OCCURRENCE OF RESISTANCE IN UNCINULA NECATOR TO TRIADIMEFON, MYCLOBUTANIL, AND FENARIMOL IN CALIFORNIA GRAPEVINES

Uncinula necator subcultures from 19 vineyards in four regions in Cali fornia were analyzed for sensitivity to triadimefon, myclobutanil, and fenarimol. The means of EC,, values to triadimefon, myclobutanil, and fenarimol of U. necator subcultures from a vineyard without previous exposure to demethylation inhibition (DMI) fungicides were 1.40, 0.15, and 0.13 mg/liter, respectively. The highest means of EC(50) values w ere found in the Central Coast region, and frequency distributions wer e skewed most toward higher resistance to all three fungicides. Sub-cu ltures with high resistance levels also were present in the other regi ons examined. A time course study performed in one vineyard, where res istant strains were reported, demonstrated a steady and significant in crease in EC(50) values for all three fungicides during the growing se ason after multiple applications of triadimefon. Increased resistance to triadimefon, but not to myclobutanil and fenarimol, was maintained in early-formed ascospores released alter the growing season.