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DCNA-Benomyl Multiple Tolerance in Strains of Botrytis cinerea. G. A. Chastagner, Postgraduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Present address: Washington State University, Western Washington Research and Extension Center, Puyallup, WA 98371; J. M. Ogawa, professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 69:699-702. Accepted for publication 29 December 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-699.

DCNA (2,6-dichloro-4-nitroaniline)-benomyl multiple-tolerant strains of Botrytis cinerea were obtained by plating conidia derived from benomyl-tolerant isolates of B. cinerea onto potato-dextrose agar (PDA) plates amended with 100 μg of DCNA per milliliter. Colonies of the DCNA-benomyl multiple-tolerant strains grown on PDA were symmetrical with unusually tough mycelial mats. Growth rates of these multiple-tolerant strains on PDA were about half that of the benomyl-tolerant isolates. The ED₅₀ for reduction of mycelial growth as measured by colony diameter ranged from 0.8 to 1.4 μg of DCNA per milliliter for the isolates tolerant to benomyl only but was 3.9 to 88.8 μg of DCNA per milliliter for the strains tolerant to both fungicides. The multiple-tolerant strains and benomyl-tolerant strains both were able to grow at 1,000 μg of benomyl per milliliter. Strains tolerant to both fungicides consistently produced fewer conidia than the isolates tolerant only to benomyl. Conidial germination of multiple-tolerant strains on PDA amended with either 10 μg of DCNA or 10 μg of DCNA plus 10 μg of benomyl per milliliter ranged from 48.7 to 92.3% after 12 hr at 20 ± 1 C. Germ tubes of multiple-tolerant strains, after 12 hr at 20 ± 1 C, on PDA amended with either 10 μg of DCNA or 10 μg of DCNA plus 10 μg benomyl per milliliter were less than half the length of those produced on PDA alone or PDA amended with 10 μg of benomyl per milliliter. All multiple-tolerant strains tested produced sclerotia on PDA and decayed DCNA or DCNA plus benomyl-treated peach fruit. Multiple-tolerant strains were pathogenic on tomato stems, but the lengths of stem colonized were less than that of the parent benomyl-tolerant strains.