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Soil solarization has been used to manage soilborne plant pathogens. Research on solarization efficacy has focused on the minimum temperature and exposure time required to kill pathogens, and most mathematical models of the effects are based on cumulative temperature over time. However, two additional factors, soil water potential and diurnal temperature fluctuation, may influence pathogen survival. Our objective was to develop a more accurate predictive model based on results from controlled lab experiments. We assessed temperature, water potential, and intermittent heat effects on survival of <i>Phytophthora ramorum</i> and <i>P. pini</i> in infested leaf inoculum that contained chlamydospores or oospores, respectively. For both pathogens, survival frequency at high temperature was greater at lower water potentials. Survival was also greater when exposure to high temperature was interrupted by a cooler temperature. Results indicate that heat effects on pathogen survival increase gradually during heat treatment, suggesting that the temperature effect is not simply cumulative. The mathematical model was tested in solarization field trials conducted in 2013 by comparing calculated heat units to recovery of <i>P. ramorum</i> and <i>P. pini</i> subjected to various heat regimes. The model was improved significantly by adding the factors water potential and temperature fluctuation, allowing for greater accuracy in predicting soil solarization efficacy.