Current Extent and Impacts of Swiss Needle Cast in Pacific Northwest Forests

Fig. 1 - Annual variation in surveyed SNC area

Since around 1990, unusually severe and persistent symptoms of Swiss needle cast have been observed in Douglas-fir forest plantations in western Oregon, particularly near the town of Tillamook (Hansen and others 2000). Annual aerial surveys conducted since 1996 by the Oregon Department of Forestry have documented the disease on about 150,000 ha of forest land in the Oregon Coast Range (Figure 1). Unlike Boyce’s (1940) characterization of the pathogen on native Douglas-fir as being inconspicuous and harmless, the fungus is abundant, trees frequently are defoliated of all but current-year needles, and attached foliage is often severely chlorotic (Figure 2). Growth reductions of 20 to 50 percent due to Swiss needle cast have been measured in the affected area (Maguire and others 2002). The severity of the problem in Oregon has brought renewed interest in understanding the biology of the pathogen and epidemiology of Swiss needle cast disease. In particular, research has focused on understanding why an inconspicuous, insignificant native pathogen has become a significant forest health problem.

Fig. 2 - Symptoms of Swiss needle cast

Aerial surveys for Swiss needle cast conducted by the Oregon Department of Forestry have classified patches of Swiss needle cast severity based on foliage discoloration, characterizing the discoloration as being severe or moderate. The affected area as determined by the aerial survey lies along the entire length of the Oregon coast, extending inland about 40 km, with most symptoms occurring within 30 km of the coast. The crest of the Coast Range forms the approximate eastern edge of the affected area. The aerial survey covers about 1.2 million ha of coastal forest, with the symptomatic area comprising between 50 and 160,000 ha (Figure 1).

Disease severity (foliage retention, discoloration, crown sparseness, pathogen abundance) has also been monitored annually in permanent plots in the Oregon Coast Range (Hansen and others 2000) equipped with temperature and leaf wetness dataloggers. Foliage retention, and abundance of P. gaeumannii ascocarps on 1- and 2-year-old needles have been monitored annually since 1996 in 9 to 12 Douglas-fir stands, initially aged 12 to 15 years. Study sites were selected to represent a range of elevations, distance from maritime influence, and disease severity. Within the area of severe disease, symptom severity is variable, but all Douglas-fir show some effects of the disease compared to healthy stands on the eastern slope of the Coast Range and in the Cascade Range. Normal needle retention in healthy coastal form Douglas-fir is about 4 years. Within the epidemic area, needle retention varies from about 1.5 to 2.6 years (Hansen and others 2000). Although symptom severity, (e.g., foliage discoloration, needle retention) for all sites varies from year to year, relative disease severity, (i.e., abundance of P. gaeumannii ascocarps on 1- and 2-year-old foliage) is fairly consistent. Disease tends to be more severe nearer the coast, at lower elevations, and on southern aspect slopes, gradually diminishing to the east (Hansen and others 2000, Manter and others 2003, Rosso and Hansen 2003).

The Swiss needle cast epidemic area appears to correspond approximately to the Sitka spruce vegetation zone, a narrow strip of coastal forest characterized by elevations generally below 150 m, proximity to the ocean, a moderate climate, and a distinct forest type (Franklin and Dyrness 1973). Although Douglas-fir is considered the early seral dominant in the western hemlock vegetation zone, which borders the Sitka spruce zone to the east, its occurrence within the Sitka spruce zone is more sporadic. There Douglas-fir occurs mainly in mixtures with Sitka spruce and western hemlock, but normally not as pure stands as is typical of early postfire succession in the western hemlock zone (Franklin and Dyrness 1973).

The correspondence between the Sitka spruce zone and the main Swiss needle cast epidemic area suggests that environmental factors that influence the development of different plant associations in western Oregon may also influence the severity of Swiss needle cast. Irruption of the disease in the coastal zone may be due to changes in abundance of Douglas-fir in coastal forests due to forest management practices that favor the cultivation Douglas-fir over other species (Hansen and others 2000). Swiss needle cast itself may be one such environmental determinant of the natural distribution of Douglas-fir in the western Coast Range, making it an inferior competitor where Swiss needle cast is severe (Hansen and Stone 2005).

But what factors affect the distribution and abundance of P. gaeumannii? It has long been suspected that local climate plays a key role in the pathogenicity of P. gaeumannii. Boyce (1940) suggested that seasonal patterns in local climate could differentially affect fungal growth and development, and this might explain the greater virulence of P. gaeumannii in Europe and the Eastern U.S. compared to the area where both P. gaeumannii and Douglas-fir are native. Within the Douglas-fir zone of the Pacific Northwest, there is also a relationship between disease severity and local climate. Hood (1982) found more P. gaeumannii in southern British Columbia and western Washington in coastal forests of Vancouver Island and the Olympic Peninsula, with lower levels in the rain shadow of eastern Vancouver Island and the interior, and attributed the difference mainly to precipitation patterns. More severe disease symptoms and greater fungal colonization are commonly observed on lower elevation sites near the coast, suggesting the possible involvement of maritime fog (Rosso and Hansen 2003).

Rosso and Hansen (2003) used a visual index of stand disease to investigate the relationship between several environmental variables and Swiss needle cast severity. The best model for predicting stand disease rating (a composite index of several factors) was a regression model that included July temperature, precipitation, and fog occurrence, which explained about 60 percent of the variation. Manter and others (2005) found a strong relationship between winter (Dec.-Feb.) mean daily temperature, spring (May- July) leaf wetness, and P. gaeumannii abundance.