An epidemic of Swiss needle cast, caused by the ascomycete Phaeocryptopus gaeumannii, is causing defoliation and growth reductions in Douglas-fir forest plantations along the Oregon Coast. The area of symptomatic plantations has been monitored annually since 1996 by aerial survey; in spring 1999, 119,500 ha were affected. Pathogen and symptom development have also been monitored on nine permanent plots in stands of differing disease severity. Infection levels and symptom severity are greatest in low elevation plantations close to the coast. In areas of severe disease, trees retain only current year needles. Defoliation is proportional to the number of stomata occluded by pseudothecia of the fungus, with needles being shed when about 50% of stomata are occupied, regardless of needle age. Fungus sporulation and premature needle abscission are greatest on the upper branches of trees. Annual application of fungicides increases needle retention significantly. Tree height and diameter growth and total tree volume are reduced by disease, and tree volume is significantly correlated with needle retention on our plot trees. The epidemic continues to be most severe in Douglas-fir plantations established on sites where Sitka spruce and western hemlock or red alder predominated in earlier times.

Little is known about indigenous Phytophthora species in natural ecosystems. Increasing evidence, however, suggests that a diverse, trophically complex Phytophthora community is important in many forests. The number of described species has steadily increased, with a dramatic spike in recent years as new species have been split from old and new species have been discovered through exploration of new habitats. Forest soil, streams, and the upper canopies of trees are now being explored for Phytophthora diversity, and a new appreciation for the ecological amplitude of the genus is emerging. Ten to twenty species are regularly identified in temperate forest surveys. Half or more of this Phytophthora diversity comes from species described since 2000. Taxa in internal transcribed spacer (ITS) Clade 6 are especially numerous in forest streams and may be saprophytic in this habitat. Three ecological assemblages of forest Phytophthora species are hypothesized: aquatic opportunists, foliar pathogens, and soilborne fine-root and canker pathogens. Aggressive invasive species are associated with all three groups.

Phytophthora ramorum is an invasive pathogen in some mixed-hardwood forests in California and southwestern Oregon, where it causes sudden oak death (SOD) on some members of Fagaceae, ramorum shoot dieback on some members of Ericaceae and conifers, and ramorum leaf blight on diverse hosts. We compared symptoms of P. ramorum infection resulting from four different artificial inoculation techniques with the symptoms of natural infection on 49 western forest trees and shrubs; 80% proved susceptible to one degree or another. No single inoculation method predicted the full range of symptoms observed in the field, but whole plant dip came closest. Detached-leaf-dip inoculation provided a rapid assay and permitted a reasonable assessment of susceptibility to leaf blight. Both leaf age and inoculum dose affected detached-leaf assays. SOD and dieback hosts often developed limited leaf symptoms, although the pattern of midrib and petiole necrosis was distinctive. Stem-wound inoculation of seedlings correlated with field symptoms for several hosts. The results suggested that additional conifer species may be damaged in the field. Log inoculation provided a realistic test of susceptibility to SOD, but was cumbersome and subject to seasonal variability. Pacific rhododendron, salmonberry, cascara, and poison oak were confirmed as hosts by completing Koch's postulates. Douglas-fir was most susceptible to shoot dieback shortly after budburst, with infection occurring at the bud.

We followed the local intensification and dispersal of Phytophthora ramorum Werres, De Cock, & Man In’t Veld in Oregon tanoak ( Lithocarpus densiflorus (Hook & Arn.) Rehd.) forests from its initial detection in 2001 through 2006, coincident with a continuing eradication effort. The initial infested area included nine scattered sites below 400 m elevation, close to the Pacific Ocean near Brookings, Oregon. In subsequent years, one-half of new infections were within 122 m of a previous infection, and 79% of the newly detected trees occurred within 300 m of a previously identified tree. Dispersal up to 4 km was occasionally recorded. Initial infection occurred in the upper crowns of tanoak trees. The pathogen was recovered in rainwater collected beneath diseased tanoak trees in every month from November 2006 through October 2007. Twenty-four multilocus microsatellite genotypes were identified among 272 P. ramorum isolates collected from Curry County. Genotypic analysis provided independent estimates of time of origin of the Oregon infestation, its clustered distribution, and dispersal distances. In all sampling years, 60%–71% of the isolates belonged to the same multilocus genotype. In 2001, 12 genotypes were detected and new genotypes were identified in each of the subsequent years, but all isolates belonged to the same clonal lineage. Knowledge of local intensification of the disease and long-distance dispersal should inform both Oregon eradication efforts and national quarantine regulations.