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Biology of Benomyl-Tolerant Strains of Cercospora beticola from Sugar Beet. E. G. Ruppel, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Crops Research Laboratory, Colorado State University, Fort Collins 80523; Phytopathology 65:785-789. Accepted for publication 20 February 1975. DOI: 10.1094/Phyto-65-785.

Benomyl-tolerant strains of Cercospora beticola as a group were no different from benomyl-sensitive strains in growth and sporulation in vitro, or in virulence and sporulation in vivo. Strains differed in their degree of tolerance, the tolerance level being stable for at least 1 year in vitro. Tolerance of one strain was unchanged after three passages through sugar beet. In mixed inoculations of sugar beet with a sensitive strain, a tolerant strain population declined, but never disappeared. Degree of cross-tolerance as measured by linear growth of three tolerant strains tested against benomyl, thiabendazole, and thiophanate depended on isolate, fungicide, and fungicide concentration. Conidial germination of a tolerant and a sensitive strain tested against three benzimidazole derivatives and six protectant fungicides showed that the sensitive strain generally was more sensitive to all the fungicides at concentrations of 10 µg or more per ml, and the protectants were somewhat more inhibitory to both strains than were the systemics. Benomyl was more toxic to conidia than to hyphae of a tolerant strain, whereas the reverse was true for thiabendazole. Ultraviolet irradiation of conidia from a sensitive strain failed to induce benomyl-tolerant mutants.