Evaluation of New Oomycota Fungicides for Management of Phytophthora Root Rot of Citrus in California.

Phytophthora root rot, caused by several species of Phytophthora, is an important disease of citrus in California and other growing regions. For chemical management, mefenoxam and potassium phosphite have been available for many years, and resistance in Phytophthora spp. has been reported for both compounds. We evaluated the efficacy of the new Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin, each with a different mode of action, against Phytophthora root rot of citrus in field and greenhouse studies. Root balls of navel orange trees on 'Carrizo citrange' rootstock were inoculated with P. nicotianae at planting in the field in fall 2013. Applications with 11 fungicide treatments were made 5 weeks after planting, in spring and fall 2014, and in spring 2015. Feeder roots and adjacent soil were collected before or after application. All of the new fungicides significantly reduced root rot incidence and Phytophthora soil populations to very low levels as compared with the control starting after the first application. Mefenoxam was only effective when a high label rate was used in the fourth application. Selected treatments also increased tree canopy size, trunk diameter, and fruit yield as compared with the control. A rate comparison with the four new fungicides was initiated in summer 2016 in another field trial using navel orange trees inoculated with P. citrophthora. Minimum effective rates to reduce Phytophthora root rot incidence and pathogen soil populations were determined after one and two applications in fall 2016 and summer 2017, respectively. Greenhouse studies confirmed the efficacy of the new fungicides. Based in part on our studies, fluopicolide recently received a federal and oxathiapiprolin a full registration for use on citrus, and registrations for ethaboxam and mandipropamid have been requested. These new compounds will provide highly effective treatment options and resistance management strategies using rotation and mixture programs for the control of Phytophthora root rot of citrus.