Isolates of Botrytis cinerea from containerized conifer seedlings from two nurseries in the Northern Rocky Mountains were exposed to the dicarboximide fungicides vinclozolin and iprodione and the chlorinated nitroaniline fungicide dicloran to evaluate occurrence and characteristics of resistant strains. The isolates were grown on test media prepared by incorporating the fungicides at various concentrations into potato dextrose agar. In this way, we were able to select B. cinerea strains resistant to increasing fungicide concentrations up to a maximum of 10,000 ug/ml. Resistance to the three fungicides occurred in isolates not previously exposed to the chemicals. Some strains were even resistant to high fungicide concentrations. Genetic stability of these resistant strains was confirmed by growing them in the absence of fungicides. Cross-resistance among the three fungicides was common. Vinclozolin and iprodione generally inhibited spore germination more than mycelial growth; the opposite was true for dicloran. Ramifications of fungicide resistance in disease control are discussed.
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