VIEW ARTICLE

Disease Control and Pest Management

Ceratocystis ulmi Tolerance to Methyl-2-Benzimidazole Carbamate and Other Related Fungicides. W. T. Nishijima, Former graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706, Present address of senior author: Department of Botany and Plant Pathology, Colorado State University, Ft. Collins, CO 80523; E. B. Smalley, professor, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706. Phytopathology 69:69-73. Accepted for publication 8 July 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-69.

An isolate of Ceratocystis ulmi which was tolerant to methyl-2-benzimidazole carbamate (MBC) concentrations as high as 2,000 μg/ml, but sensitive to lower concentrations of several other benzimidazole fungicides was discovered in a routine bioassay plate. The inheritance of this high level of MBC tolerance was controlled by a single gene. American elm (Ulmus americana) seedlings inoculated in the greenhouse with the MBC-tolerant isolate developed typical Dutch elm disease (DED) symptoms, but of a greater intensity than that produced with an MBC-sensitive wild-type isolate. Fungicide-treated greenhouse-grown elm seedlings also developed typical DED symptoms when inoculated with the MBC-tolerant isolate. Comparisons of 205 C. ulmi isolates from untreated elms and elms treated with various benzimidazole or thiourea-type fungicides failed to indicate increases in the frequency of MBC or TBZ tolerance as a result of the treatment. The results of inoculation and reisolation studies with greenhouse-grown American elm seedlings pretreated with benomyl, MBC-H₃PO₄, M&B 21914C, or TBZ-H₂PO₂ suggested that benomyl, but not the other compounds tested, induced a significant level of tolerance in a wild-type C. ulmi isolate.