Abstract
The resistance to four fungicides used for the management of Botrytis rot (gray mold) in strawberry was evaluated among 65 isolates of Botrytis cinerea from coastal California. Fungicide resistance was tested by agar diffusion assay on Czapek-Dox agar. Isolates not showing inhibition zones around wells at a discriminatory concentration were considered resistant. Concentrations of active ingredients for detecting resistance were 50 mg a.i./liter for thiophanate-methyl, 10 mg a.i./liter for fenhexamid, 50 mg a.i./liter for cyprodinil and fludioxonil (Switch pre-mix), and 100 mg a.i./liter for boscalid and pyraclostrobin (Pristine pre-mix). Most isolates (92%) in all fields surveyed were resistant to thiophanatemethyl. Resistance to the more recent products was also widespread and occurred in most fields. Overall, incidence of resistance among isolates was 25% for fenhexamid, 28% for cyprodinil/fludioxonil, and 66% for bocalid/pyraclostrobin. Resistant isolates remained uninhibited by higher concentrations, suggesting that they had become insensitive to these products when re-tested at twice and four times the discriminatory concentration. Among isolates of B. cinerea,85% were resistant to at least two products. This widespread occurrence of resistance to single site mode of action fungicides suggests that their effectiveness to control Botrytis rot might become impaired. Accepted for publication 21 June 2010. Published 6 August 2010.
Published Version
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