Recovery of Phytophthora ramorum following exposure to extreme temperatures

Authors: Tooley, Paul; Browning, Marsha; Berner, Dana

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2007
Publication Date: 3/1/2008
Citation: Tooley, P.W., Browning, M.E., Berner, D.K. 2008. Recovery of Phytophthora ramorum following exposure to extreme temperatures. Plant Disease. 92:431-437

Interpretive Summary: We studied conditions under which the destructive fungus-like pathogen Phytophthora ramorum can withstand extreme high and low temperatures. We studied survival of a certain type of thick walled spore called chlamydospores, as well as survival of the pathogen while inside infected rhododendron leaf pieces for up to 7 days at some hot and cold temperatures. Chlamydospores did not survive treatment at some very cold temperatures (equal to minus 4 and 14 degrees Farenheit), but did survive treatment at 32 degrees Farenheit for 7 days. P. ramorum was able to survive in infected rhododendron leaf pieces for 7 days at 86 degrees Farenheit, but showed less survival at 95 degrees and none at 104 degrees Farenheit. P. ramorum was also able to survive in leaf pieces at some cold temperatures such as 32 and 14 degrees Farenheit. Thus, we found that this pathogen should be able to survive winter and summer conditions in parts of the Eastern U.S. were it to be introduced there via long distance movement from infested areas in the Western U.S.

Technical Abstract: We examined the impact of exposure to high and low temperature extremes on survival of P. ramorum both as free chlamydospores and within infected rhododendron tissue over a 7 day period. Infected Rhododendron - Cunningham’s White - leaf disks held in a sandy loam, loam, or sand at 2 different soil moisture levels were subjected to various constant temperatures for 7 days and to a variable temperature regimen for 12 weeks. Survival was characterized by growth of P. ramorum on selective agar medium following exposures to temperature treatments. Chlamydospores held in moistened sand showed a high rate of survival at 30 degrees C, steadily declining survival at 35 degrees C, and no survival at 40 degrees C over the 7 day period. Chlamydospores also survived at 0 degrees C for 7 days, but not at minus 10 degrees C or minus 20 degrees C. In infected rhododendron tissue, P. ramorum survived at 20 degrees C and 30 degrees C for 7 days, but at 35 degrees C the pathogen showed a decline within 2 days and no survival by 4 days. A 40 degrees C treatment was lethal to P. ramorum in infected tissue after 2 days. P. ramorum survived in infected leaf disks at 0 degrees C, and minus 10 degrees C for 7 days. At minus 20 degrees C, survival declined rapidly after 1-3 days and no recovery was obtained after 4 days. The results indicate that P. ramorum is capable of surviving some highly adverse temperature conditions for at least 7 days both as free chlamydospores in sand and within infected host tissue.