Occurrence of Resistance in Uncinula necatorto Triadimefon, Myclobutanil, and Fenarimol in California Grapevines. W. D. Gubler, Department of Plant Pathology, University of California, Davis 95616. H. L. Ypema, Department of Plant Pathology, University of California, Davis 95616; D. G. Ouimette, Dow Elanco, 9330 Zionsville Rd., Indianapolis, Indiana; and L. J. Bettiga, University of California Cooperative Extension, Salinas 93901. Plant Dis. 80:902-909. Accepted for publication 9 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0902.

Uncinula necator subcultures from 19 vineyards in four regions in California were analyzed for sensitivity to triadimefon, myclobutanil, and fenarimol. The means of EC50 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 EC50 values were found in the Central Coast region, and frequency distributions were skewed most toward higher resistance to all three fungicides. Subcultures 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 EC50 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 after the growing season.