UC Riverside

Phytophthora citrophthora and P. syringae are currently the major brown rot pathogens of citrus in California. The diversity and structure of populations of these species has never been characterized using genomic techniques. Recently, four new fungicides (ethaboxam, fluopicolide, mandipropamid, oxathiapiprolin) have been registered or are pending registration to manage Phytophthora diseases of citrus, whereas others currently used include mefenoxam and potassium phosphite. We evaluated Phytophthora populations in three California growing regions for fungicide sensitivity and resistance potential using traditional and genomic methods and studied the mobility of mandipropamid in citrus plants. The results of this dissertation are presented in three chapters: 1) genomic characterization of populations of Phytophthora citrophthora and P. syringae causing brown rot of citrus in California; 2) regional comparisons of sensitivities of these Phytophthora spp. to six fungicides; and 3) mobility of mandipropamid in citrus plants as compared to oxathiapiprolin and mefenoxam. Genomic analyses of isolates of both species in California identified regional differentiation of populations. P. citrophthora was found to have a clonal population structure with low levels of diversity and limited evidence of sexual recombination. In contrast, comparatively greater inter- and intra-population diversity was evident in P. syringae which is indicative of some level of sexual recombination. P. syringae isolates collected from non-citrus hosts were found to be distinct from those from citrus, indicating that isolates from non-citrus hosts are not contributing to brown rot of citrus. Regional comparisons of sensitivities of isolates of both species expanded the previously established baseline sensitivities. Population genomics analyses identified P. citrophthora and P. syringae isolates from the southern growing regions to have an increased risk for resistance development to mandipropamid and ethaboxam, respectively. Studies on the mobility of mandipropamid showed that it has limited acropetal mobility. The fungicide was absorbed by the roots and translocated at low levels into stem and leaf tissues of young plants, but not back into untreated roots (i.e., no basipetal movement). Oxathiapiprolin had similar acropetal mobility, whereas the previously described limited ambimobile properties of mefenoxam were confirmed.