Seasonal Variation in Isolation of Phytophthora from Wildland Soils.

Phytophthora species are responsible for a variety of plant diseases including Sudden Oak Death (SOD) and Port Orford-cedar root disease. Phytophthora species were isolated from four soil/vegetation combinations in Northwestern California over four seasons for two consecutive years. Soils were sampled from oak woodlands, mixed-conifer forests on serpentine soils, mixed conifer soils on non-serpentine soils, and a moist control site known to have Phytophthora lateralis-infected Port Orford-cedar. We baited Phytophthora from the soil samples using rhododendron and Port Orford-cedar leaf baits. We isolated oomycete colonies growing from the baits on Phytophthora-selective PARPNH media and then transferred single hyphal tips to clarified V8 broth. We subsequently freeze-dried the individual colonies, extracted DNA, amplified the ITS region using the primer pair ITS 6F and ITS 4R, purified the PCR products, and sequenced them for species identification. Our data indicate we were most successful in baiting Phytophthora from soils collected during the spring period. BLAST searching of two Phytophthora-specific databases as well as Genbank showed that almost all Phytophthora isolated was P. cambivora; furthermore, it came exclusively from oak woodland soils. Mixed conifer, serpentine, and control soils yielded primarily other oomycetes, zygomycetes, and a Penicillium species that grew on the selective medium. Baiting and incubating samples in cold vs room-temperature conditions showed that room temperatures are necessary for Phytophthora culture growth, whereas Pythium species can survive and dominate in cold temperatures. We plan to continue our seasonal tracking of Phytophthora recovery from a variety of soil and forest types in northwestern California.