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.