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Baseline sensitivity of Botrytis cinerea isolates to natamycin and its control of gray mold on stored mandarin fruit

Seiya Saito: USDA ARS

<div>Gray mold caused by <em>Botrytis cinerea</em> is an emerging postharvest disease of mandarin fruit in California. Control of postharvest diseases in mandarins relies on postharvest fungicides. However, resistance to multiple mode-of-action fungicides is common in <em>B. cinerea</em> populations, 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 <em>B. cinerea</em> obtained from decayed mandarin with known resistance phenotypes to other fungicides were tested. The effective concentration of natamycin for 50% reduction in growth relative to control (EC<sub>50</sub>) 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 1.0 µg/ml for germination and 10.0 µg/ml for mycelial growth. No cross resistance between natamycin and other mode-of-action fungicides was detected. Control tests on mandarin fruit inoculated with five different fungicide-resistant phenotypes of <em>B. cinerea</em> isolates 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 postharvest gray mold control and management of fungicide resistance in <em>B. cinerea</em>.</div>