Forest & Grassland Health

Tomentosus Root Rot

Onnia tomentosa (Fr.) P. Karst. (=Inonotus tomentosus)

Hosts in Alaska:

Primarily white and black spruce (Picea glauca and P. mariana),
but also Sitka spruce (P. sitchensis), lodgepole pine (Pinus contorta) and eastern larch (Larix larisona)


Click on the image for a larger version.

Onnia tomentosa fruiting bodies on white spruce in the Kenai NWR.

Onnia tomentosa fruiting on white spruce in the Kenai National Wildlife Refuge.

Honey-combed decay typical of Onnia tomentosa.

Honey-combed decay typical of the white rot Onnia tomentosa.

A stump with stain typical of incipient heartrot decay of Onnia tomentosa.

A stump with incipient (early) heartwood stain of Onnia tomentosa.

A tomentosus root disease center with a cluster of fallen spruce trees.

A tomentosus root disease center (a cluster of diseased, fallen spruce with substantial root decay).

Mushroom and decay of shore pine near Haines, Alaska.

Onnia tomentosa mushroom and decay of shore pine near Haines in Southeast Alaska.


Current Status (2020 Update)

We recorded five occurrences of O. tomentosa in 2020, all in Southcentral. A further eight research grade observations were made in 2020 between Anchorage and Cooper Landing in iNaturalist. The pathogen is presumed to be widespread throughout spruce stands of Southcentral and Interior Alaska. However, because the fruiting structures are ephemeral and it is difficult to confidently identify without them, it has only been confirmed and mapped in 29 locations (see Map). Post-harvest stump surveys in Interior Alaska have shown very high incidence of decay and stain symptoms consistent with Tomentosus; however, fruiting bodies of the fungus are seldom found at the time of survey. The lack of above-ground diagnostic features are obstacles to detection and inclusion in comprehensive surveys. In Southeast Alaska, this pathogen has been reported on Sitka spruce and shore pine near Skagway and Haines. Near Hoonah, Onnia circinata (=Inonotus circinatus) has been detected on shore pine.

Symptoms, Biology & Impacts

Annual fruiting bodies are produced in the late-summer and fall, developing on the ground near the base of infected trees. Fruiting bodies are leathery and small (usually less than 4 inches in diameter), and often have leaf litter embedded in them.The upper surface is distinctly felty, yellow-brown to rusty-brown with a blunt, rounded, yellowish-white margin. The lower surface is cream to yellow-brown with small round pores that extend part way down the thick central stalk. There is often brown felt along the portion of the stalk closest to the ground. The incipient/early stage of root decay is characterized by a red-brown to pink stain in the heartwood. Wood with latent/advanced decay contains elongated spindle shaped pits, with poorly defined margins, separated by red-brown firm wood. The cross section of infected roots with advanced white pocket rot decay appears honeycombed. Stump surfaces of infected trees often exhibit stain and honeycombing.

Damage includes outright mortality, butt decay of up to one third of the gross volume, premature uprooting from loss of structural roots, and growth reduction. Host trees are susceptible to infection at any age. The fungus efficiently spreads through root contacts between infected and healthy roots, thus diseased trees occur in groups and mortality results in stand openings. Following death of an infected tree, the fungus may remain alive for decades. Spruce trees planted or growing within active tomentosus root rot centers become infected through root contact with diseased stumps/roots and may be  killed outright.

Avoid planting spruce trees in root disease centers as infected stumps and roots serve as inoculum sources for succeeding generations of spruce trees. Losses can be mitigated by establishing less susceptible species (e.g., hardwoods) on infected sites, and planting susceptible species (e.g., spruce) at least 3 meters from known inoculum sources. Thinning infected stands may increase the incidence and damage of this root disease. Careful diagnosis through root inspection is important because advanced decay can be mistaken for other pocket rots, particularly Porodaedalea pini. Canada’s neighboring Yukon Territory has conducted limited surveys that suggest more than 40% of trees were infected. This root disease can be very difficult to diagnose, especially  because conks are often absent. The causal fungus was formerly known as Inonotus tomentosus.

Detection Locations in Alaska

Observations of Onnia tomentosa in Alaska as of 2020.

Georeferenced locations of Onnia tomentosa in Alaska as of 2020 with the distribution of black spruce, shore pine, Sitka spruce, and white spruce. Modeled host tree layers were developed by the Forest Health Technology Enterprise Team in 2011 (240m-resolution, presence based on dominant tree species by tree diameter). Click image for larger version.

Links and Resources

Hunt, R.S., White, T. 1998. First Report of Inonotus tomentosus, the Cause of Tomentosus Root Disease, from the Yukon Territory. Plant Dis. 82(2):264. Available here.

Lewis. K.J. 1997. Growth reduction in spruce infected by Inonotus tomentosus in central British Columbia. Can. J. For. Res. 27: 1669–1674. Available here.

Lewis, K.J. and B.S. Lindgren. 2002. Relationship between spruce beetle and tomentosus root disease: two natural disturbance agents of spruce. Can. J. of For. Res. 32:31–37. Available here.

Lewis, K.J., Trummer, L.M. and R.D. Thompson. 2004. Incidence of tomentosus root disease relative to spruce density and slope position in south-central Alaska. Forest Ecology and Management 194(1–3):159–167. Available here.

Lewis, K.J., Thompson, R.D., and L. Trummer B.S. 2005. Growth response of spruce infected by Inonotus tomentosus in Alaska and interactions with spruce beetle. Can. J. of For. Res. 35:1455–1463. Available here.

Reich, R. and K.J. Lewis. 2013. Tomentosus Root Rot Forest Health Stand Establishment Decision Aid. Journal of Ecosystems and Management 14(1): 8pp. Available here.

Trummer, L.M. 1999. Tomentosus Root Rot Alaska Forest Health Leaflet. R10-TP-80. Available here as pdf and website.

Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection,

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