Technical Abstract: We investigated the temperature and moisture conditions that allow P. ramorum to infect 'Cunningham's White' rhododendron. For whole plants incubated in dew chambers at 10-31C, the greatest percentage diseased leaves occurred at 22C, followed by 16, 25, and 19C. Significantly less infection occurred at 10, 13, 28 and 31C. At 20C, a dew period as short as 1 hour resulted in a small amount of disease on whole plants following 7 day incubation at 20C. However, at least 4 hours of dew were required for more than 10 percent of the leaves to become diseased by 7 days. Moisture periods of 24 h and 48 h resulted in the greatest number of diseased leaves. The relationship between moisture period and percent diseased leaves fit a nonlinear regression and all effects in the model, including the intercept, were highly significant. In temperature gradient plate experiments using detached leaves, significant temperature effects, moisture period effects, and temperature x moisture period interactions were observed. At 18C, the greatest amount of disease occurred at 24 h moisture, while at 22C the greatest amount of disease occurred at 48 h moisture. A predictive model for disease development was generated with multiple regression analyses of the combined non-linear regressions for temperature and moisture period. From this model, the predicted optimum moisture period for percent diseased leaves was 72 hours, and the predicted optimum temperature was 20C. Temperature had a greater influence on disease development than did moisture period. Our results show that P. ramorum can cause disease on susceptible hosts over a wide range of temperatures. Thus, P. ramorum may have the potential to become established in many parts of the U.S. which are outside of its current range.