Sterile Conk Trunk Rot of Birch

Inonotus obliquus (Pers.:Fr.) Pilát
(=Polyporus obliquus (Pers.:Fr.) Fr.)
(=Poria obliqua (Pers.:Fr.) P. Karst.)

Basidiomycotina, Aphyllophorales, Polyporaceae

Hosts: In B.C., Inonotus obliquus has been reported on paper birch and rarely on cottonwood. Elsewhere in North America it has also been found on alder, hickory, beech, and Ostrya (American leverwood or hophornbeam).

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

Identification: The most prominent character used to identify this fungus is the presence of "sterile conks." These are conspicuous perennial black masses of fungal tissue, commonly 20-30 cm in diameter, that erupt from bark cankers (Fig. 44a). The conk surface is rough and cracked, and the internal tissue of the conk is yellow-brown to rust-brown, with a punky texture (Fig. 44b). The tree trunk is often thickened at the site of the conk, a result of increased wood production and thicker bark. In contrast, fertile fruiting bodies are less conspicuous and annual, forming in the summer and early fall under the bark or outer layers of wood surrounding sterile conks on dead standing or fallen trees. As the sporophores develop, the bark and outermost wood rings split and lift away exposing the spore-bearing surface. The fertile sporophores quickly deteriorate through insect and weather damage and are usually difficult to find. The fruiting body is resupinate, 1-3 mm thick, with a grey to reddish-brown pore surface. Pores appear as oblique openings to vertically aligned tubes, 6-8 per mm.

Incipient decay is yellowish white in irregular zones. Advanced decay occurs in the heartwood, moving to the sapwood after trees die, and appears in alternating zones of white and light reddish-brown wood. White veins of mycelium are common near the cankers. Dark zone lines are often present throughout the decayed wood (Fig. 44c).

Microscopic Characteristics: Hyphae in the context of the fruiting body thin- to moderately thick-walled, frequently branched, simple septate, basidiospores broadly ellipsoid to ovoid, hyaline to pale brownish, IKI-, 9-10 x 5.5-6.5 µm. Growth in culture slow, mat white becoming yellow to brown, hyphae hyaline, thin-walled, simple septate, thick-walled setae, laccase positive. Stalpers: 1 3 (4) (9) (10) (11) (12) (13) (14) 17 21 22 25 (26) 30 (31) (34) 35 (38) 48 52 53 67 69 (70) 83 (88) 89.

Damage: Infected trees are severely damaged. The presence of a single sterile conk indicates extensive heartwood decay; 50-"100%" cull is assumed.

Remarks: Infection occurs through dead branch stubs, trunk wounds, or through pre-existing cankers (e.g., Nectria) by spores produced by fertile fruiting bodies. Decay characteristics and small sterile conks are similar in appearance to those of Phellinus igniarius.


Campbell, W. A. and R. W. Davidson. 1938. A Poria as the fruiting stage of the fungus causing the sterile conks on birch. Mycologia 30: 553-560.

True, R. P., E. H. Tyron, and J. F. King. 1955. Cankers and decays of birch associated with two Poria species. J. For. 53: 412-415.

Zabel, R. A. 1976. Basidiocarp development in Inonotus obliquus and its inhibition by stem treatments. For. Sci. 22: 441-437.


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Figure 44a: A sterile conk of Inonotus obliquus on paper birch.






Figure 44b: The yellow-brown colour of the context of a sterile conk of Inonotus obliquus.






Figure 44c: Advanced decay in paper birch caused by Inonotus obliquus.