Integrated Postharvest Strategies for Management of Phytophthora Brown Rot of Citrus in the United States

Abstract

Brown rot caused by several species of Phytophthora is an economically important disease of citrus fruit. Crop losses may occur in the orchard or after harvest. Recently, brown rot has become a quarantine issue with some of California’s trading partners, and effective management programs are needed. The disease can be controlled with field applications of copper or with systemic phosphonate (e.g., fosetyl-Al; potassium phosphite) and phenylamide (e.g., mefenoxam) fungicides. To potentially increase the degree of brown rot control in an integrated approach, we evaluated the efficacy of postharvest treatments. Using orange fruit inoculated with P. citrophthora in laboratory and packingline studies, potassium phosphite, mandipropamid, fluopicolide, and azoxystrobin were highly effective as pre-infection treatments. Only potassium phosphite was also effective as a post-infection treatment and thus, has the potential to prevent decay from pre-existing infections occurring shortly before harvest in the orchard. A dip of inoculated fruit in water at an average temperature of 56 °C was similarly effective to potassium phosphite applied at ambient temperature. Brown rot incidence was lowest using a combination treatment of heated potassium phosphite (with or without imazalil added) and a spray application of imazalil and TBZ in packing fruit coating. Our data indicate that postharvest potassium phosphite can be effective for the management of brown rot of citrus fruit using existing packing line equipment. Incidence of disease can be reduced to very low levels with an integrated management approach that includes orchard practices (e.g., tree skirting, Phytophthora root rot control), preharvest fungicide applications (e.g., copper or newer modes of action), and postharvest phosphite at ambient temperature or as a heated treatment. Potassium phosphite has an exempt from residue status in the United States but not in many export markets. Efforts are underway to establish international maximum residue limits (MRLs) with our trade partners. Fluopicolide and mandipropamid are proposed for preharvest use in citrus orchards for managing Phytophthora root rot and brown rot. Registrations of several fungicides will help in the implementation of resistance management strategies such as rotations or mixtures to ensure the long-term usage of these compounds.