Evaluation of Reduced Fungicide Applications for Disease Management in Cold-Climate Wine Grapes in Wisconsin

Cold-climate wine grapes are produced on 8,000 ha in the North Central region of the United States. Wisconsin has experienced considerable growth, with a 26% increase in acerage since 2017. Chemical management of fungal diseases in cold-climate, interspecific hybrid grapes mirrors that of traditional Vitis vinifera cultivars despite significant differences in disease susceptibility. Most cold-climate cultivars display disease tolerance or resistance to key pathogens such as Plasmopara viticola (downy mildew), Erysiphe nectar (powdery mildew), and Phyllosticta ampelicida (black rot). Current fungicide programs in Wisconsin's cold-climate grape industry underutilize genetic resistance, resulting in overreliance on at-risk fungicides and an increased threat of fungicide resistance development. In vineyard trials, the impacts of a reduced fungicide application number compared with current grower standard spray programs were assessed for disease incidence and severity for five diseases: anthracnose, black rot, downy mildew, Phomopsis cane and leaf spot, and powdery mildew. In 2022, with moderate disease pressure at both vineyard sites, there were no significant differences observed when fewer fungicides (six or five applications versus four applications) were applied. In 2023, higher disease incidence was observed in the "Standard" spray program at one study location that received a greater number of fungicide applications. In both years, grape cultivar was a significant factor, with the cv. LaCrosse displaying greater average disease severity than cv. St. Pepin in both the "Standard" and "Reduced" programs. These findings present a promising opportunity for cold-climate grape growers to reduce the number of fungicide applications while maintaining disease control and marketable yield.