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

Authors: Saito, Seiya; MICHAILIDES, THEMIS - Kearney Agricultural Center; Xiao, Chang-Lin

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2018
Publication Date: 1/5/2019
Citation: Saito, S., Michailides, T.J., Xiao, C. 2019. Fungicide-resistant phenotypes in Botrytis cinerea populations and their impact on control of gray mold on stored table grapes in California. European Journal of Plant Pathology. 154(2):203-213. https://doi.org/10.1007/s10658-018-01649-z.
DOI: https://doi.org/10.1007/s10658-018-01649-z

Interpretive Summary: Gray mold caused by the fungus Botrytis cinerea limits the storage and shelf life of table grapes grown in the Central Valley of California. Control of gray mold in table grapes has long been dependent on postharvest fumigation with sulfur dioxide. 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. Determining the frequency and phenotypes of fungicide resistance in the current B. cinerea populations is important to the development of effective chemical control programs. In this study, isolates of B. cinerea were collected from table grape vineyards and tested for resistance to selected fungicides on fungicide-amended media, and baseline sensitivity to the newer fungicide fluopyram was also tested. 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. All tested isolates were sensitive to fluopyram. 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.

Technical Abstract: 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 0.054 µg/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.