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The grape powdery mildew conundrum: fungicide selection and timing
Brent Warneke: Oregon State University; Lindsey Thiessen: North Carolina State University; Tara Neill: USDA; Walter Mahaffee: USDA
<div>Grape powdery mildew (GPM, causal agent <i>Erysiphe necator</i>) is the most economically important disease of grapevine in the Western U.S. The inflorescence and early infructescence growth stages are highly susceptible to GPM and its management during these stages is critical to preventing economic losses. Targeting these stages with mobile fungicides could significantly reduce fruit infections. A small-plot experiment was conducted in Corvallis, Oregon during the 2015 and 2016 growing seasons to examine the interaction between fungicide chemistry and application timing on GPM berry infection. Five commonly used fungicides with varying degrees of mobility were applied at three different growth stages around bloom. The controls consisted of a non-treated and sulfur, each applied every 14 days. The proportion of grape berries infected with GPM was significantly influenced by the interaction between fungicide selection and application timing (Drop-in-deviance test, χ<sup>2</sup> = 33.1, df = 8, P<0.01). All treatments significantly reduced fruit infection compared to the non-treated control (Z test, P<0.01). The trifloxystrobin, quinoxyfen, and fluopyram applications made at berry set were most effective at reducing the proportion of berries infected, having 4.8, 8.0, and 11.5 times lower odds of berry infection, respectively, compared to the earliest timing (Z test, P<0.01). This research indicates that using mobile fungicides during fruit development may improve disease control.</div>

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