Characterization of fungicide resistance in <i>Plasmopara viticola</i> isolates from Maryland and Pennsylvania vineyards

Plasmopara viticola, the causal organism of grapevine downy mildew, poses a significant threat to Eastern US grape production. In this study, we determined fungicide resistance profiles of vineyards across Maryland and Pennsylvania between 2019 and 2023. A total of 352 downy mildew samples were collected from 27 vineyards and 32 different cultivars and tested for resistance to various fungicides commonly used for downy mildew management at field application rates using whole leaf bioassays. DNA was also extracted and used to confirm known resistance mechanisms to azoxystrobin (G143A) and mandipropamid (G1105S) using Sanger sequencing and PCR-RFLP. Resistance to azoxystrobin, mandipropamid, and phosphorous acids was found throughout all regions tested, with bioassays resulting in 69%, 39%, and 33% of isolates displaying resistance to respective chemicals. In addition, all isolates subsampled and sequenced displayed the G143A mutation regardless of resistance phenotype, indicating bioassays could underestimate the frequency of resistance. Of the 52 isolates tested using PCR-RFLP to confirm the presence of the G1105S mutation, 46 displayed genotypes consistent with their observed phenotypes. This is the first report of resistance to phosphorous acid in North America, which was found to be widespread throughout regions tested and detected as early as 2020. Multi-chemical class resistance was also commonly detected, with 20% of isolates displaying resistance to all three chemicals simultaneously. This study highlights the escalating threat posed by P. viticola to grape production in the Northeast, as rising populations resistant to multiple chemical classes considerably diminish the fungicide options available to growers.