ABSTRACT
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.
