Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2008
Publication Date: 3/1/2008
Citation: Browning, M.E., Englander, L., Tooley, P.W., Berner, D.K. 2008. Survival of Phytophthora ramorum hyphae following exposure to temperature extremes and various humidities. Mycologia. 100:236-245. Interpretive Summary: We studied the survival of the fungus-like pathogen named Phytophthora ramorum, responsible for causing a plant disease called sudden oak death. The disease has killed thousands of oak trees in the Western U.S. and also infects many additional plant species. The pathogen has the potential to move to the Eastern U.S. where it may become established and threaten the vast oak forests there. Information about its potential to survive high and low temperatures is thus critical to determining where the disease may become established in the Eastern U.S. We found by exposing the pathogen to high and low temperatures that exposure to 32.5 and 35 C resulted in reduced survival over an 8 h period, while 30 C had no impact on 3 of 4 pathogen strains. The pathogen also demonstrated considerable tolerance to cold, with all strains tested surviving a 24 hr exposure to –5 C. The results indicate that the pathogen can survive exposure to extreme temperatures and that it is likely that temperatures found in the northeastern US will not prevent establishment of the sudden oak death pathogen.
Technical Abstract: We examined the impact of short-term exposure to high and low temperatures and a range of relative humidities on survival of Phytophthora ramorum hyphae. Spore-free hyphal colonies were grown on dialysis squares atop V8 medium. Relative humidity ranged from 41 – 93% at 20 C and 43 – 86% at 28 C. Survival in all trials was characterized by growth from dialysis squares into V8 medium. Temperatures of 37.5–40 C were lethal to P. ramorum hyphae within several hours, and temperatures of 42.5–50 C were lethal within minutes. Exposure to 32.5 and 35 C resulted in reduced survival over an 8 h period, while 30 C had no impact on 3 of 4 isolates. Hyphal colonies demonstrated considerable tolerance to cold, with all isolates surviving a 24 hr exposure to –5 C. Survival diminished over time at lower temperatures, however, a few colonies survived a 24 h exposure to –25 C. Temperature also affected the ability of hyphal colonies to withstand reduced humidity. A RH of 41 to 43% was lethal in 2 hr at 28 C compared to 8 hr at 20 C. Three of 4 isolates were unaffected by an 8 h exposure to 81 and 95% RH at 20 C, and 73 and 86% RH at 28 C. Isolate differences were apparent in tolerance to freezing temperatures and reduced humidity. The results indicate that the cold temperatures found in the northeastern US are not likely to prevent the establishment of P. ramorum. There is also the potential for hyphae, and presumably spores, to survive periods of high humidity on the leaf surface in the absence of free water.