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

The antifungal polyene macrolide natamycin was evaluated as a postharvest biopesticide for citrus fruit. Aqueous spray applications with 1,000 µg/ml were moderately to highly effective against green mold incidence after inoculation but did not reduce sporulation of Penicillium digitatum on infected fruit. Treatments with natamycin were significantly more effective against green mold on grapefruit and lemon than on orange and mandarin, with 92.9, 88.5, 57.5, and 60.9% reductions in decay, respectively, as compared with the control. 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 with the untreated control (with 81.9%). The incidence of sour rot of lemon and mandarin was also significantly reduced from the untreated control by natamycin (1,000 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 natamycin-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 antiresistance strategy and an effective treatment in mixtures with other fungicides for the management of major postharvest decays of citrus.