By Norma Kline, Oregon State University Extension Forester for Coos and Curry Counties
If you live or work in southern Oregon, the dead and dying Port-Orford-cedar (Chamaecyparis lawsoniana) along Highway 101 are hard to miss. The culprit behind this widespread mortality of Port-Orford-cedar is an introduced invasive pathogen called Phytophthora lateralis. Phytophthora lateralis causes Port-Orford-cedar root disease. Phytophthora means plant destroyer in Greek and is part of a larger order of microorganisms called Oomycetes or water molds. These pathogens are related to fungi, but do not develop obvious fruiting bodies. If the genus Phytophthora sounds familiar, it is because a related pathogen called Phytophthora ramorum causes sudden oak death. Phytophthoras thrive in the moist conditions in coastal Oregon. There are many other notable Phytophthoras including Phytophthora infestans, the pathogen that causes late blight of potato and tomato.
Port-Orford-cedar root disease was first detected in nursery plants in the 1920’s and was identified in natural forests in Coos County in the 1950’s. Since that time, Port-Orford-cedar root disease has spread across much of the native range of Port-Orford-cedar. While the primary host in Oregon for Phytophthora lateralis is Port-Orford-cedar, Pacific Yew (Taxus brevifolia) can also be infested. Fortunately, the disease does not infest our other native forest species. For example, the pathogen will not spread from an infested Port-Orford-cedar to a neighboring Douglas-fir (Pseudotsuga menziesii) or Western-redcedar (Thuja plicata).
Unfortunately, Phytophthora lateralis is easily transported on muddy road building machinery or maintenance equipment and even shoes. When the pathogen is moved to a new area, it typically first infests tree roots and then progresses up to the base of the tree where it girdles and kills the tree. The pathogen then progresses from one Port-Orford-cedar to the next via interconnected roots. A common series of events is for an infestation to start with trees at the edge of a road or driveway and then slowly spread into the adjacent forest. The disease seems to travel in waves across the landscape.
What do you do if you observe yellow and brown foliage on a Port-Orford-cedar? First, confirm that the tree is indeed Port-Orford-cedar. Western redcedar and Port-orford-cedar can look similar from a distance. Cones, if you can find them, are the best diagnostic tool. Both Port-Orford-cedar cones and western redcedar cones are very small (about ½ inch in size). Port-Orford-cedar cones are round (Figure 1) while Western redcedar cones are shaped like a rosebud, and bend up and back towards the tree (Figure 2). The foliage and bark also have characteristic differences. Turn the foliage over to observe the “X” shape pattern on Port-Orford-cedar or the butterfly or bowtie shape distinctive of Western redcedar (Figures 3 and 4). The bark can also be helpful in differentiating between the two species; Port-Orford-cedar has thicker more fibrous bark than the finer textured bark of western redcedar.
Once you have determined that the tree in question is a Port-Orford-cedar, then Port-Orford-cedar root disease might be responsible for the observed decline and mortality. On a newly dead tree, you can investigate further by carefully using a hatchet to remove the outer bark near the base of the tree. Port-Orford-cedar root disease causes a characteristic stain on the cambium (the living tissue beneath the bark) (Figure 5). Just remember, it’s not a good idea to chop into a living tree as it can expose the tree to pathogens or insects. If you see areas of loose bark on a dead tree, you might notice insect galleries (tunnels) under the bark. In many cases, these are indications of beetles that aid in wood decomposition. These “secondary” beetles invade trees that are already dead and dying. You might also see signs of cedar bark beetles, which are also considered a secondary beetle (as they are not the primary cause of tree mortality). Cedar bark beetles are attracted to stressed cedars (such as Port-Orford-cedars stressed from root disease), and stressed junipers and redwoods. Cedar bark beetle galleries are distinctive (Figure 6).
Port-Orford-cedar root disease is difficult to treat. Some landowners choose to remove obviously infested Port-Orford-cedar as well as nearby apparently healthy Port-Orford-cedar in order to disrupt disease spread from interconnected roots. This is not likely to be effective in areas where the disease is widespread. In which case, property owners will need to assess safety and fuels hazards created by dead and dying trees. Leaving some snags (dead trees) in the landscape is beneficial for both cavity nesting birds and bats. Likewise, downed logs serve as habitat for amphibians and benefits soil structure as the logs slowly rot.
If Port-Orford-cedar root disease symptoms are caught very early in the disease progression, a homeowner might opt to have a notable Port-Orford-cedar in the home landscape treated with fungicides. Continued treatments would be required to keep the infestation in check. This approach is not likely to be practical in situations where large numbers of trees are infested. Read more about chemical treatments in the PNW handbook (see References).
Luckily there is good news. Disease resistant Port-Orford-cedar seedlings are available for landowners interested in restoring Port-Orford-cedar to the landscape. Disease resistant Port-Orford-cedar are the result of a long term and very successful breeding program at the USFS Dorena Genetic Nursery in southern Oregon. Landowners interested in planting disease resistant Port-Orford-cedar in Coos or Curry Counties should contact their extension forester (norma.kline@oregonstate.edu). Just remember that planting season in western Oregon is in the rainy winter months.
As always, use herbicides safely! Wear protective clothing and safety equipment as recommended on the label. Bathe or shower after each use. Read the pesticide label – even if you have used the pesticide before. Follow the instructions on the label (and any other directions you have). Be cautious when you apply pesticides. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use. Use herbicides responsibly.
References
Port-Orford-Cedar Root Disease Leaflet https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5346825.pdf
PNW Handbook – Phytopthora lateralis treatment options
