Spatial and Temporal Analysis of Populations of the Sudden Oak Death Pathogen in Oregon Forests.

Sudden oak death caused by the oomycete Phytophthora ramorum was first discovered in California toward the end of the 20th century and subsequently emerged on tanoak forests in Oregon before its first detection in 2001 by aerial surveys. The Oregon Department of Forestry has since monitored the epidemic and sampled symptomatic tanoak trees from 2001 to the present. Populations sampled over this period were genotyped using microsatellites and studied to infer the population genetic history. To date, only the NA1 clonal lineage is established in this region, although three lineages exist on the North American west coast. The original introduction into the Joe Hall area eventually spread to several regions: mostly north but also east and southwest. A new introduction into Hunter Creek appears to correspond to a second introduction not clustering with the early introduction. Our data are best explained by both introductions originating from nursery populations in California or Oregon and resulting from two distinct introduction events. Continued vigilance and eradication of nursery populations of P. ramorum are important to avoid further emergence and potential introduction of other clonal lineages.

Stream Monitoring for Detection of Phytophthora ramorum in Oregon Tanoak Forests.

Stream monitoring using leaf baits for early detection of Phytophthora ramorum has been an important part of the Oregon Sudden Oak Death (SOD) program since 2002. Sixty-four streams in and near the Oregon quarantine area in the southwest corner of the state were monitored in 2008. Leaves of rhododendron (Rhododendron macrophyllum) and tanoak (Lithocarpus densiflorus) were placed in mesh bags, and bags were floated in streams. Leaf baits were exchanged every 2 weeks throughout the year. Leaves were assayed by isolation on selective medium and by multiplex rDNA internal transcribed spacer polymerase chain reaction (ITS PCR). The two methods gave comparable results, but multiplex PCR was more sensitive. P. ramorum was regularly recovered at all seasons of the year from streams draining infested sites 5 years after eradication treatment. In streams with lower inoculum densities, recovery was much higher in summer than in winter. P. ramorum was isolated from streams in 23 watersheds. When P. ramorum was detected, intensive ground surveys located infected tanoaks or other host plants an average of 306 m upstream from the bait station. P. ramorum was isolated from stream baits up to 1,091 m from the probable inoculum source.

Sudden Oak Death Caused by Phytophthora ramorum in Oregon.

Sudden oak death, caused by Phytophthora ramorum (1,2), has been found for the first time in Oregon, killing tanoak, Lithocarpus densiflorus, trees. To our knowledge, this is the first report of the disease outside of the San Francisco to Monterey area in California, (300 km to the south). Nine areas of infestation, all within a 24-km2 area, were discovered on forest lands near Brookings, in southwest Oregon. Mortality centers ranged in size from 0.2 to 4.5 ha and included 5 to approximately 40 diseased trees. P. ramorum was isolated from stem cankers using Phytophthora-selective medium. Isolates had distinctive morphological features characteristic of P. ramorum, including abundant production of chlamydospores and caducous, semipapillate sporangia on solid media. Internal transcribed spacer (ITS) sequences of isolates of P. ramorum from Oregon were identical to ITS sequences of isolates from California (1). The pathogen also was isolated from necrotic lesions on leaves and stems of native Rhododendron macrophyllum and Vaccinium ovatum growing beneath diseased tanoaks. In July 2001, the disease was located by an aerial survey conducted cooperatively by the USDA Forest Service and Oregon Department of Forestry. All lands within 1.6 km (1 mile) of the mortality centers are subject to Oregon quarantine, which bars the transport of any host plant materials. An eradication effort is currently underway. Symptomatic plants and all known host plants within 15 to 30 m of symptomatic plants are being cut and burned in the first phase of this operation. The total treated area is approximately 16 ha. References: (1) D. M. Rizzo et al. Plant Dis. In press. (2) S. Werres et al. Mycol. Res. 105:1155, 2001.

Incidence and Impact of Swiss Needle Cast in Forest Plantations of Douglas-fir in Coastal Oregon.

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.