Resistance of Coccomyces hiemalis to Benzimidazole Fungicides. A. L. Jones, Professor, Pesticide Research Center and Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. G. R. Ehret, Research Technician, Pesticide Research Center and Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 64:767-769. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-767.

Benomyl-resistant strains of the cherry leaf spot fungus (Coccomyces hiemalis) were detected in a sour cherry orchard in 1975, in another in 1976, and in three sweet and four sour cherry orchards and a fruit tree nursery in 1979. Sensitive isolates were strongly inhibited in a disk assay at 1.0 µg/ml benomyl; resistant strains were weakly inhibited at 800–1,000 µg/ml. Resistant strains developed in orchards where benomyl had been used exclusively for 3–5 yr and where mixtures of benomyl and nonsystemic fungicides had been applied after 1975, the year a resistant strain was first detected. The isolates were also resistant to carbendazim and thiophanate-methyl.

Keyword(s): Prunus avium, P. cerasus.