Pathology > Forest Pathology > Root Disease > Laminated Root Rot

Laminated Root Rot (LRR)

The causal organism, its distribution and hosts

Laminated root rot, caused by the fungus Phellinus weirii, is widespread in southern British Columbia, Washington, Oregon, northern California, western Montana, and northern Idaho. The fungus is also present in Alaska and has been reported from Japan and Siberia. There are three forms of the fungus: an Asian form and North American Douglas-fir and Cedar forms. In North America the Douglas-fir form affects several conifer species (e.g., Abies, Tsuga, Picea, and Pinus spp.) while the Cedar form principally affects species of cedar. All hardwoods are immune to P. weirii infection.

The problem

Laminated root rot is one of the most damaging root disease affecting conifers in northwestern North America. The disease reduces the productivity and volume yield of commercial forests by causing mortality, growth reduction, and butt rot and by increasing the susceptibility of infected trees to windthrow and insect damage. While research on LRR has been conducted by the Canadian Forest Service (CFS) and other agencies (e.g., USDA Forest Service, universities) since 1940, there remain many questions about the disease.

Windthrown Douglas-fir in a Phellinus weirii disease center. The fungus has caused significant decay in the roots of these trees, leaving only compact root wads.

Current state of knowledge

Phellinus weirii was originally described in 1914 on western redcedar in Idaho. The first report of the fungus on Douglas-fir was in 1940 from specimens collected in the Cowichan Lake area of British Columbia. The geographic and zonal distribution of the Douglas-fir and Cedar forms of P. weirii in eastern British Columbia is poorly known.

The impact of the disease on volume production in forest stands is well documented; annual wood volume losses in British Columbia are estimated to be 1.4 million cubic metres. There has been little research on the role of P. weirii as a native inhabitant in forest ecosystems or on its interactions with other organisms such as mycorrhizae and insects. The removal of Douglas-fir overstory by P. weirii may have major effects on the diversity of the plant community.

Laminate decay in Douglas-fir infected with Phellinus weirii.

Much is known about the growth of P. weirii and spread of LRR in susceptible forest stands. The fungus first infects and kills its hosts, then colonizes and uses them as food sources. The disease begins in a stand when roots contact infected stumps or roots left from the previous stand. Phellinus weirii may survive in undisturbed stumps and large roots for up to 50 years.Phellinus weirii (Douglas-fir form) produces an annual, flattened-type fruiting body and numerous basidiospores but long range aerial spread of LRR has not yet been demonstrated.

It is known that P. weirii can colonize healthy (i.e. undamaged) root bark, but the infection process is not well understood. Susceptible hosts of any age can be killed by P. weirii, although older trees and trees infected later in life are better able to tolerate infection.

Survival and spread of P. weirii and damage attributable to LRR are likely influenced by site conditions. Soil factors affecting LRR, including temperature, pH, and moisture content, have been investigated. As yet no strong evidence exists that any individual site factor (or group of factors) is a reliable predictor of either the presence or intensity of LRR in a given stand.

In maturing coastal forests where P. weirii is active, diseased trees generally occur in centers around inoculum sources. Dead and dying trees are innermost in the center; trees with crown symptoms occur as one moves away from the mid-point of the center, and infected but asymptomatic trees occur at the outer margins of the center. The actual area affected by LRR may be double that revealed by crown symptom trees.

Infection and mortality caused by Phellinus weirii in a juvenile stand of Douglas-fir. Note the scattered dead (red) and infected (yellowing) trees.

Disease dynamics of LRR following harvest will be determined by the harvesting system used and whether or not some mode of disease management is applied to the cut stand. The effects of harvesting systems, particularly alternative silvicultural systems such as partial cutting, on the disease dynamics of LRR, are not known. The effects of silvicultural treatments such as thinning and spacing in LRR-affected plantations and maturing stands also are not known. A concept derived in British Columbia known as 'bridge tree removal' proposes that the removal of healthy, susceptible hosts around disease centers in maturing stands will prevent the spread of P. weirii. The bridge tree removal concept has not been definitively proven.

Management techniques for LRR are best applied at the time of harvest. Planting of species not susceptible to P. weirii is documented in the literature and is a practice well accepted by foresters. The efficacy of stump removal (stumping) and push-falling in reducing P. weirii inoculum also has been tested and the results of these trials have been published.

Biological control agents for P. weirii such as Trichoderma viride have been tested in the laboratory and in the field but a suitable field delivery system remains to be developed.

Chemical inactivation of P. weirii inoculum with fumigants such as chloropicrin has been demonstrated, but their use will likely be severely restricted because of current attitudes and policies about the use of pesticides in forests.

Genetic-based resistance is another possible management strategy for LRR. Empirical evidence of differential resistance of Douglas-fir to attack by P. weirii has been reported for many years and has recently been demonstrated for Douglas-fir seedlings from British Columbia's coastal Douglas-fir tree improvement program.

More information on this particular root disease is available on the Common Tree Diseases of British Columbia web site.

Posters

Pathology: Detection of a chitinase-like protein in the roots of infected Douglas-fir trees

Pathology: Screening Coastal Douglas-fir for Resistance to the Laminated Root Rot Pathogen, Phellinus weirii

Pathology: Resistance of Coastal Douglas-fir to Phellinus weirii: an update on research activities

Pertinent publications by CFS research staff

Nelson, E.E., and Sturrock, R.N. 1993. Susceptibility of western conifers to laminated root rot (Phellinus weirii) in Oregon and British Columbia field tests. West. J. Appl. For. 8: 67-70.

Robinson, R. M., Sturrock, R.N., Davidson, J.J., Ekramoddoullah, A.K.M., and Morrison, D.J. 2000. Detection of a chitinase-like protein in the roots of Douglas-fir trees infected with Armillaria ostoyae and Phellinus weirii. Tree Physiol. 20:493-502.

Sturrock, R.N. 2000. Management of root diseases by stumping and push-falling. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, B.C. Technology Transfer Note 16. 8 p.

Sturrock, R.N., and Fraser, R.G. 1994. Commercial thinning in coastal forest stands infested with laminated root rot: preliminary guidelines. Forestry Canada, B.C. Ministry of Forests, Canada-British Columbia Partnership Agreement on Forest Resource Development: FRDA II. FRDA Memo No. 219.

Sturrock, R., and Garbutt, R. 1994. Laminated root rot of Douglas-fir. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Forest Pest Leaflet 3. 8 p.

Sturrock, R.N., Phillips, E.J., and Fraser, R.G. 1994. A trial of push-falling to reduce Phellinus weirii infection of coastal Douglas-fir. Forestry Canada, B.C. Ministry of Forests, Canada-British Columbia Partnership Agreement on Forest Resource Development: FRDA II. FRDA Report 217. 22 p.

Sturrock, R.N., and Reynolds, G. 1998. A new technique for inoculation of conifer seedling roots with the laminated root rot pathogen Phellinus weirii. Can. J. Plant Pathol. 20: 324-330.

Thies, W.G., and Sturrock, R.N. 1995. Laminated root rot in western North America. USDA Forest Service, Pacific Northwest Research Station General Technical Report PNW-GTR-349. 32 p. In cooperation with: Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre.

Thomson, A.J., Goodenough, D.G., Barclay, H.J., Lee, Y.J., and Sturrock, R.N. 1996. Effects of laminated root rot (Phellinus weirii) on Douglas-fir foliar chemistry. Can. J. For. Res. 26: 1440-1445