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White Spongy Trunk Rot

Fomes fomentarius (L.:Fr.) J. Kickx fils

Basidiomycotina, Aphyllophorales, Polyporaceae

Hosts:Fomes fomentarius has been reported in B.C. on birch, alder, balsam poplar, and cottonwood. Elsewhere in North America it has also been found on maple, Douglas-fir (rarely), oak, apple, willow, and Prunus spp.

Distribution: This fungus is widely distributed throughout the range of its hosts in B.C.

Identification: The fruiting bodies are perennial, woody or leathery, and usually hoof shaped (Fig. 10a). The upper surface is zoned (an indicator of perennial growth), grey to brown or grey to black, and smooth with a thick crust. The lower surface is concave, pale-brown, and poroid; the pores are small (4-5 per mm) and regular in outline. The context is a thin brown layer between the surface crust and the old tube layers. Conk age can be fairly accurately estimated as the layers of tubes laid down annually are quite distinct and can be readily counted. Fruiting bodies are found on standing living or dead trees, or on slash.

Decay first appears as a light brown discoloration, the wood remaining quite firm. Wood with advanced decay is yellow-white, soft and spongy, and frequently containing brown to black zone lines (Fig. 10b). Small radial cracks filled with yellow mycelium may develop giving the decay a mottled appearance.

Microscopic Characteristics: Hyphae in the context of the fruiting body thin-walled with clamp connections Basidiospores are cylindric, hyaline, smooth, IKI-, 12-18 (20) x 4-7 m. Growth in culture rapid, mat first white-cream, then brown, clamp connections, fibre hyphae, laccase positive. Stalpers: 1 2 3 (4) (6) (7) (11) 12 (14) 17 18 21 22 24 25 (26) 30 31 34 (35) (38) 39 42 44 45 36 (47) 48 50 (51) 52 53 (61) (63) 67 83 89 (94).

Damage: Fomes fomentarius causes decay in both living and dead timber, producing a white rot that is present in both sapwood and heartwood. If fruiting bodies are visible, there is little merchantable heartwood in a tree.

Remarks: Fungal spread is by air-borne spores, and infection occurs through exposed dead wood tissue. Some control might be achieved by minimizing damage to living trees, and through the removal of dead trees bearing sporophores. The fruiting bodies of F. fomentarius might be confused with those of Phellinus igniarius, but the upper surface of the former is smoother, lighter in colour, and more "hoof-shaped," with the pore layer generally at an angle of 90 to the tree stem.


Hilborn, M. I. 1942. The biology of Fomes fomentarius. Bull. Me. Agric. Exp. Sta. 1942. No. 409.

Stillwell, M. A. 1954. Progress of decay in decadent Yellow Birch trees. For. Chron. 30:292-298.


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 Fruiting bodies on birch - Click on the image to see a larger version

Figure 10a: Fruiting bodies on birch.





A cross-section of a F. fomentarius fruiting body -  Click on the image to see a larger versionFigure 10b: A cross-section of a F. fomentarius fruiting body on a birch stem in an advanced stage of decay.