Grape powdery mildew (GPM, causal agent Erysiphe necator) is the most economically important disease of grapevine in the Western U.S. Low levels of GPM infection on clusters (1-5%) can negatively affect wine sensory quality, so intensive fungicide programs are common. This may lead to problems like excessive fungicide use and buildup of resistant E. necator strains. Fungicide redistribution could improve fungicide protection of grape clusters, but has not been investigated for many products. Many fungicides effectively control GPM cluster infections, but optimal timing regimes for specific products remain poorly characterized. Increased knowledge of fungicide redistribution and application timing may alleviate the need for narrow application windows during bloom. Bioassays determined that all fungicides tested could redistribute by some mechanism, with xylem and translaminar mechanisms being common. Some fungicides performed as expected, while others redistributed at a lower level or via mechanisms not previously demonstrated, such as fluopyram redistributing via vapors. Vapor redistribution appeared to protect clusters in the field and three fungicides could translocate through flower caps. In small plot trials, most fungicide treatments timed to the end of bloom provided significantly better GPM control on clusters than non-treated and wettable sulfur control treatments (P<0.01). The application of two sequential treatments with a 14-day interval of either trifloxystrobin, quinoxyfen, or fluopyram at different bloom stages showed that applications initiated at end of bloom resulted in the lowest respective berry infection probabilities of 0.073, 0.097, and 0.020. Rotations between quinoxyfen and fluopyram initiated at end of bloom resulted in berry infection probabilities of 0.059 when quinoxyfen was applied first, and 0.076 when fluopyram was applied first. After industry reported GPM control failures when using quinone outside inhibitor (QoI) products, a survey was conducted to determine if QoI resistant strains were present in Oregon viticultural regions. Survey results using a competitive TaqMan qPCR assay showed that 62% of field samples had populations of E. necator carrying the G143A mutation, and it was present in all three regions sampled. The qPCR assay determined that 87% of isolates contained the G143A mutation and phenotypic characterization validated that isolates containing G143A were resistant and could withstand high concentrations of two QoI products (EC50 > 100μg/ml).
This research showed that an improved understanding of fungicide redistribution could improve disease management recommendations, especially if crop phenology is also considered. Integrating two carefully timed applications of redistributing fungicides initiated at end of bloom into a fungicide program may be an effective strategy for wine grape growers in western Oregon seeking to produce fruit with low GPM infection levels and minimum synthetic chemical input. This strategy would have the added benefit of reducing selection for fungicide resistant strains of E. necator.