Susceptibility of Pacific Yew to Phytophthora lateralis.

In 1991, Pacific yew was reported as a new host for Phytophthora lateralis, an aggressive root pathogen thought previously to be specific to Port Orford cedar. This study was designed to compare the pathogenicity of P. lateralis on the two hosts through seedling, stem, branch, and rootlet inoculations, and field survey. Mortality of inoculated plants averaged 72% for cedar and 4% for Pacific yew, and root colonization by P. lateralis was significantly greater in cedar seedlings than in Pacific yew seedlings. Lesion length on the cedar seedling stems was twice the lesion length on Pacific yew stems, and cedar branches had lesions four times longer than Pacific yew branches. Abundant zoospore aggregation occurred on cedar rootlets along the zone of elongation and the region of maturation. In comparison, far fewer zoospores encysted on Pacific yew rootlets, and they were concentrated on the root hairs. A field survey along 0.8-km stretches of three infested streams in southwest Oregon and northwest California revealed a total of 1,199 dead Port Orford cedar (46% mortality), and 86 dead Pacific yew (10% mortality). We conclude that Pacific yew is less susceptible to P. lateralis than Port Orford cedar.