What is laminated root rot disease?

A 40-acre section of forest surrounding Walton Lake is temporarily closed while the Forest Service figures out the best approach to deal with the diseased trees and provide for public safety. Here is an explanation of the disease from our forest pathologist:

Laminated root rot is considered the most damaging root disease of forest trees in Oregon and Washington, and because of that, it is also one of the most hazardous to people and property in popular developed recreation areas. This native tree disease is caused by a fungus (Phellinus sulphurascens) that rots/decays the root system of various conifers. The disease gets its name from the decayed appearance of wood in the roots and base of trees that are infected, which have thin, laminated sheets of wood that separate at the annual rings. Trees that are highly susceptible to laminated root rot can be killed and include conifers like Douglas-fir, white and grand firs, and mountain hemlock. This root disease occurs in parts of Oregon, Washington, British Columbia, Idaho, western Montana and northern California. On the Ochoco National Forest, it is more common in mixed conifer settings with Douglas-fir and grand fir in northern portions of the Forest.


Once present on a site, the fungus that causes laminated root rot grows along the roots of susceptible trees and spreads by root-to-root contact underground from one live tree to the next. The fungus grows about a foot or two a year on roots. However, it does not spread through roots of dead trees much at all. The fungus spreads very rarely by windblown spores. As the roots decay, an infected tree loses its anchor and may die standing before falling over, or live trees can break at the base due to decay and fall without warning. Even though the strength of an infected tree’s roots is greatly reduced, that tree may not show aboveground symptoms maintaining green foliage until half or more of the root system is affected. Because live trees can fall without warning, in addition to dead trees falling with decay, this disease is a serious safety concern in popular developed recreation areas.


The fungus can survive in decaying roots and stumps of larger trees for 50 years or more in some cases. Laminated root rot is considered a “disease of the site” because once established, it can remain long-term if enough susceptible hosts exist. Fire does not usually eliminate the fungal pathogen because it does not get hot enough underground to kill the fungus in colonized roots.


The abundance of susceptible trees affects the impact of this disease where it occurs. Deciduous or broadleaf trees (hardwoods) are immune and some conifers like ponderosa pine are resistant to infection. Other conifers are considered tolerant of laminated root rot and some have intermediate susceptibility compared to highly susceptible Douglas-fir, grand fir and other true firs (Abies species). Laminated root rot can kill trees of all sizes and create pockets or centers of on-going tree mortality up to many acres in size. Mortality centers grow and expand about a foot a year.


Openings in the forest canopy associated with root disease can enhance diversity. In some forests, a variety of plants regenerate in openings created by root disease and provide habitat for different organisms. Standing dead trees and down wood created as a result of laminated root rot provide wildlife habitat. The disease also influences fuel loading in the event of a fire. There can be more surface fuels from dying trees and those falling over where the disease is widespread. Losses in timber volume can result from laminated root rot as well. Since this native disease can create both desirable and undesirable effects, whether or not to manage the disease depends on the setting.


Depending on management objectives, there are options to curb laminated root rot in forest stands and recreation areas to reduce adverse effects associated with the disease. When enough trees are present that are immune, resistant or tolerant of the disease, those species can be favored and the highly susceptible tree species could be removed. Logging does not spread or exacerbate laminated root rot. Removing stumps of infected trees to reduce levels of the fungal pathogen on a site is expensive, requires additional equipment that could potentially result in more soil disturbance, and typically is not done in the U.S. When the disease is present in stands with an abundance of trees that are highly susceptible, a common approach to promote a healthy stand long-term is to identify the boundary of where the disease exists and remove all highly susceptible trees where it occurs and a buffer of at least 50 feet beyond where the disease was confirmed to stop its spread. Then the site is planted with tree species that are much less susceptible to laminated root rot to provide lasting vegetation. Another option in recreation areas is to close a portion of the area affected or close the entire area due to unacceptable risk to public safety. 


In developed recreation areas, management of laminated root rot is important for long-term prevention of future hazard trees and to maintain safe public access for enjoyment of recreational opportunities. Where the public is invited to enjoy the great outdoors safely in developed recreation areas, land managers have a responsibility to minimize hazards and reduce the risk of property damage and injury from occurring. This includes reducing the risk of trees posing significant hazards. There are many past examples of State and City Parks along with federal campgrounds in parts of Oregon, Washington, Idaho and British Columbia having to close portions of recreation areas for a period of time to address safety concerns related to the presence of laminated root rot. Careful planning where laminated root rot occurs is necessary to manage this disease in developed recreation areas in the Northwest. To maintain public access, land managers often choose to remove infected trees at risk of falling and other highly susceptible trees where the disease has been confirmed. Then planting occurs with tree species that will not succumb to the disease and to promote healthy vegetation into the future.