Despite its name, Douglas fir (Pseudotsuga menziesii) is a conifer -- an evergreen -- but not a fir. It belongs to its own genus, Pseudotsuga, meaning hemlock-like, in the Pineaceae, or pine, family. This magnificent, fragrant tree grows as tall as 300 feet. Oregon’s state tree provides dense softwood to build Western homes and a favorite Christmas tree for the entire country. The tree thrives in the cool, moist forests of the Pacific Northwest.
Fungus Among Douglas Firs
Evergreen plants face a host of possible pathogens when they live in moist environments. Some are life threatening, but the organism will adapt to many of them over time. While the latter group of pathogens may disfigure the plant or weaken it against other infections, the plant typically survives the disease. The American Phytopathological Society identifies three dozen fungal pathogens that affect Douglas fir. Five of those -- Annousis and Armillaria root rots, Cytospora and upper stem canker, and damping off -- can result from several pathogens. Douglas fir even has a parasite, dwarf mistletoe (Arceuthobium douglasii “Engelmann”).
Three diseases pose major hazards to Douglas firs. The first, rhabdocline needlecast, is caused by Rhabdocline pseudotsugae or Rhabdocline weirii fungus. The spores travel on wind during rainy weather, beginning in late fall or early winter and becoming evident as yellow spots on needles in spring. The spots grow and turn reddish-brown, eventually falling off the branch. Good air circulation helps fight the spread of Rhabdocline needlecast. Spray trees with a fungicide containing chlorothalonil as soon as green buds appear at the ends of branches in late winter. Trees may require as many as four applications when spring weather is cold and wet.
Phaeocryptopus gaeumannii causes Swiss needlecast. Oregon State University researchers Bryan Black, David Shaw and Jeffrey Stone suggest that a warming climate may be responsible for the spread of the disease in Pacific Northwest forests. Lines of black fruiting spores appear on the underside of the flat Douglas fir needles in early spring and disperse during wet weather. The disease causes year-old needles to turn brown and drop off the tree and stunting trees. Good air circulation and quarantine of new trees, the most common source of Phaeocryptopus gaeumannii, help minimize risk. Spray with chlorothalonil in spring as soon as new shoots reach 1/2 to 2 inches in length and repeat in 14 to 21 days. Two years of treatment may control an outbreak if no new infected stock is introduced to the area.
Sudden Oak Death
In 1995, sudden oak death was discovered in Marin County in California. The disease, caused by the soil-borne pathogen Phytophthora ramorum, has spread to tanoaks and other species along the Pacific coast. In 2002, it was discovered affecting young Douglas fir trees. The disease appears as cankers on branches and foliage lesions, eventually killing the tree. So far, quarantines of affected plants and soil are the only known management strategies.
- U.S. National Arboretum: State Trees
- Tillamook Forest Center: Douglas Fir
- Durango Nature Studies: How to Identify Colorado's Native Evergreens? Shake Hands
- American Phytopathology Association: Diseases of Douglas Fir
- Penn State College of Agricultural Sciences: Douglas Fir Diseases
- U.S. Forest Service: How to Identify and Control Rhabdocline and Swiss Needlecasts of Douglas-Fir
- Oregon Live.com: Douglas Fir Disease Spreading Along Oregon and Washington Coasts
- Forest Pathology: Sudden Oak Death
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