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A Comparison of the Modes of Action of Three Benzimidazoles. Janice Bartels- Schooley, Department of Botany, University of Guelph, Guelph, Ontario, Canada; Blair H. MacNeill, Department of Botany, University of Guelph, Guelph, Ontario, Canada. Phytopathology 61:816-819. Accepted for publication 12 February 1971. DOI: 10.1094/Phyto-61-816.

ed50 values representing the toxicity to Fusarium oxysporum f. sp. melonis of benomyl [methyl-1-(butylcarbamoyl)-2-benzimidazolecarbamate], thiabendazole [2-(4-thiazolyl)-benzimidazole], and furidazole [2-2(furyl)-benzimidazole] were in the approximate ratio of 1:4:13. Toxicity of all three compounds was enhanced as the pH was raised to neutrality or above. Under the selection pressure of benomyl, naturally occurring benomyl-tolerant mutants occurred at the rate of 1 in 8.6 × 107 spores screened. In ultraviolet-irradiated populations, tolerance to benomyl occurred at the rate of 1 in 4.6 × 105 spores surviving irradiation. Benomyl- and thiabendazole-tolerant mutants exhibited cross-tolerance with all three benzimidazoles, but the furidazole-tolerant mutant was as sensitive as the wild-type Fusarium to benomyl and thiabendazole. Reduction in fungitoxicity of the benzimidazoles occurred in the presence of purines and certain other compounds involved in nucleic acid synthesis. On the basis of the demonstration of cross-tolerance among benzimidazole mutants, and from evidence that the fungicides act as antimetabolites, it is concluded that furidazole exhibits a mode of action that is common to all three compounds, but benomyl and thiabendazole have an additional mechanism.

Additional keywords: fungicide tolerance.