Hosts:  Spruce and Douglas-fir

Figure 274. Leathery O. tomentosa fruiting body.

Symptoms/signs:  Fruiting bodies of Onnia tomentosa most commonly develop in August and September. They are small (usually less than 10 cm in diameter), mushroom shaped, and have a lower pore surface from which the spores are released. Although leathery, they are annual. The upper surface is yellow-brown to rust-brown and velvety, and becomes dark brown with age. The early stage of decay is characterized by a red-brown discoloration in the heartwood of roots. The late stage is also reddish brown but appears lighter because of numerous small elongate pockets with pointed ends, filled with white mycelium. This type of decay is known as a white pocket rot. The most distinctive characteristic of Tomentosus rot is that a cross section of an infected stem in advanced stages of decay has a honeycomb appearance.

Biology:  Tomentosus root disease spreads primarily by root-to-root contact, however, infection by spores can occur through deep wounds in roots. There is no evidence that spores colonize stump surfaces as with Annosus root disease. Diseased trees occur singly or in groups. The fungus spreads both outward in roots and up into the butt. Diseased roots have dead, decayed distal portions and red-brown, resin-soaked wood extending into the living portion and the butt. Advanced decay develops slowly and expands to involve both deep-lying wood and sapwood near the cambium. Infected trees often die standing, but sometimes blow over while alive because of advanced decay in major roots or butt. Onnia tomentosa has been found to persist for more than 50 years in decaying wood in soil.

Figure 275. Red-brown discoloration of roots decayed by O. tomentosa.

Effects:  Although Tomentosus root disease has been observed in spruce/fir and transition forests throughout the Southwest, there is little information available on the extent and damage from this disease. In other parts of the country, blowdown caused by Tomentosus root disease has initiated spruce beetle (Dendroctonus rufipennis) outbreaks but that has not been reported here.

Similar Insects and Diseases:  Onnia. tomentosa and O. circinata are nearly identical and difficult to differentiate macroscopically. Microscopically, O. circinata can be differentiated from O. tomentosa based on hooked structures the latter does not produce. Onnia. tomentosa and O. circinata are sometimes called false velvet-top fungi, because of the similarity of the conks of these fungi with that produced by Phaeolus schweinitzii, which is the real velvet-top fungus.

References:  30, 93, 103, 111