Heterobasidion annosum

Heterobasidion annosum is among the most economically consequential fungi of the northern conifer forest, and it is more often met as a hidden decay than as an obvious mushroom. The fruiting bodies are perennial polypores that develop at the base of trunks, on exposed roots, on stump cut-surfaces, and frequently in dark cavities beneath moss and litter where they can be easy to overlook entirely.

The genus Heterobasidion belongs to the Bondarzewiaceae and is now understood to be a complex of closely related species separated largely by host preference and by mating tests: H. annosum in the strict sense favours pines, H. parviporum spruce, and H. abietinum firs, with further members in North America and Asia. In the field they are effectively indistinguishable, so the collective name is what most observers apply.

What makes it distinctive as a polypore is its irregular, often half-resupinate architecture. Rather than a tidy shelf, it commonly spreads as a lumpy crust that lifts away from the substrate into shelving lobes wherever it meets air, giving a bracket that seems poured onto the wood rather than projecting from it.

Fruiting body

Highly variable: fully resupinate (flat against the substrate), effused-reflexed, or forming shelving brackets 3–20 cm across and up to about 3 cm thick. It is perennial, adding a new tube layer each year, so old specimens become thick and misshapen. The upper surface is rough, uneven and often knobbly, concentrically ridged, and coloured red-brown, chocolate-brown to almost black in the older parts. A crisp white to cream growing margin runs around the active edge and is one of the best field characters.

Pores

The pore surface is white to cream, ageing pale buff or straw, and bruises only faintly. Pores are small and round to slightly angular, roughly 3–5 per millimetre, with tubes 3–6 mm deep in each annual layer. On resupinate specimens the pore layer may cover an irregular patch many centimetres long on the underside of a root or stump.

Stipe & flesh

There is no stalk. The context is white to pale buff, tough, corky to leathery, and clearly demarcated from the darker upper crust. It does not change colour appreciably when cut. Fruiting bodies attach broadly and are often awkward to detach cleanly from the wood.

Spore print

White. Spores are small, subglobose to broadly ellipsoid, hyaline and finely roughened. In culture and on rotten wood the fungus also produces a distinctive conidial (asexual) stage with small stalked spore-bearing structures.

This fungus is found at the root collar, on exposed lateral roots, on the butt of standing trunks and on the cut faces of freshly felled stumps. Its principal hosts are conifers — pine, spruce, fir and larch above all — though it also occurs on juniper and, less often, on broadleaved trees such as birch and beech growing in infected conifer stands. Plantation forest on former agricultural or sandy ground is a classic setting.

It is widespread across Europe, northern Asia and North America wherever conifers grow, and is especially conspicuous in managed forests. Because the brackets are perennial, they can be found in any month; new growth and heavy spore release peak in autumn, and fruiting bodies frequently develop in the sheltered gap between root buttresses, hidden under needle litter.

Heterobasidion annosum causes a white rot of the roots and butt of conifers. Decayed wood first shows a violet-grey stain, then becomes pale, soft and pitted with small black-flecked cavities, eventually breaking down to a fibrous white mass. Spread happens in two ways: airborne spores landing on freshly exposed wood, particularly stump surfaces created by thinning operations, and vegetative growth of mycelium along root contacts and root grafts from an infected tree to its neighbours.

This root-to-root spread is why the fungus produces expanding disease centres in even-aged plantations, with a ring of thin-crowned or windthrown trees around an older opening. As a decay organism it is also a major recycler of conifer stumps and roots after felling, returning structural carbon to forest soils, and the hollowed butts it creates provide habitat for invertebrates and small vertebrates in older stands.

Fomitopsis pinicola occupies similar conifer stumps but forms a hard, hoof-shaped bracket with a resinous, lacquered crust banded in red, black and white, and causes a brown cubical rot with a brownish context — the opposite of Heterobasidion's white, fibrous decay and dull, unlacquered upper surface. Ganoderma applanatum is broader and flatter with a grey-brown crust and drops a brown spore print, and it is a hardwood specialist.

Phaeolus schweinitzii also fruits at conifer bases but is an annual, soft, spongy, rusty-brown fungus with a broad yellow-green margin and large angular pores, often on the ground over roots. Trametes and Bjerkandera species can appear on the same stumps but are thin, flexible, annual brackets with a hairy or velvety zoned cap and no thick perennial crust.