Potential climatic suitability for establishment of Phytophthora ramorum within the contiguous United States

Phytophthora ramorum has caused extensive mortality to tanoak and several oak species in coastal California. This pathogen has infected at least 72 plant species under natural conditions and 32 additional species in the laboratory. Many infected hosts have been distributed across the United States by the horticultural industry. We developed a simulation model using CLIMEX software to evaluate the suitability of the climate in the United States for establishment of P. ramorum. CLIMEX was driven by monthly climate normal data for 1971–2000 collected from >5300 weather stations in the contiguous United States. CLIMEX growth-requirement and stress-response parameters were derived from literature data. Values for the ecoclimatic index (EI), a measure of overall climatic suitability based on temperature and soil moisture, were between 0 and 53. Much of the Intermountain West and the Great Plains was climatically unsuitable for establishment of P. ramorum (EI = 0). Many states bordering the Great Lakes were marginal (0 < EI < 11). Areas considered climatically highly favorable (EI > 25) for establishment of P. ramorum were common in the Gulf States, and areas considered favorable (10 < EI < 26) extended into southern Illinois, southern Indiana, and northwards into southern Maine. Predictions derived from CLIMEX matched known occurrences of P. ramorum in California and Oregon. Finds of the pathogen were 3.4-times more likely in areas classified as favorable or very favorable than in areas classified as marginal or unsuitable. Model results were only modestly sensitive to changes in values assigned to temperature parameters for growth but were more sensitive to changes in values assigned to moisture parameters for growth. Additional research is needed to determine the effects of low moisture on population growth of the pathogen. Nevertheless, our model distinguishes some areas within the contiguous United States that do not have a suitable climate for the pathogen. Such information could be used to refine survey and detection programs. # 2006 Elsevier B.V. All rights reserved.