Persistence of the Swiss Needle Cast Outbreak in Oregon Coastal Douglas-Fir and New Insights from Research and Monitoring

Swiss needle cast (SNC), caused by Nothophaeocryptopus gaeumannii, is a foliage disease of Douglas-fir (Pseudotsuga menziesii), that reduces growth in native stands and exotic plantations worldwide. An outbreak of SNC began in coastal Oregon in the mid-1990s and has persisted since that time. Here we review the current state of knowledge after 24 years of research and monitoring, with a focus on Oregon, although the disease is significant in coastal Washington and has recently emerged in southwestern British Columbia. We present new insights into SNC distribution, landscape patterns, disease epidemiology and ecology, host-pathogen interactions, trophic and hydrologic influences, and the challenges of Douglas-fir plantation management in the presence of the disease. In Oregon, the SNC outbreak has remained geographically contained but has intensified. Finally, we consider the implications of climate change and other recently emerged foliage diseases on the future of Douglas-fir plantation management. Study Implications: Douglas-fir tree growers need to consider Swiss needle cast (SNC) and other emerging foliage diseases as SNC has not abated over the past 24 years, and along with other emerging diseases, it continues to pose a threat to Douglas-fir plantation productivity. Douglas-fir management in western Oregon remains important, such that a knowledge of disease impacts and effective silvicultural responses is key. Managers should carefully consider whether alternative species may be ecologically or economically beneficial in some situations while tree improvement programs must continue to breed for tolerance to SNC. Research shows that regional scale foliage disease outbreaks can result in trophic cascades and hydrologic changes that affects more than just the trees. The environmental controls on the SNC epidemic imply that climate change could strongly influence future directions of the outbreak, with the greatest threats to trees at higher elevations.

Different radial growth rate effects on outerwood properties of coastal Douglas-fir in healthy trees vs. trees impacted by Swiss Needle Cast

A common belief is that rapid growth in coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) results in decreased outerwood quality. In Oregon, the opposite pattern has been reported for stands with Swiss Needle Cast, in which a native fungus causes pre-mature needle drop and an increase in latewood proportion (LW%), wood density, stiffness (MOE), and strength (MOR). Using a combination of new and existing data, we compared properties of outerwood of about 25 yrs age from 18 healthy and 14 diseased stands with direct tests (6-8 beams from 7-12 trees/stand, 2 614 beams total) and indirect SilviScanII tests (1 sample for each of the 366 trees). As seen before, diseased stands showed a decrease in wood quality with growth rate: ring count was strongly and positively correlated with density, MOE, and MOR (r2 = 0.74, 0.65, and 0.63), and LW% was positively correlated with ring count, density, MOE, and MOR (r2=0.50, 0.62, 0.30, and 0.44). In contrast, healthy stands had no significant effect of ring count on density, MOE, or MOR. LW% was weakly and significantly correlated with MOE (r2=0.25) but not with ring count, density, or MOR. Among healthy stands, growth acceleration had no adverse effects on outerwood properties.

Douglas-fir foliage retention dynamics across a gradient of Swiss needle cast in coastal Oregon and Washington

Swiss needle cast (SNC) is an important foliage disease of Douglas-fir (Pseudotsuga menziesii) caused by the native pathogen Nothophaeocryptopus gaeumannii, that has been present in epidemic proportions since the 1990s in coastal Douglas-fir forests. Under conducive environmental and stand conditions, the fungal fruiting bodies emerge on young needles, inhibiting gas exchange and causing premature needle casting and subsequent growth losses. Using a new regional plot network, which extends and approximately doubles the area of SNC-susceptible coastal forest sampled, we investigated the distribution of SNC disease indices across the region, and throughout individual tree crowns. Foliage retention varied from 1.15 to 3.9 years and disease severity (incidence x % occluded stomata) ranged from 0.05 to 52.11%. Foliage retention was positively correlated with distance from the coast and elevation. Foliage retention and disease severity were found to be negatively associated across the study area. Within crowns, disease severity was negatively associated with crown depth, and foliage retention was positively associated with crown depth, regardless of distance from coast. Across the entire study, foliage retention was found to decrease and disease severity increase with latitude, all else being equal. Tree growth metrics are positively associated with increasing foliage retention, and normal growth occurs greater than ~3.2 years.