Phytophthora cinnamomi, an unlikely threat to Douglas-fir forestry

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Abstract:

Phytophthora cinnamomi has damaged forests around the world in regions with mild climates. Its pathogenesis on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in southern Europe and southeastern United States suggested a potential threat to Douglas-fir forests of the Pacific Coast. Concern increased in 1950 with discovery of P. cinnamomi in nurseries growing ornamentals and in landscape plantings in western Oregon and Washington and research was undertaken to evaluate the threat. A temperature of 60° F was found necessary for infection. While summer soil temperatures on south exposures are above 60° F most of the time from June to October soil moisture is continuously below field capacity and too dry for infection. On north exposures summer soil moisture is adequate but temperatures are too low; 60° F is reached for only a few hours near the first of August. Forest soils of western Oregon are generally too dry for infection during the summer. The fall rains provide necessary moisture but concurrently depress temperatures below the critical 60° F. Adverse soil temperature and moisture influence disease by impeding infection rather than reducing survival of P. cinnamomi. The disease in ornamentals is sustained by summer irrigation of warm locations such as the south side of buildings. The data show that P. cinnamomi is unlikely to become a problem in forests of the Pacific Northwest.

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