Occurrence of resistance in Uncinula necator to triadimefon, myclobutanil, and fenarimol in California grapevines

Uncinula necator subcultures from 19 vineyards in four regions in California 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 were found in the Central Coast region, and frequency distributions were skewed most toward higher resistance to all three fungicides. Sub-cultures with high resistance levels also were present in the other regions examined. A time course study performed in one vineyard, where resistant strains were reported, demonstrated a steady and significant increase in EC(50) values for all three fungicides during the growing season 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.