Fungicide-resistant phenotypes in Botrytis cinerea populations and their impact on control of gray mold on stored table grapes in California

Gray mold caused by Botrytis cinerea is the major postharvest disease in table grapes grown in the Central Valley of California. Preharvest use of fungicide sprays may provide an alternative to the control of postharvest gray mold. However, fungicide resistance in B. cinerea can result in the failure of disease control. In this study, 212 isolates of B. cinerea were collected from table grape vineyards in three table grape-producing counties in the region and tested for resistance to selected fungicides on fungicide-amended media. In addition, 80 isolates were tested to establish baseline sensitivity to the newer fungicide fluopyram. Seven fungicide-resistant phenotypes were detected; 85.0%, 23.1%, 13.7%, and 94.8% of the isolates were resistant to boscalid, cyprodinil, fenhexamid, and pyraclostrobin, respectively. All isolates were sensitive to fludioxonil. Only 5.2% of the isolates were sensitive to all fungicides tested, whereas 8.9%, 56.1%, 23.6% and 6.1% were resistant to one, two, three, and four modes-of-action fungicides, respectively. Of the 80 isolates tested, all were sensitive to fluopyram with EC50 values ranging from 0.001 to 0.054g/mL. Most fungicides failed to control gray mold on detached table grapes inoculated with respective fungicide-resistant phenotypes. Our results suggest that alternation of sprays using different classes of fungicides will be needed to control postharvest gray mold, and that fludioxonil and fluopyram could be effective fungicides integrated into a preharvest fungicide spray program for control of gray mold in table grapes in the Central Valley of California.