The efficacy of myclobutanil, triadimenol, and propiconazole was significantly reduced when recommended rates were mixed with cupric hydroxide and applied to control powdery mildew (Uncinula necator) on grape. In 6 field experiments conducted over 2 growing seasons and on 4 grape cultivars, the incidence and severity of powdery mildew on bunches increased following addition of copper to the sterol demethylation inhibiting fungicides. On Rhine Riesling, 4 applications of either 31.25 mg myclobutanil/L or 25 mg triadimenol/L controlled powdery mildew, whereas, 1.25 g cupric hydroxide/L with these fungicides resulted in 53 and 62% of bunches infected, respectively.