Resistance to Benzimidazole Fungicides in the Cereal Eyespot Pathogen, Pseudocercosporella herpotrichoides, in the Pacific Northwest 1984 to 1990

Isolates of Pseudocercosporella herpotrichoides resistant to benzimidazole fungicides were detected in commercial winter wheat fields in the Pacific Northwest region (Washington, Oregon, and Idaho) of the United States for the first time in the spring of 1989. Benzimidazole-resistant isolates were found in nine of 62 fields sampled in 1989 and in 17 of 167 fields sampled in 1990, which represents 24 and 19% of those fields yielding the pathogen, respectively. In 1989 and 1990, respectively, 96 and 70% of all isolates collected from fields where fungicide resistance was detected were resistant to the benzimidazole fungicides. All fields where fungicide-resistant strains of the eyespot fungus were found had at least four previous applications of a benzimidazole fungicide. In 1989 and 1990, respectively, 24 and 15% of the P herpotrichoides cultures collected had a slow growth rate with feathery colony margins on potato-dextrose agar and corresponded to rye-type isolates. However, benzimidazole-resistant rye-type isolates represented only 7 and 4% of the total resistant isolates collected in 1989 and 1990, respectively.