Heterobasidion annosum Identification Guide

Where You Will Be When You Meet It

Heterobasidion annosum is not a fungus you spot from a path. It fruits at ground level, in the dark, wet spaces where a conifer stump meets the soil, and it is usually covered by needles, moss or a slab of loose bark. The setting is a conifer plantation, a thinned stand, a windthrow, or any place where conifers have been cut: pine and spruce above all, also fir and larch, and occasionally broadleaf stumps such as birch.

Finding it is an act of deliberate searching. Kneel at a stump three or four years after felling, pull the litter back from where the root buttresses run out, and slide your fingers into the gap under a lifted flap of bark or under the overhang of the stump collar. What you are feeling for is a tough, cool, slightly leathery crust with a suede-soft, flat lower face.

Growth Form: Crust, Bracket, or Both

The species is highly plastic in shape, and this trips people up. Where the fungus grows flat against wood it is resupinate - a crust of white pore surface spread over the substrate, with only a narrow rim of paler tissue at the margin. Where there is an air space - under a stump overhang, in a root cavity, between a buttress and the soil - it lifts away from the wood and forms a bracket, and on a single stump you may find both forms continuous with one another.

Brackets are irregular, lumpy and often lopsided rather than neat semicircles, 3-15 cm across and 0.5-3 cm thick. Because they frequently form in a confined pocket, they can be moulded around roots and stones, and they routinely have soil grains, needles and moss embedded in the upper surface.

Reading the Upper Surface

Where an upper surface exists, examine it under a lens. It is dry, hard and crusty rather than felted, uneven and knobbly, and marked by concentric ridges and shallow furrows that record successive seasons of growth. The centre is the oldest and darkest - chocolate to reddish-brown, sometimes almost blackish and often cracked - grading outward to a lighter zone and then to a distinct, narrow white to cream growing margin, often 1-3 mm wide, which is frequently the brightest and most conspicuous feature in a dim root hollow.

That two-tone contrast, a dark crusty back with a clean white rim, is the single most useful visual signature in the field.

The Pore Surface

Turn a bracket over, or look at the exposed face of a resupinate patch. The pore layer is white to pale cream when actively growing, ageing to buff or dirty straw and darkening where handled or where slugs have grazed it.

The pores themselves are small and neat: roughly 3-4 per millimetre, round to slightly angular, with even, thick-looking walls. Under a lens the surface has a fine, regular, almost printed texture. Press a fingernail into it - it dents rather than staining, and there is no sudden colour change.

Because the fungus is perennial, a mature specimen has several tube layers stacked one behind the other. Slice a bracket vertically with a sharp knife and hold the cut face to the light: you should be able to count two to five distinct tube strata, each only a few millimetres deep, separated by thin lines of sterile flesh.

Flesh, Texture and Colour Change

The flesh is the character that settles most arguments. It is white to very pale cream, tough, corky and leathery, and it is thin - often only 1-5 mm even in a thick bracket, because most of the thickness is tube layers. It does not turn brown, it does not turn red, and it does not go black.

Cutting is informative in itself. A dry specimen resists the blade like cork or hard rubber and can be sliced cleanly; it does not crumble and it does not shatter.

There Is No Stipe

Worth stating explicitly, because it eliminates a whole set of comparisons: Heterobasidion has no stalk of any kind. It is either broadly attached along one edge or spread flat on the substrate.

Taking a Spore Print

A crust fungus is awkward to print but not impossible. Cut a piece with an intact, actively growing pore surface, and lay it pore-down on a sheet of black paper or a glass slide, covered, somewhere cool and humid for six to twelve hours. In autumn, when sporulation peaks, a good specimen will drop a visible white deposit.

The print is white. Under a microscope the spores are the confirmation: small, 4.5-6 x 3.5-4.5 microns, colourless, subglobose to broadly ellipsoid, and - crucially - finely spiny or warted rather than smooth. Very few pale-fleshed white-pored polypores have ornamented spores, so this character is close to decisive.

Substrate, Season and Ecology

The ecology is worth understanding because it predicts where to search. Airborne spores land on freshly exposed conifer wood, above all the cut surface of a stump within days of felling, and germinate there. The mycelium then colonises the stump and travels outward along the roots, crossing to neighbouring living trees wherever root contacts or natural root grafts occur.

The result is an expanding centre of infection: a roughly circular patch of thin, declining or windthrown trees that widens outward by a metre or so each year, ringed by apparently vigorous stand. If you find one fruit body, look for that pattern in the canopy overhead - it is a landscape-scale field character.

Inside the wood the fungus causes a white rot of the heartwood in roots and butt, which shows on a cut face as a soft, stringy, bleached column, often bordered by a darker line and frequently peppered with small black flecks in the decayed tissue. That flecking is a good secondary clue on any freshly cut stump.

The fruit bodies are perennial and can be found in any month, but they add a fresh tube layer and release spores most heavily in autumn.

Taxonomically the old broad species has been split by host preference - H. annosum in the strict sense principally on pine, H. parviporum on spruce, H. abietinum on fir - and these are not reliably separable in the field by eye.

Separating It From Its Look-alikes

Red-belted bracket (Fomitopsis pinicola) shares conifer stumps but is much thicker and hoof-shaped, with a hard resinous crust that shows a characteristic orange-red band just behind the margin. Its flesh is pale but thick and woody, and the rot it causes is a brown cubical rot, not a bleached stringy white one. Its pore surface often bruises yellowish.

Artist's bracket (Ganoderma applanatum) also builds a grey-brown crusty perennial bracket with a white pore surface, but its pore layer bruises instantly and permanently brown when scratched - the reason for the name - its flesh is brown rather than white, and its spore deposit is a conspicuous cocoa-brown dust that coats the bracket and everything above it. It is mostly a broadleaf species.

Pine bracket (Porodaedalea pini, formerly Phellinus pini) grows on standing living conifer trunks, well above ground, and is at once separated by rusty-brown flesh and a brown pore surface with larger, irregular, somewhat maze-like pores.

Violet-toothed polypore (Trichaptum abietinum) covers conifer logs in great numbers, but it is thin, flexible, hairy above and violet-tinged below, annual, and never crusty or perennial.

Climacocystis borealis occurs at conifer stump bases with a white pore surface, but it is soft, watery, fleshy and annual, with a hairy white to cream upper surface, and it collapses when squeezed - the opposite of the corky toughness of Heterobasidion.

Field Sequence

1. Search the base of conifer stumps and root collars, under litter and lifted bark.
2. Find a tough, thin bracket or crust with no stalk.
3. Confirm a dark crusty, zoned upper surface with a clean white margin.
4. Confirm small round white pores, 3-4 per mm, and white corky flesh.
5. Cut vertically and count stacked tube layers - a perennial.
6. Look up: a widening gap or a ring of declining conifers supports the identification.