The parasitic fungus chaga often develops on the trunks of birch trees, also known as Inonotus obliquus (Pers.) Pilat., form sterilis, which belongs to the basidiomycetes.
The spores of the fungus, dispersed in the air, land on the tree in areas where the bark is damaged (broken branches, frost damage, etc.) and begin to germinate, forming mycelium. The fungal hyphae of the mycelium penetrate the wood, gradually destroying it and causing white heartwood rot. In the areas where the spores initially penetrate the bark, black growths, known as chaga, develop from the hyphae of the fungus. These growths gradually expand and, over a period of 10–15 years, can reach large sizes and masses—up to 3–5 kg or more. The growths represent the sterile mycelium of the fungus, while the fruiting body, which produces basidiospores, develops under the bark and is not visible on the outside of the trunk.
Chaga is found in forests on old, living birches or on felled trees in logging areas. The fungus does not develop on young birch trees.