Disease Note

Tolerance in Pseudocercosporella herpotrichoides to Benzimidazole Fungicides in Washington. G. W. Bruehl, Department of Plant Pathology, Washington State University, Pullman 99164. R. Machtmes, and T. Murray, Department of Plant Pathology, Washington State University, Pullman 99164. Plant Dis. 69:360. Accepted for publication 9 December 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-360e.

Foot rot (Pseudocercosporella herpotrichoides (Fron) Deighton) lesions were found on a few stems of winter wheat (Triticum aestivum L.) heavily sprayed with benomyl (Benlate) in experimental plots in Pullman, Washington. Single conidial isolates from four of these stems grew on potato-dextrose agar containing 3 μg a.i./ml of benomyl (Benlate), thiabendazole (Mertect 340-F), or thiophanate-methyl (Topsin M). At this concentration, the fungicides were highly fungistatic but not fungicidal; higher concentrations were not tested. The four isolates were virulent when tested on wheat in the greenhouse. All other isolates tested were inhibited in culture by all three fungicides. Benlate has been extensively used in eastern Washington for about 5 years. We have no reports of fungicide failure in farm fields. No survey of farm fields has been made, but we suspect that isolates tolerant to benzimidazoles exist and that the useful life of these fungicides will be curtailed. Alternative, different mode-of-action fungicides should be vigorously sought. Tolerance to these fungicides is a problem in parts of the British Isles where they have been repeatedly applied to wheat for several years, especially with more than one application per crop.