Ceratocystis ulmi Tolerance to Methyl-2-Benzimidazole Carbamate and Other Related Fungicides

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Abstract
An isolate of C. ulmi tolerant to methyl-2-benzimidazole carbamate (MBC) concentrations as high as 2000 .mu.g/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 [thiabendazole] 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 and B 21914C [1-methoxycarbonyl-3-(2 dimethylaminoacetamidophenyl) thiourea hydrochloride] 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.

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