Identification of five new hosts of Phytophthora ramorum in an infested forest in California

Phytophthora ramorum causes stem and bole cankers (sudden oak death) and foliar and twig dieback (ramorum blight) of susceptible plants. To date, more than 100 tree, shrub and herbaceous hosts of P. ramorum have been identified. In March 2015, plant samples were submitted to the CDFA Plant Pest Diagnostics Lab from the Marin Municipal Water District for disease analysis. The samples were collected near Bolinas Ridge in Marin County, CA in a maritime chaparral-live oak woodland forest with a history of P. ramorum and P. cinnamomi. The collectors noticed a large amount of unusual die-back in many plant species earlier in the drought year and were concerned that the plants were infected with P. cinnamomi or other Phytophthora root pathogens. Manzanita species (Arctostaphylos canescens, A. sensitiva, A. virgata), chinquapin (Chrysolepis chrysophylla) and chaparral pea (Pickeringia montana) were submitted for diagnosis. Isolation from the roots onto PARP media from samples was attempted even though the roots appeared healthy. No Phytophthora sp. grew on isolation plates and no Phytophthora spp. were detected from the roots using the Agdia Phytophthora spp. specific immunoassay. The Arctostaphylos and Pickeringia montana samples contained foliar tissue with leaf spot, vein necrosis and stem canker symptoms which were tested separately for Phytophthora. Phytophthora spp. was detected from the leaf spots and stem cankers from the Pickeringia montana and some of the Arctostaphylos spp. by immunoassay. In addition, P. ramorum was detected in culture and confirmed by sequence analysis from the symptomatic foliage of these hosts. In April 2015, a second visit was made to the site to obtain official samples for regulatory purposes. In addition to the hosts collected initially, Arctostaphylos glandulosa and Rubus ursinus (California blackberry) were collected for testing for P. ramorum. On some Arctostaphylos plants, the leaves on entire branches were completely brown while the cambium was still green. Both mature and young foliage were affected. On some manzanita plants, the older foliage seemed more broadly affected, while on others the newest foliage on the growing tips was the most symptomatic. Chaparral pea plants exhibited small leaves that were necrotic with some partially defoliated. Cankers on the plants were located near the nodes with numerous necrotic areas on the short thorny stems. Phytophthora ramorum grew from isolated foliar and stem blight symptoms of Pickeringia montana, A. virgata and A. glandulosa. Phytophthora ramorum DNA was also detected from foliar leaf spot symptoms of blackberry using a species specific qPCR. Isolations from blackberry were unsuccessful on PARP media. Declining chinquapin trees appeared drought stressed with dry, chlorotic, and off-colored leaves. Affected branches and root suckers were collected and dark vascular streaks were seen in the xylem tissue when the bark was removed. These symptoms were seen in fully grown trees and young seedlings. Phytophthora 1 A version of this paper was presented at the Sixth Sudden Oak Death Science Symposium, June 20-23, 2016, San Francisco, California. 2 California Department of Food and Agriculture, Plant Pest Diagnostics Lab, Sacramento, CA 95832. 3 Marin Municipal Water District, Corte Madera, CA. 4 National Ornamentals Research Site at Dominican University, San Rafael, CA. Corresponding author: suzanne.latham@cdfa.ca. gov. GENERAL TECHNICAL REPORT PSW-GTR-255 84 ramorum DNA was isolated in culture onto PARP medium from the vascular streaking in a single tree but tested positive by PCR for P. ramorum from multiple trees. Although the Bolinas Ridge area in Marin County, CA is known to be infested with P. ramorum, five new associated hosts of P. ramorum were identified at this site. Arctostaphylos virgata is endemic to Marin County and is listed on the California Native Plant Society Inventory of Rare and Endangered Plants. The severe leaf and stem blight symptoms seen on A. virgata and A. glandulosa differ from those previously described on A. manzanita, but are similar to those on other ericaceous hosts. Chaparral pea (Fabaceae) is an endemic shrub to California that occurs along the coast and in the foothills of the Sierra Nevada Mountains. Difficulty in sampling its thorny stems may have contributed to it being overlooked as a host of P. ramorum in the past. Chinquapin has been suspected as a host of P. ramorum; log inoculations indicated it was susceptible. This is the first report of P. ramorum being isolated from symptomatic chinquapins in the wildlands. The dark vascular streaking associated with infection by P. ramorum suggests that P. ramorum may also be a wilt pathogen on some members of the Fagaceae. Pathogenicity experiments are ongoing and are dependent on obtaining the proper species for testing.