Sensitivity of Uncinula necator to Benomyl, Triadimefon, Myclobutanil, and Fenarimol in California

Sensitivity of Uncinula necator subcultures to benomyl and the demethylation-inhibiting (DMI) fungicides triadimefon, myclobutanil, and fenarimol was assessed in 1993, 1994, and 1995 with leaf disk bioassays. In 1993, 1994, and 1995, 81.8, 96, and 96.7% of the subcultures, respectively, did not grow on leaf disks treated with 30 mg of benomyl per liter, whereas growth of the remaining subcultures was inhibited by more than 90%. Median EC₅₀ values of triadimefon, myclobutanil, and fenarimol decreased from 1993 to 1994, and those of triadimefon decreased again from 1994 to 1995. In the same period, median EC₅₀ values of all three DMI fungicides increased in a vineyard never exposed to DMI fungicides. The highest means and ranges of EC₅₀ values found were those of triadimefon. Means and ranges were lower for myclobutanil and lowest for fenarimol, reflecting differences in inherent activities of the fungicides and po-tential for development of resistance. Pairwise correlations between EC₅₀ values of each DMI fungicide were positive and confirmed earlier indications of cross resistance.