New Preharvest Treatments and Strategies in Managing Phytophthora Brown Rot of Citrus in California

Brown rot caused by Phytophthora citrophthora, P. nicotianae, P. syringae, and P. hibernalis is an important fruit disease of citrus in California, and the latter two species are quarantine pathogens in some important export markets. The newly registered fungicides oxathiapiprolin (OXA) and mandipropamid (MAN), as well as a premixture of the two (MAN+OXA) were compared to standard fixed copper and potassium phosphite (KPO3) treatments under field conditions in two citrus production regions of California. Fruit were sampled periodically over 8 weeks after application in winter or spring seasons, inoculated with zoospores of P. citrophthora or P. syringae, and brown rot incidence was evaluated. Single applications with all fungicides significantly reduced brown rot incidence of fruit harvested after 8 weeks as compared with the control in seasons with different amounts of precipitation (i.e., 17.2 to 153.9 mm between application and the 8-week sampling). MAN and OXA were similarly or significantly more effective than copper or KPO3. Two applications done in November and January significantly improved the efficacy of KPO3 and copper when compared to a single application of each fungicide done in January. For MAN and OXA, however, a single application was similar in efficacy as two applications. Two-application rotations of MAN, OXA, MAN+OXA, or copper significantly reduced the disease incidence by >84% from the control for at least 8 weeks after the second application. Low-volume (935 liters/ha) applications of MAN, OXA, MAN+OXA, or KPO3, but not copper, were significantly more effective than industry standard high-volume (3,740 liters/ha) applications. Thus, our studies identified and supported registration of new preharvest fungicide treatments to manage brown rot of citrus that are highly effective and persistent, and we optimized treatment strategies. Additionally, rotational programs with fungicides with different modes of action will minimize resistance development in pathogen populations and extend the usage of these fungicides.