Location: Foreign Disease-Weed Science
Title: The effect of temperature on germination of chlamydospores of Phytophthora ramorum Authors
Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 5, 2013
Publication Date: May 28, 2014
Citation: Tooley, P.W., Browning, M.E., Leighty, R.M. 2014. The effect of temperature on germination of chlamydospores of Phytophthora ramorum. Mycologia. 106:424-430. Interpretive Summary: To acquire knowledge of the threat potential posed by the sudden oak death pathogen, Phytophthora ramorum, to oak forests of the Eastern U.S., we assessed the range of temperatures over which its thick-walled resting spores (chlamydospores) are able to germinate. Chlamydospores which survive the winter provide the source of new epidemics, and their germination ability over a range of temperatures has not been determined. We performed germination experiments and determined the optimal temperature for germination of chlamydospores obtained from a range of P. ramorum strains. We observed a high degree of variation in germination of the chlamydospores among strains of P. ramorum at the temperatures tested. We also observed that specific strains of P. ramorum show chlamydospore germination rates substantially higher or lower than observed for other strains. Thus, a temperature considered too low to allow germination by one strain may be adequate to allow germination by a different strain. This variation must be taken into account when predicting the likelihood that P. ramorum chlamydospores will be capable of germination and epidemic initiation in new regions of the U.S. into which the pathogen may be introduced.
Technical Abstract: Mycelium-free chlamydospores of twelve isolates of P. ramorum representing three clonal lineages (NA-1, NA-2, and EU-1) were produced using a method involving incubation in non-sterile sand at 20 C in darkness for 30 days. Chlamydospores were incubated on selective agar medium in incubators at 5, 10, 15, 20, 25, and 30 C. Dishes were examined for chlamydospore germination after 1, 2, 4, 6, 8, 10, 12, and 14 days incubation. After 8 days, counts became difficult because of colony overgrowth. The optimal temperature for germination based on the 8 day data was 20 C for all clonal lineages, and mean germination rates over all isolates tested were 2, 21, 44, 67, 32, and 0 percent at 5, 10, 15, 20, 25, and 30 C, respectively. The highest germination rate obtained (85 percent) at 8 days incubation was for isolates of the EU-1 clonal lineage at 20 C. Substantial variation was observed among isolates within each clonal lineage. Over all temperatures and days of incubation upon which germination was assessed, the NA-1 clonal lineage showed the lowest overall mean germination, even though one isolate showed the highest germination of any isolate in any lineage. The results indicate that 20 C is the optimal germination temperature for P. ramorum chlamydospores, and that a great disparity in germination percentage can exist within isolates of a single clonal lineage.