Field Tests of the Capacity of Phytophthora Root Rot to Damage Douglas-Fir

Abstract When Phytophthora cinnamomi was discovered on ornamentals in the Pacific Northwest, questions were raised as to its potential for damage to commercial forests of the region. Was the forest industry to prepare to combat a dangerous epiphytotic; to develop a stand management program to reduce chronic losses; or simply to keep a watchful eye? Absence of the fungus from Oregon's forests was shown by negative isolation results from 1800 forest soil samples whereas 30 percent of check samples from certain nurseries yielded P. cinnamomi. To test pathogenesis under field conditions, inoculum of P. cinnamomi as liquid cultures, agar cultures, and infested soil was introduced into the soil at 53 inoculation points in logged-off Douglas-fir sites of two general types and into the soil of a thicket of fir saplings. The fungus was not recoverable following the dry summers characteristic of the region. It was not recovered on south slopes after late June nor on north slopes, with two exceptions persisting until October, after August. Measurable growth from inoculum into forest soil occurred only during the spring when infested soil served as the inoculum. Inoculations in the root zone of the sapling thicket failed to produce symptoms in the samplings and the fungus could not be recovered later than June. One group of inoculated sites tended to be cool and moist, the other to be warm and dry. Neither completely represented the more moderate soil micro-climate obtaining under full Douglas-fir cover and on more gentle topography; therefore, the conclusion that P. cinnamomi will not survive the summer climate of this region should not be generalized to all sites without reservation.