Reduced sensitivity of Uncinula necator to sterol demethylation inhibiting fungicides in southern Australian vineyards

Isolates of Uncinula necator with reduced sensitivity to triadimenol and, to a lesser extent, fenarimol have been detected in Australian vineyards. Grapevine leaves and berries affected by powdery mildew were collected from a home garden and 13 vineyards in nine viticultural regions in Australia between 1993 and 1998. Single-spore isolates of U. necator established from vines, either not exposed to DMIs ('unexposed' collection) or treated with DMIs ('selected' collection), were maintained on micropropagated grapevines in vitro. A bioassay for fungicide sensitivity was used to test 60 single-spore isolates of U. necator for sensitivity to triadimenol. Of these, 34 were tested for sensitivity to fenarimol. Mean EC50 values for the 12 unexposed isolates were 0.07 and 0.08 mg/L for triadimenol and fenarimol, respectively. For the selected isolates, mean EC50 values were 0.83 mg/L for triadimenol and 0.19 mg/L for fenarimol. In comparison with the unexposed population, there appeared to have been a shift in the selected population towards reduced sensitivity. Examination of more isolates from vineyards not exposed to DMIs may strengthen this conclusion. Cut-off EC50 values, used to define individual isolates as having reduced sensitivity, were 0.42 mg/L (resistance factor, RF = 6) for triadimenol and 0.12 mg/L (RF = 1.5) for fenarimol. The findings were consistent with the existence of cross-resistance between triadimenol and fenarimol.