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Release of Erysiphe necator Ascospores and Impact of Early Season Disease Pressure on Vitis vinifera Fruit Infection

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. The temporal distribution of ascospore maturity was monitored and 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. The influence of primary inoculum quantity and how that influences subsequent fruit disease severity was assessed. Cumulative ascospore release was positively correlated with accumulated wetting events and heat units; however, in most situations, >50% of the season-total ascospores were released before the date of local budbreak of Vitis vinifera. Both abbreviated and season-long fungicide programs suppressed mildew on fruit across a 100-fold gradient of inoculum dose when seasonal weather was relatively unfavorable for epidemic development. 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.