Efficacy of Natamycin against gray mold of stored mandarin fruit caused by isolates of Botrytis cinerea with multiple fungicide resistance

Authors: Saito, Seiya; WANG, FEI - University Of California, Davis; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2019
Publication Date: 1/15/2020
Citation: Saito, S., Wang, F., Xiao, C. 2020. Efficacy of Natamycin against gray mold of stored mandarin fruit caused by isolates of Botrytis cinerea with multiple fungicide resistance. Plant Disease. 104(3):787-792. https://doi.org/10.1094/PDIS-04-19-0844-RE.
DOI: https://doi.org/10.1094/PDIS-04-19-0844-RE

Interpretive Summary: Gray mold caused by the fungus Botrytis cinerea is an emerging postharvest disease of mandarin fruit in California. Control of postharvest diseases in mandarins relies on postharvest fungicides. However, resistance to multiple modes-of-action fungicides is common in B. cinerea populations from mandarin, leading to failure of decay control. Natamycin is a newly registered biorational fungicide for postharvest use on citrus and other fruits, and it belongs to a mode-of-action of chemistry different from other citrus postharvest fungicides. In this study, we established baseline sensitivity to natamycin in B. cinerea obtained from mandarins and tested the efficacy of natamycin for control of gray mold on mandarin fruit. We found that natamycin is effective in controlling B. cinerea on mandarin fruit regardless of whether the fungal strains are resistant to other citrus postharvest fungicides azoxystrobin, pyrimethanil, or thiabendazole. Our results suggest that natamycin is an effective tool for control of postharvest gray mold and for management of resistance to other modes-of-action fungicides in B. cinerea.

Technical Abstract: Gray mold caused by Botrytis cinerea is an emerging postharvest disease of mandarin fruit in California. Control of postharvest diseases in mandarins relies on postharvest fungicides. However, resistance to multiple modes-of-action fungicides is common in B. cinerea populations from mandarin, leading to failure of decay control. Natamycin is a newly registered biorational fungicide for postharvest use on citrus and other fruits. To establish baseline sensitivity to natamycin, 64 isolates of B. cinerea obtained from decayed mandarin fruit with known resistance phenotypes to other citrus postharvest fungicides (azoxystrobin, fludioxonil, pyrimethanil and thiabendazole) were tested. The effective concentrations of natamycin for 50% reduction in growth relative to control (EC50) ranged from 0.324 to 0.567 µg/ml with a mean of 0.444 µg/ml for conidial germination and from 1.021 to 2.007 µg/ml with a mean of 1.578 µg/ml for mycelial growth. The minimum inhibitory concentrations (MIC) were 0.7 to 1.0 µg/ml for conidial germination and 5.0 to 10.0 µg/ml for mycelial growth. No cross resistance between natamycin and other modes-of-action fungicides such as azoxystrobin, pyrimethanil, and thiabendazole was detected. Control tests on mandarin fruit inoculated with B. cinerea isolates exhibiting five different fungicide-resistant phenotypes showed that natamycin significantly reduced incidence and lesion size of gray mold on fruit regardless of fungicide-resistant phenotypes. Our results suggest that natamycin is an effective tool for control of postharvest gray mold and for management of resistance to other modes-of-action fungicides in B. cinerea.