Gary Mold: Fungicide resistance in the pathogen botrytis cinerea and its impact on control

Authors: Saito, Seiya; Xiao, Chang-Lin

Submitted to: Citrograph
Publication Type: Trade Journal
Publication Acceptance Date: 3/11/2021
Publication Date: 10/1/2021
Citation: Saito, S., Xiao, C. 2021. Gary Mold: Fungicide resistance in the pathogen botrytis cinerea and its impact on control. Citrograph. 12(4):60-64.

Interpretive Summary: Gray mold caused by the fungus Botrytis cinerea is an emerging postharvest fruit rot disease of mandarin fruit in California. Because of its recent emergence, historically B. cinerea has not been among the target pathogens in postharvest disease control programs for citrus. In order to develop postharvest disease control programs to control gray mold, information is needed as to whether isolates of B. cinerea from mandarins are resistant to the currently registered postharvest fungicides that are known effective against B. cinerea such as azoxystrobin, pyrimethanil, fludioxonil, and thiabendazole. We found that resistance to azoxystrobin, pyrimethanil, and thiabendazole is widespread but no fludioxonil resistance was detected. There were five fungicide resistance phenotypes in B. cinerea from mandarins with the one that is simultaneously resistant to azoxystrobin, pyrimethanil and thiabendazole being most prevalent. Efficacy tests showed that fungicides failed to control gray mold on mandarin fruit inoculated with their respective fungicide resistance phenotypes, but fludioxonil remained effective because of no resistance. Alternatively, efficacy of natamycin, a newly registered biofungicide, was evaluated for the control of gray mold. Natamycin effectively controlled gray mold on mandarin fruit regardless of fungicide resistance phenotypes. Natamycin appeared to be a promising tool to be integrated into postharvest disease control programs for not only control of gray mold but also management of fungicide resistance in B. cinerea.

Technical Abstract: Gray mold caused by the fungus Botrytis cinerea is an emerging postharvest fruit rot disease of mandarin fruit in California. Because of its recent emergence, historically B. cinerea has not been among the target pathogens in postharvest disease control programs for citrus. In order to develop postharvest disease control programs to control gray mold, information is needed as to whether isolates of B. cinerea from mandarins are resistant to the currently registered postharvest fungicides that are known effective against B. cinerea such as azoxystrobin, pyrimethanil, fludioxonil, and thiabendazole. We found that resistance to azoxystrobin, pyrimethanil, and thiabendazole is widespread but no fludioxonil resistance was detected. There were five fungicide resistance phenotypes in B. cinerea from mandarins with the one that is simultaneously resistant to azoxystrobin, pyrimethanil and thiabendazole being most prevalent. Efficacy tests showed that fungicides failed to control gray mold on mandarin fruit inoculated with their respective fungicide resistance phenotypes, but fludioxonil remained effective because of no resistance. Alternatively, efficacy of natamycin, a newly registered biofungicide, was evaluated for the control of gray mold. Natamycin effectively controlled gray mold on mandarin fruit regardless of fungicide resistance phenotypes. Natamycin appeared to be a promising tool to be integrated into postharvest disease control programs for not only control of gray mold but also management of fungicide resistance in B. cinerea.