Benomyl-Resistant Strains of Uncinula necator on Grapes. R. C. Pearson, Assistant Professor, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456. E. F. Taschenberg, Professor, Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Fredonia, NY 14603. Plant Dis. 64:677-680. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-677.

Benomyl sprays provided 97–100% control of grape powdery mildew caused by Uncinula necator in western New York during 1973-1976 but was much less effective in some vineyards in 1977 and 1978. Isolates of U. necator were collected in 1978 and 1979 and maintained on potted grapevines in biotrons or on detached grape leaves in plastic boxes. Resistance was determined by dipping leaves in a benomyl suspension and inoculating them with a spore suspension of the test isolate. After 7, 14, and 21 days, the amount of mildew on benomyl-treated leaves was not significantly different from that on untreated leaves inoculated with U. necator isolates from two western New York vineyards and one central New York vineyard. In a field trial in one of the western New York vineyards, four sprays of Benlate 50W at 1.12 kg/ha provided only 5 and 8% control of mildew on foliage and fruit clusters, respectively, in 1979. Field trials indicated that control of benomyl-resistant strains of U. necator with combinations of benomyl and one of several other fungicides, each at half the recommended rate, depended on the efficacy of the nonbenzimidazole component, but the half rate of benomyl alone was adequate to control benomyl-sensitive strains.

Keyword(s): Vitis spp., fungicide resistance.