Onnia tomentosa

The cap is the most useful identifying feature: flat when young, with a blunt, rounded, yellowish-white margin, and later developing a slightly depressed center with a wavy, contoured rim that becomes sharp-edged with age. It grows up to 10 cm across and is covered in felt-like fuzz that shifts in color from grey in youth to rusty brown as it matures.

The stem, when present, is short, thick, and dark brown to near-black, though the caps may also grow directly from the base of the tree without a distinct stalk. The flesh beneath the cap surface is ochre brown, softer above and hard and fibrous below.

Onnia tomentosa is frequently found in coniferous forests at higher altitudes, typically growing in large groups, and is comparatively rare at lower elevations. As a plant pathogen, it causes tomentosus root rot primarily in spruce, with white spruce and black spruce identified as the most susceptible species in inoculation testing carried out in Saskatchewan.

In Canada, the fungus attacks most commercially important conifers, and its impact has been documented in white spruce plantations such as those at Grand-Mère, Quebec, where it contributed to high losses. In white spruce plantations generally, mortality in clusters of two or three trees tends to occur around age 30 to 35, though younger trees, occasionally as young as 10 years old, can also be killed.

A six-year study of seventeen white spruce plantations aged 43 to 58 years in Ontario recorded average annual mortality of 0.7%, with accumulated mortality of dominant and codominant trees reaching 10.3% across all plantations. At a plantation in Searchmont, Ontario, tomentosus root rot was found in more than half the stumps after clearcutting, though decay and stain had not yet reached stump height.

Observation records show a strong seasonal peak in late summer into early autumn, suggesting this is when fruiting bodies are most often noticed and recorded, though these figures reflect presence observations rather than actual abundance or the fungus's complete range.

1. Onnia tomentosa produces a white pocket rot in the roots and butts of the conifers it infects, a distinctive pattern of decay tied to its common name, tomentosus root rot.
2. It is among the pathogens attacking most commercially important conifers grown in Canada, including white spruce.