Pathology > Forest Pathology > Root Disease > Tomentosus Root Disease

Tomentosus Root Disease

The causal organism, its distribution and hosts

Inonotus tomentosus fruiting body.

Inonotus tomentosus, the causal agent of Tomentosus root disease, was first described in Ontario in 1922 and is now known to be the most widespread root disease in the boreal and sub-boreal forests of the northern hemisphere. The host range is quite large with spruce (Picea) and larch (Larix) being the most severely affected species. Research on this fungus is still in its infancy.

The problem

The disease causes mortality, growth reduction, butt rot, and predisposition to windthrow and possibly insects, mainly in spruce. Results of research on growth loss due to I. tomentosus root infections, butt rots, and windthrow indicate that Tomentosus root disease is a serious economic concern for forest managers. Unfortunately it is difficult to convince forest managers of the disease impact because of the long-term nature of the problem. This is serious since the fungus can build-up in the stumps with continued cutting. Spores can pose a threat because they can spread the disease quickly to new areas. Inonotus tomentosus has been confused with another root pathogen, I. circinatus, which has confounded the knowledge of the geographical and host distribution of the fungus.

Current state of knowledge

Inonotus tomentosus can survive in stumps for up to 30 years and can spread from these stumps via root contacts and by spores to the new stand. Evidence suggests that new infections started by spores only occur within a limited distance from fungus-infected stumps.

Preliminary observations suggest that the effects of Tomentosus root disease vary with geographic region; however, relations between soil factors such as temperature, nutrient status, and moisture and pathogen spread, pathogenesis, and host response, are poorly understood.

Stumps can become colonized by the fungus after harvesting. A stump survey for decayed wood can be used to detect the fungus. In clearcuts, the fungus spreads to new hosts as young tree roots touch infected stumps. It is not known how partial cuts will affect the disease since, in this situation, roots will contact infected stumps much sooner.

Research has lead to control strategies that can be applied across Canada. Several strategies, focusing on host resistance and removal of infected stumps, have been used to control this disease. Infected stands might be clearcut and regenerated to a resistant species. In stands where there are several tree species, precommercial or commercial thinning of susceptible species can encourage establishment of more resistant species. Partial cuts are not recommended where susceptible species are still present.

Crown symptoms in mature spruce (Picea spp.) caused by Inonotus tomentosus.

Infected stumps can be removed by several methods. Stumps can be removed in conjunction with pushover tree-harvesting. Here, trees are pushed over by an excavator that leverages roots out of the soil; this is similar to a blowdown event. Infected stumps already in place can be removed with an excavator. Pop-up spacing can be done with a modified feller-buncher head that pulls whole trees, including roots, out of the soil. This method is suitable for precommercial and commercial thinning operations where the fungus is present.

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

Pertinent publications by CFS research staff

Hunt, R.S., and Unger, L. 1994. Tomentosus root disease. Canadian Forest Service, Pacific Forestry Centre. Forest Pest Leaflet 77. 7 p.

Whitney, R.D. 1977. Polyporus tomentosus root rot of conifers. Fisheries and Environment Canada, Canadian Forest Service, Great Lakes Forest Research Center. Technical Report 18. 12 p.

Whitney, R.D. 1976. Root rot spruce and balsam fir in northwestern Ontario I. Damage and implications for forest management. Department of Environment, Canadian Forest Service, Sault Ste. Marie., Ont. Report 0-X-241.

Whitney, R.D. 1964. Inoculation of eight Saskatchewan trees with Polyporus tomentosus. Canadian Department of Forestry. Bimonthly Progress Report 20(5):3.