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Efficacy of New Benzimidazole Fungicides Against Sensitive and Benomyl-Resistant Botrytis cinerea. Mikio Chiba, Research Station, Agriculture Canada, Vineland Station, Ontario, Canada L0R 2E0; John Northover, Research Station, Agriculture Canada, Vineland Station, Ontario, Canada L0R 2E0. Phytopathology 78:613-618. Accepted for publication 30 November 1987. Copyright 1988 Department of Agriculture. Government of Canada. DOI: 10.1094/Phyto-78-613.

Four alkyl isocyanate homologues of benomyl were synthesized and compared for their toxicities towards benomyl-sensitive (S) and benomyl-resistant (R) isolates of Botrytis cinerea using a spore germination test. For the inhibition of germ tube length, methyl, ethyl, and propyl isocyanate homologues of benomyl (MBC-MIC, MBC-EIC, and MBC-PIC, respectively) were as effective against the S isolate as benomyl (MBC-BIC), but they were more effective than MBC-BIC against the R isolate. The hexyl isocyanate homologue of benomyl (MBC-HIC) was less effective than MBC-MIC, MBC-EIC, and MBC-PIC against both isolates. MBC-MIC was equally effective in inhibiting spore germination of both the S and R isolates and was more effective than the other homologues against the S isolate. The sensitivity of spore germination of the S and R isolates to MBC-EIC and MBC-PIC was the reverse of their sensitivity to benomyl, constituting an example of negative cross resistance. For the protection of wounded apples, MBC-EIC, MBC, and benomyl were compared. MBC-EIC was more effective than benomyl and MBC against the R isolate but was slightly less effective than benomyl and MBC against the S isolate. Accordingly, MBC-EIC and possibly MBC-MIC are benzimidazole compounds with potential for use against fungal strains both sensitive and resistant to benomyl.