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Tolerance of Verticillium dahliae to Benzimidazoles. J. B. McHugh, Former Research Associate, Department of Plant Pathology, Ohio State University, Columbus 43210. L. R. Schreiber, Supervisory Plant Pathologist, USDA, ARS, Nursery Crops Research Laboratory, Delaware, OH 43015. Plant Dis. 68:424-427. Accepted for publication 9 December 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/PD-68-424.

Microsclerotial and album isolates of Verticillium dahliae and dark mycelial isolates of V. alboatrum were screened for tolerance to MBC phosphate (MBC-P), thiabendazole, thiophanate-methyl (TM), MBC-HCl, and thiabendazole hydrochloride at 1, 5, 10, 100, and 500 μg/ml. Album isolates grew at all concentrations, whereas microsclerotial and dark mycelial isolates were inhibited by all benzimidazoles except TM at concentrations higher than 1 μg/ml. Sectors from album isolates grew on 1,000 μg/ml of MBC-P. A subculture from a microsclerotial isolate grew at 500 μg/ml MBC-P. Chemical tolerance remained unchanged after 40 wk of storage at 2 C or after 13 serial subcultures onto unamended potato-dextrose agar. Pathogenicity studies with chemically sensitive and tolerant microsclerotial isolates and tolerant album isolates from elm and maple indicated that tolerant and sensitive microsclerotial isolates were more pathogenic than album isolates on both American elm (Ulmus americana) and red maple (Acer rubrum).