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Evaluation of new fungicides for management of Phytophthora root rot of citrus
W. Hao (1), M. Gray (1), H. Förster (1), J. Adaskaveg (1) (1) University of California, Riverside, U.S.A.

Phytophthora root rot (PRR), caused by Phytophthora spp., is an important disease of citrus in California. For its chemical control, only mefenoxam and potassium phosphite are currently registered. Mefenoxam resistance is widespread, and overuse of phosphonates may lead to resistance in Phytophthora spp. Thus, we evaluated the efficacy of new chemicals belonging to different modes of action (i.e., ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin) against PRR. Navel orange trees on Carrizo rootstock were inoculated with P. parasitica at planting. Applications with 12 fungicide treatments were made five weeks after planting, in spring and fall 2014, and in spring 2015. Feeder roots and adjacent soil were collected before or after application. PRR incidence and pathogen populations were determined by plating root segments and soil suspensions, respectively, on a selective medium. All of the new fungicides reduced PRR incidence and soil populations significantly to very low levels as compared to the control starting with the first application. Mefenoxam was only effective when a high label rate was used in the fourth application. Most of the new fungicides also increased tree canopy size and trunk diameter compared to the control. Greenhouse studies confirmed the efficacy of these new fungicides. Once registered, these new compounds will provide highly effective treatment options and resistance management strategies in the control of PRR of citrus.