Recovery Frequency of Phytophthora ramorum and Other Phytophthora spp. in the Soil Profile of Ornamental Retail Nurseries.

We tested the hypothesis that inoculum of the aboveground exotic plant pathogen Phytophthora ramorum would be limited to the organic layer (top layer of plant debris) of soils at infested retail nurseries located outside of the area where the pathogen has become established in the landscape. To test this hypothesis and compare inoculum levels of P. ramorum with levels of other Phytophthora spp. in the soil profile, soil cores were collected and sampled from three Washington State retail nurseries at which the soil had previously tested positive for P. ramorum. Phytophthora was isolated from soil using rhododendron leaves as bait, and pure cultures were obtained and stored on V8 juice agar. Isolates were identified to species using a combination of DNA sequencing of the internal transcribed spacer (ITS) region of rDNA, real-time polymerase chain reaction (PCR) diagnostic testing, and culture morphology. Recovery frequencies were tabulated and compared by species at the organic layer, 0 to 5 cm, 5 to 10 cm, and 10 to 15 cm depth classes. The three most common Phytophthora spp. recovered from the soil cores were P. citricola (32%), P. drechsleri (32%), and P. ramorum (27%). P. citricola and P. drechsleri were more evenly distributed throughout the soil profile, whereas P. ramorum was primarily recovered from the organic and 0 to 5 cm depth class (86% of recoveries). P. ramorum was not detected below 10 cm.