Conifer-base Polypore

The fruiting bodies of the fungus, which are also known as basidiocarps, are normally brackets which are whitish around the margins and dark brown on the uneven, knobbly upper surface. They can blacken in age. They can also take a resupinate form, consisting only of a white crust which corresponds to the underside of the bracket. Basidiocarps are up to about 40 cm in diameter and 3.5 cm thick. The fertile surface of the fruiting body is white, easily bruising brown, and has barely visible pores, with 3-4 per mm. The flesh, which has a strong fungus smell, is elastic when young but becomes woody when older. Sexual spores called basidiospores are created in the fertile layer on the lower surface of the basidiocarps, whilst conidiospores occur in the asexual stage and are produced on microscopic "conidiophores" which erupt through the surface of the host tree. Conidiospores and basidiospores are both produced by this fungus, the latter being more important for infecting the conifers. The species is inedible.

The species is endemic to the United States. It was introduced into Italy during the Second World War, vectored in wooden crate material.

In the summertime, basidiospores, the primary infective propagules, are released. These basidiospores are carried long distances by wind currents. They infect trees (usually conifers) through damage such as freshly cut stumps. Once on the stump the fungus colonizes and moves into the root via mycelium. Heterobasidion annosum moves short distances from the roots of an infected stump through root grafts with other trees. It can also spread through insects that feed on roots. Since this fungus can not move very far through soil, it relies on tree roots to help it infect neighbouring trees. In these roots, it can grow 0.1–2.0 m per year. This results in a spread of the fungus and disease gaps in the forest. These disease gaps are produced when the trees die and fall, creating gaps in the forest canopy. These gaps affect the moisture and sunlight available, altering the habitats for plants and animals on the forest floor. Spiniger meineckellus, the name for the asexual stage of this fungus, is produced on stumps when the conditions are moist, and the conidiospores that are produced will be able to live in the soil for up to ten months. The role of conidiospores is unknown in the infection process and is not thought to be important.