Cedar, Western Red (Thuja plicata)-Yellow Root Rot

Latest revision: 
March 2023

Cause Perreniporia subacida, a fungus, causes yellow root rot of several conifer species in Oregon and Washington. It is usually found on suppressed or weakened trees. It is rarely seen in vigorous trees. Hosts other than western red cedar include Douglas-fir, western hemlock, mountain hemlock, lodgepole pine, larch, and grand fir. Most infection results from contact by colonized roots with susceptible roots. Tree roots that are severely stressed appear to be most susceptible to infection and decay. The role of spores in initiating new infections is not known.

Symptoms Stressed trees die and may have minor amounts of butt rot. Advanced stages of wood decay have irregularly shaped pockets in the spring wood that run together, forming masses of stringy fibers. Small black flecks may appear in the pockets or masses of fibers. The annual rings separate easily and may be confused with laminated root rot.

Appressed conks are white when fresh and become cream to dirty yellow-orange as they age. They may be up to half-inch thick and can vary from a few inches to 2 or 3 feet long. Conks are found on the underside of root crotches or exposed roots and under fallen trees.

Cultural control Damage has not been serious enough for treatment techniques to have been developed.

  • Remove affected trees during harvesting operations.
  • Favor tree species not showing damage when thinning or planting sites with yellow root rot.

References Hadfield, J. S., Goheen, D. J., Filip, G. M., Schmitt, C. L., and Harvey, R. D. 1986. Root Diseases in Oregon and Washington Conifers. USDA, Forest Service, Pacific Northwest Region, Forest Pest Management, Portland, Oregon. R6-FPM-250-86.