Temporal occurrence and niche preferences of Phytophthora species causing brown rot of citrus in the Central Valley of California

Brown rot of citrus fruits is caused by several species of Phytophthora and is currently of serious concern for the California citrus industry. Two species, P. syringae and P. hibernalis, are quarantine pathogens in China, a major export market for California citrus. To maintain trade and estimate the risk of exporting a quarantine pathogen, the distribution and frequency of Phytophthora species causing brown rot of oranges in major growing areas of California was investigated. Symptomatic fruit were collected from navel (winter to late spring) and Valencia (late spring to summer) orange orchards from 2013 to 2015. Species identification of isolates was based on morphological characteristics, random amplified polymorphic DNA banding patterns, and sequencing of the ITS and the partial cox2/spacer/cox1 regions from axenic cultures, or directly in DNA from fruit tissue using a multiplex TaqMan qPCR assay. In winter samplings, the incidence of P. syringae based on the number of fruit with Phytophthora spp. de...