Enhanced recovery of Phytophthora ramorum from soil following 30 days storage at 4C

Authors: Tooley, Paul; CARRAS, MARIE - Retired ARS Employee

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2011
Publication Date: 8/17/2011
Citation: Tooley, P.W., Carras, M.M. 2011. Enhanced recovery of Phytophthora ramorum from soil following 30 days storage at 4C. Journal of Phytopathology. 159:641-643.

Interpretive Summary: Phytophthora ramorum is a destructive pathogen of both forest and nursery crops that can attack and cause a serious disease on many plant species. Methods of assaying soil to detect this pathogen are very important to determine whether it remains present in nurseries into which it has been introduced on infected plant material. The USDA-Animal and Plant Health Inspection Service (APHIS) has recommended soil sampling methods but little experimental evidence exists that validates the suggested protocols. We infested soil with known densities of P. ramorum spores and used two different assays to determine how much of the pathogen was present at the time the soil was infested and also following 30 days storage in the cold. We found that P. ramorum can be recovered at enhanced levels from soil following 30 days of cold storage which is in keeping with methods suggested by APHIS. The results provide information about recovery methods for P. ramorum detection from soil that will be useful to other workers, and also provide evidence that supports the APHIS soil sampling recommendations.

Technical Abstract: Chlamydospores of Phytophthora ramorum produced by mixing 20 percent V8 juice broth cultures with sand and incubating over a 30 day period were used to infest field soil at densities ranging from 0.2 to 42 chlamydospores per cubic centimeter of soil. Chlamydospore recovery was determined by baiting with rhododendron leaf discs and dilution plating both at time 0 (when soil infestation was performed) and following 30 days storage at 4 degrees Celsius, as recommended by APHIS. Baiting was slightly more sensitive than dilution plating in recovering P. ramorum immediately following infestation of soil, and allowed detection from samples infested with as little as 0.2 chlamydospores per cc compared with 1 chlamydospore per cc for dilution plating. Following 30 days of infested soil storage at 4 degrees Celsius, P. ramorum was detected at significantly (P = 0.05) higher levels than at time 0. The results indicate that storage of P. ramorum-infested soil at 4 degrees Celsius for 30 days may allow the pathogen to increase its biomass, and thus can enhance recovery from soil.