Six research grade observations of Onnia tomentosa were submitted to iNaturalist in 2021, all in the vicinity of Anchorage. Over the last two years, 18 iNaturalist observations spanned across Southcentral and the Interior, from Anchorage to Cooper Landing, complementing FHP detections farther west on the Kenai, the Chugach Mountain, and near Fairbanks (see Detection Map). Since O. tomentosa produces fruiting structures that are both uncommon and ephemeral, iNaturalist observations could enhance our understanding of this pathogen’s distribution in Alaska. In Southeast Alaska, we have found this fungus on Sitka spruce and shore pine near Skagway and Haines. Near Hoonah, Onnia circinata (=Inonotus circinatus) has been detected on shore pine.

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.