TY - JOUR T1 - Release of <em>Erysiphe necator</em> Ascospores and Impact of Early Season Disease Pressure on <em>Vitis vinifera</em> Fruit Infection JF - American Journal of Enology and Viticulture JO - Am. J. Enol. Vitic. DO - 10.5344/ajev.2014.13111 SP - ajev.2014.13111 AU - Michelle M. Moyer AU - David M. Gadoury AU - Wayne F. Wilcox AU - Robert C. Seem Y1 - 2014/04/11 UR - http://www.ajevonline.org/content/early/2014/04/09/ajev.2014.13111.abstract N2 - Populations of Erysiphe necator cleistothecia can dehisce and release ascospores over an extended period ranging from fall through late spring, and are an important source of primary inoculum for grapevine powdery mildew epidemics. In this study, we monitored the temporal distribution of ascospore maturity, measured as ascospore release from field-stored samples in a controlled-environment assay. Assays were conducted over multiple seasons, in multiple locations, using multiple source populations. We assessed the impacts of primary inoculum dose (quantity), and the impact on subsequent disease severity of fruit in a research vineyard. Cumulative ascospore release was positively correlated with accumulated wetting events and heat units; however, in most situations, more than 50% of the season-total ascospores were released prior to the date of local budbreak of Vitis vinifera. An abbreviated fungicide program suppressed mildew on fruit as well as theseason -long program, across a 100-fold gradient of inoculum dose when seasonal weather was relatively unfavorable for epidemic development; but suppression was degraded with progressive 10-fold increases of ascosporic inoculum dose when seasonal weather was more conducive to epidemic development. Combined, these findings suggest that differences between severe and mild years in grape powdery mildew can relate to the amount of primary inoculum present in the vineyard, that the levels of primary inoculum can be influenced by pre-budbreak weather conditions, and that effectiveness of spray programs at controlling primary infection events is related to the favorability of in-season weather conditions. ER -