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 and exotic plantations worldwide. An outbreak began in coastal Oregon in the mid-1990’s 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 emerged in SW British Columbia. We have new insights into the spread of SNC, landscape patterns, epidemiology, ecology, host-pathogen interactions, as well as trophic and hydrologic influences, while silviculture and management remain nuanced and closely associated with local site conditions. In Oregon, the SNC outbreak has remained geographically contained, but has intensified within the epidemic area. Finally, we consider the implications of climate change and recently emerged foliage diseases on the future of Douglas-fir plantation management.

Impact/Purpose

Swiss needle cast is a plant disease caused by a fungus which adversely affects the needle-like leaves of Douglas-fir trees in the Pacific Northwest, USA. The disease causes trees to prematurely shed, or cast, its needles. An outbreak began in coastal Oregon in the mid-1990’s and has persisted since that time. This manuscript reviews the current state of knowledge after 24 years of research and monitoring, with a focus on Oregon. The manuscript provides new insights into the spread of SNC, landscape patterns, epidemiology, ecology, host-pathogen interactions, as well as trophic and hydrologic influences. The objectives of this research project are to confirm that reduced needle retention is related to increased disease severity, determine which portion of the tree crown best indicates disease severity, develop a map of foliage retention for the Oregon Coast Range based on observations of leaf loss and factors such as land elevation. The relationships between needle retention, tree crown level, disease severity, and land elevation were examined. A spatial analysis of needle retention and elevation revealed that foliage retention increased with increasing elevation. A needle retention map for the Oregon Coast Range will aid managers in estimating the potential for disease impacts and help ensure that Douglas-fir trees remain a productive component of the coastal forests of Oregon and Washington in the presence of Swiss needle cast disease. This research will contribute to the objectives of EPA’s Air Climate and Energy (ACE) research program under the Climate Impacts, Vulnerabilities and Adaptation Tree Mortality Survey Task (CIVA-2.4) by developing forest indicators of climate-related changes to forest growth, mortality, and pest, disease and fire disturbances.

Citation

Shaw, D., G. Ritóková, Y. Lan, D. Mainwaring, A. Russo, R. Comeleo, S. Navarro, D. Norlander, AND B. Smith. Persistence of the Swiss Needle Cast Outbreak in Oregon Coastal Douglas-fir, and New Insights from Research and Monitoring. Society of American Foresters, Bethesda, MD, 119(4):407-421, (2021). [DOI: 10.1093/jofore/fvab011]

DOI: Persistence of the Swiss Needle Cast Outbreak in Oregon Coastal Douglas-fir, and New Insights from Research and Monitoring