APS Homepage

Poster: Epidemiology: Pathogen Dispersal


A leaf litter and fruit brown rot life style of Phytophthora syringae in California citrus
W. HAO (1), H. Förster (1), T. Miles (2), F. Martin (3), G. Browne (4), J. Adaskaveg (1) (1) University of California, Riverside, U.S.A.; (2) California State University, Monterey Bay, U.S.A.; (3) USDA-ARS, U.S.A.; (4) University of California, Davis and

In recent years, Phytophthora syringae has become a major pathogen causing brown rot of citrus fruit in California during the winter season. This is a serious concern for the citrus industry because P. syringae is a quarantine pathogen in some export countries. The source of P. syringae inoculum infecting citrus fruit has never been clearly established. Roots and rhizosphere soil from trees where brown rot was found to be caused by P. syringae were sampled in three orchards. Samples were plated onto a Phytophthora-selective medium and TaqMan qPCR and Droplet Digital PCR were performed. P. citrophthora and P. parasitica were consistently detected by isolation in roots and soil and by PCR in roots. P. syringae was not found causing root rot and was not detected in the soil using these methods. Leaf litter on the ground and soil were sampled from the same locations and immersed in water. Mature green pears were used as baits and incubated at 12°C for 1 to 2 wk. Brown lesions that developed on pears at the water line were plated onto selective medium. P. syringae was detected in most leaf litter samples but only in two soil samples. Thus, leaf litter is a major inoculum source for P. syringae and changes our understanding of the disease cycle of this important pathogen and separates it from other Phytophthora species that cause citrus root and brown rots. Recognition of the distinct epidemiology of the disease opens the opportunity for new cultural management practices.