Survival, spread, and pathogenicity of Phytophthora spp. on Douglas-fir seedlings planted on forest sites.

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Douglas-fir seedling stock infected in the nursery with Phytophthora cryptogea, P. drechsleri, P. megasperma, P. cactorum, and an unidentified Phytophthora sp. were outplanted on commercial forest sites to test survival of the diseased trees and of the pathogens. Mortality of trees initially classified in severe, moderate, and inconspicuous symptom classes at outplanting reached 61, 26, and 11%, respectively, after 18 mo. Phytophthora was recovered about equally from roots of trees in each symptom class (15, 13, and 12%). Surviving trees regenerated healthy roots above old lesions even though Phytophthora persisted. Disease spread was limited. None of 360 healthy trees planted 0.6 m downslope from diseased trees became infected, and only 2 of 720 healthy trees became infected after each was paired with a diseased tree in the same planting hole.

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