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.

Fruit Coating with Chitosan and Beeswax to Increase Papaya Shelf Life

Papaya (Carica papaya) is a climacteric fruit with an increased respiration rate during ripening process. Papaya respiration rate can be inhibited by providing a coating on the surface of the fruits, including chitosan, a polysaccharide derived from shrimp shell waste, or beeswax.The purpose of this research was to examine the effects of chitosan and beeswax coating on the shelf life and quality of papaya Callina fruit during storage. Results of the experiments showed that fruit coating with chitosan and beeswax can extend the shelf life of papaya Callina by four to five days compared to control due to inhibition respiration rate of papaya fruits during storage.The use of chitosan and beeswax was beneficial to maintain the physical and chemical quality of papaya Callina fruits.

Postharvest Strategies for Managing Phytophthora Brown Rot of Citrus using Potassium Phosphite in Combination with Heat Treatments

Phytophthora brown rot, caused by several species of Phytophthora, is an economically important disease of citrus in areas with rainfall during the late stages of fruit development. Recent export restrictions of California orange fruit to China due to the presence of brown rot caused by the quarantine pathogen Phytophthora syringae have mandated more rigorous disease management. We evaluated postharvest applications with the phosphonate fungicide potassium phosphite in combination with heat treatments. In timing studies, potassium phosphite at 1,500 μg/ml was most effective when applied within 18 h after inoculation of orange fruit with P. citrophthora, reducing the incidence of decay by >96% as compared with the control. Potassium phosphite was also highly effective in inoculations with P. syringae. Heated water treatments at 60°C were consistently and highly effective in reducing the incidence of brown rot after inoculation with P. citrophthora, whereas treatments at 55 or 50°C were more variable and generally less effective. Two-stage treatments of fruit were conducted in the laboratory to simulate current packinghouse practices and to evaluate any interaction of the efficacy of potassium phosphite with treatments of two commonly used postharvest fungicides (i.e., imazalil and thiabendazole [TBZ]) or a postharvest carnauba-based fruit coating. In these studies, an aqueous imazalil-potassium phosphite (2,000 μg/ml) dip at ambient temperature that was followed by a spray treatment of imazalil and TBZ prepared in fruit coating significantly reduced the incidence of brown rot from the control. When the aqueous dip was applied at 54°C, brown rot developed in only 1% of the fruit as compared with 76% in the water control. The efficacy of potassium phosphite was also demonstrated in commercial packinghouse treatments. Based on our research, this fungicide was registered for postharvest use against brown rot of citrus and is exempt from tolerance in the United States and many other countries.