Natamycin, a Biofungicide for Managing Major Postharvest Fruit Decays of Citrus

The antifungal polyene macrolide natamycin was evaluated as a postharvest biopesticide for citrus fruits. Aqueous spray applications with 1000 µg/ml were moderately to highly effective against green mold after inoculation but did not reduce sporulation of Penicillium digitatum on infected fruits. Treatments with natamycin were significantly more effective against green mold on grapefruit and lemons than on oranges and mandarins with 92.9%, 88.5%, 57.5%, and 60.9% reductions in decay as compared with the control, respectively. The biofungicide was compatible with a storage fruit coating but was less effective when applied in a packing coating. However, when either fruit coating was applied following an aqueous natamycin treatment (i.e., staged applications) the incidence of decay was reduced to ≤10.7% as compared to the untreated control with 81.9%. The incidence of sour rot of lemons and mandarins was also significantly reduced from the untreated control by natamycin (1000 µg/ml), but propiconazole (540 µg/ml) and propiconazole + natamycin (540 + 500 µg/ml) mixtures generally were significantly more effective than natamycin alone when using a severe inoculation procedure. Experimental and commercial packingline studies demonstrated that natamycin-fludioxonil or -propiconazole mixtures applied in a storage fruit coating or as an aqueous flooder treatment were highly effective and typically resulted in a >85.0% reduction of green mold and sour rot. Resistance to natamycin has never been documented in filamentous fungi. Thus, the use of natamycin, in contrast to other registered postharvest fungicides for citrus, can be an anti-resistance strategy and an effective treatment in mixtures with other fungicides for the management of major postharvest decays of citrus.