Reduced control of powdery mildew (Uncinula necator Schw.) when sterol demethylation inhibiting fungicides are mixed with cupric hydroxide

Abstract

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.