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

Authors: HAO, WEI - University Of California; MILES, TIMOTHY - California State University; Martin, Frank; Browne, Greg; FORSTER, HELGA - University Of California; ADASKAVEG, JAMES - University Of California

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2017
Publication Date: 2/16/2018
Citation: Hao, W., Miles, T.D., Martin, F.N., Browne, G.T., Forster, H., Adaskaveg, J.E. 2018. Temporal occurrence and niche preferences of Phytophthora spp. causing brown rot of citrus in the Central Valley of California. Phytopathology. 108(3):384-391. https://doi.org/10.1094/PHYTO-09-17-0315-R.
DOI: https://doi.org/10.1094/PHYTO-09-17-0315-R

Interpretive Summary: There are several species of Phytophthora that can infect citrus fruit and cause a fruit rot. Several of these are of regulatory concern for shipments to China due to quarantine restrictions banning their importation. This manuscript describes a study that monitored where the pathogens were in production orchards and what time of year they were most likely to infect fruit. This knowledge will be useful for improving control measures to manage fruit infection in the field.

Technical Abstract: 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. detection ranged from 73.6% to 96.1% for the two counties surveyed. The remaining isolates were identified as P. citrophthora. In late spring/summer, only P. citrophthora was recovered. P. hibernalis and P. nicotianae were not detected in any fruit with brown rot symptoms. These results indicate that P. syringae is currently an important brown rot pathogen of citrus fruit in California during the cooler seasons of the year. In the winter of 2016 and 2017, P. syringae was recovered by pear baiting at a high incidence from leaf litter, from a small number of rhizosphere soil/root samples, but not from living leaves on the tree. In contrast, P. citrophthora, was rarely found in leaf litter, but was commonly detected in the rhizosphere. Thus, leaf litter is a major inoculum source for P. syringae and this species occupies a distinct ecological niche.