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<i>Erysiphe necator</i>, the causal agent of grapevine powdery mildew, overwinters as chasmothecia in many grapevine growing regions. In the following spring, chasmothecia discharge ascospores that cause primary infections and trigger powdery mildew epidemics. In organic viticulture, only sulphur and biocontrol agents (BCAs) are available to control this disease, and the efficacy of these products must be maximized to obtain acceptable control. The possibility of including sanitation (via application of a BCA against the overwintering chasmothecia) in powdery mildew control was evaluated over a 4-year period in experimental vineyards. A biocontrol product “AQ10” that contained <i>Ampelomyces quisqualis</i> and that was applied twice (before and after harvest, i.e., during the formation and maturation of the chasmothecia), halved disease severity on bunches until the pea-sized berries stage in the following season. When sanitation with <i>A. quisqualis</i> was coupled with early season (i.e., between bud break and fruit set) sulphur applications scheduled according to the output of a mathematical model predicting ascospore infection based on weather data, disease severity on bunches was reduced by 98% (vs. an 80% reduction with sulphur sprays alone). In conclusion, the use of sanitation with <i>A. quisqualis</i> and a weather-driven model for ascospore infection can increase the efficacy of sulphur for control of powdery mildew in organic viticulture. <p><p>Keywords: Fungus, Fruits-Nuts, Grape