Thermal Inactivation of Inoculum of Two Phytophthora Species by Intermittent Versus Constant Heat.

Research on solarization efficacy has examined the critical temperature and minimum exposure time to inactivate soilborne pathogens. Most mathematical models focus on survival of inoculum subjected to a constant heat regime rather than an intermittent heat regime that better simulates field conditions. To develop a more accurate predictive model, we conducted controlled lab experiments with rhododendron leaf disks infested with Phytophthora ramorum and P. pini. Focused in vitro experiments with P. ramorum showed significantly longer survival of inoculum exposed to intermittent versus constant heat, indicating that intermittent heat is less damaging. A similar trend was observed in soil. Damage was evaluated by comparing the reduction in subsequent survival time of inoculum subjected to different intensities of sublethal heat treatments. Inoculum exposure to continuous heat reflected an increasing rate of damage accumulation. Multiple sublethal heat events resulted in a constant rate of damage accumulation which allowed us to calculate total damage as the sum of damage from each heat event. A model including a correction for an intermittent heat regime significantly improved the prediction of thermal inactivation under a temperature regime that simulated field conditions.