials conducted over four years at two locations in Wisconsin, USA, we found that most of the 15 cold-climate cultivars evaluated were not highly sensitive to copper, sulfur, or difenoconazole, although there were important exceptions. Sensitivity was expressed in relative terms, with comparisons made among the cultivars tested, and more weight given when injury was observed after a small number of applications. Regarding copper: Brianna was highly sensitive, showing injury in seven of 11 trials, sometimes after three or fewer applications; Léon Millot, and Maréchal Foch were moderately sensitive, each showing injury in three of six trials; and Frontenac, Frontenac gris, La Crescent, Marquette, and St. Croix were slightly sensitive, each showing injury in one or two trials. Regarding sulfur: Brianna, Léon Millot, and Maréchal Foch were highly sensitive, each showing injury in three trials, sometimes after three or fewer applications; and La Crescent and St. Croix were slightly sensitive, each showing injury in one trial. With the exception of Noiret, which showed injury in one trial, none of the cultivars was sensitive to difenoconazole. It should be possible for growers to integrate these fungicides into disease management programs that will control important diseases of wine grape and delay the emergence of pathogens resistant to major classes of synthetic fungicides.