P-296 - Assessing QoI Resistance of Plasmopara viticola populations from Wisconsin vineyards

Fungicides are a critical tool for preventing and mitigating fungal epidemics in vineyards. The humid summers in Wisconsin serve as an ideal environment for Plasmopara viticola, the causal agent of downy mildew. Without management, P. viticola infections cause economic damage to fruit. Widely used in Wisconsin vineyards, quinone outside inhibitors (QoI) fungicide are classified as high-risk for resistance due to their single-site mode of action. The G143A mutation, caused by a single nucleotide polymorphism (SNP) at amino acid position 143, drives complete resistance, highlighting the fragility of fungicide-based disease management as evidenced by widespread P. viticola QoI resistance. The objectives of this study were to determine the current state of QoI resistance in Wisconsin vineyards and correlate mutant population detection with vineyard traits like neighboring fields and seasonal fungicide spray frequency. In 2024, samples of P. viticola were collected using Tough-Spots from seventeen vineyards across ten counties in Wisconsin (N = 134). Primers of the cytochrome b (cytb) target gene were used for amplification for SNP detection in pooled vineyard-cultivar samples. Eight samples (6%) were confirmed to have the sensitive SNP, while twenty-five samples (18.6%) contained the resistant SNP. Mutant detection was observed primarily in vineyards that rely solely on synthetic fungicides for disease management. Understanding the prevalence of QoI resistance of P. viticola populations is imperative for the refinement of management programs and preserving these chemistries.