Recovery of Phytophthora ramorum Following Exposure to Temperature Extremes

We examined the impact of exposure to high and low temperature extremes on recovery of Phytophthora ramorum both as free chlamydospores and within infected rhododendron tissue over a 7-day period. Chlamydospores held in moistened sand were incubated at 30, 35, 40, 0, -10, and -20°C for up to 7 days. Infected Rhododendron 'Cunningham's White' leaf disks held in sandy loam, loam, or sand at two different soil moisture levels also were subjected to these temperatures for up to 7 days, and to a variable temperature regimen for 12 weeks. Recovery 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 recovery at 30°C, steadily declining recovery at 35°C, and no recovery at 40°C over the 7-day period. Chlamydospores were recovered from 0°C after 7 days, with little or no recovery observed at -10 or -20°C. In infected rhododendron tissue, P. ramorum was recovered at 20 and 30°C after 7 days but, at 35°C, the pathogen showed a decline within 2 days and no recovery by 4 days. A 40°C treatment allowed no recovery of P. ramorum from infected tissue after 2 days. For cold treatments, P. ramorum was recovered from infected leaf disks at 0 and -10°C after 7 days. At -20°C, recovery declined rapidly after 1 to 3 days and no recovery was obtained after 4 days. P. ramorum showed nearly 100% recovery from leaf disks subjected to a 12-week variable temperature treatment based on ambient summer temperatures in Lewisburg, TN. The results suggest 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. Thus, P. ramorum present as free chlamydospores or within tissue of infected plants shipped to the eastern United States has the potential to survive some of the adverse conditions encountered in summer and winter in many eastern states.