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)

Current Status (2023 Update)

The common name for this pathogen’s fruiting body is the wooly velvet polypore. We observed Onnia tomentosa at nine locations this year, all in Southcentral Alaska near Anchorage and Kenai Lake on the Kenai Peninsula associated with dead and dying spruce. Almost all fruiting bodies were attached directly to rotting tree roots. Twenty-five research grade observations of O. tomentosa were recorded in iNaturalist in 2023, in Chickaloon to the north, Palmer, Wasilla, Anchorage, Eagle River, and Cooper Landing in Southcentral Alaska, and on the Chilkat Peninsula outside of Haines in Southeast. Observations of this fungus span Interior, Southcentral, and parts of Southeast Alaska (see Detection Map). Since O. tomentosa produces fruiting structures that are both uncommon and ephemeral, iNaturalist observations enhance our understanding of this pathogen’s distribution in Alaska. A collaborative project with Drs. Patrick Bennett (Rocky Mountain Research Station) and Jane Stewart (Colorado State University) is underway to investigate Onnia species diversity. All submitted samples from Southcentral Alaska were identified via DNA analyses as Onnia tomentosa, characterized by dark brown, straight setae

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

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, robin.mulvey@usda.gov

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