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Rapid sampling techniques to determine QoI fungicide resistance in Erysiphe necator

Sarah Lowder: Oregon State University

<div>Control failures of grape powdery mildew (GPM) after the use of quinone outside inhibitor (QoI) fungicides (FRAC Group 11) have been reported by vineyard managers throughout OR, WA, and CA. Quick, cost-effective methods to detect fungicide resistance in <em>Erysiphe necator</em> are necessary to reduce the selection for resistant alleles and mitigate ineffective fungicide applications. <em>E. necator</em> samples were collected via linear stratified sampling techniques by placing a ToughSpot adhesive dot directly onto a mildew colony (n=119) or by wiping gloves with sterile cotton swabs (n=65) after simulated vine training. A competitive TaqMan qPCR assay, that was previously demonstrated to be in 100% agreement with germination assays, was used to detect the G143A mutation in the <em>cytb</em> gene conferring QoI resistance. Both methods detect similar distributions (of samples with resistant, sensitive and mixed genotypes: ToughSpots: 48.7% resistant, 43.7% sensitive, 7.6% mixed; Swabs: 59.2.0% resistant, 24.5% sensitive, 16.3% mixed), indicating that either could be a viable, rapid sampling technique with minimal interruption to grower production schedules. A rapid sampling technique for chasmothecia (syn. cleistothecia) overwintering in grape bark is under development to provide managers with an assessment of the risk of fungicide resistance in the next season. The resistant genotype can be determined from a single chasmothecium in 2 g of dry bark.</div>