Fungicide efficacy for <i>Plasmopara viticola</i> control and development of quinone outside inhibitor resistance in juice grapes

Grape downy mildew, caused by the oomycete pathogen Plasmopara viticola, is one of the most destructive diseases of cultivated grapes worldwide. While fungicide efficacy and fungicide resistance has been documented in P. viticola in wine grapes (i.e. Vitis vinifera and Vitis interspecific hybrids), limited information exists for juice grapes (i.e. Vitis labrusca). Small plot trials were conducted in 2021 and 2022 in a mature ‘Niagara’ vineyard in Fennville, MI, to evaluate fungicide efficacy and monitor resistance development. Disease pressure differed markedly between years: the untreated control (UTC) reached 34% incidence and 26% severity in 2021 versus 90% incidence and 57% severity in 2022. Across both years, mandipropamid (FRAC 40), cyazofamid (FRAC 21), and fenamidone (FRAC 11) provided the most consistent control, reducing incidence and severity AUDPC values by &gt;70% compared with the UTC (2149.5 for incidence, 807.4 for severity). Molecular analysis of 24 P. viticola isolates collected in 2022 confirmed all belonged to P. viticola clade aestivalis. The G1105S mutation associated with resistance to carboxylic acid amides was not detected but the G143A mutation conferring resistance to quinone outside inhibitor fungicides was present in 12 isolates. These findings provide fungicide efficacy data for juice grape downy mildew management and highlights the need for continued resistance monitoring to sustain long-term control.