Tuberous Polypore Identification Guide

First impression and setting

Tuberous Polypore is an early season find in broadleaved woodland, and it usually appears to be growing straight out of the ground, which is the first thing that makes it memorable. You are walking a damp deciduous wood in late spring or summer, often on limestone or another base rich soil, when a single stalked, scaly, buff coloured cap catches the eye among the herbs. There is no obvious log in sight.

The deception is worth understanding straight away, because it drives the identification. The fungus is not a soil dweller. It is rooted to a buried hardwood branch or, more strikingly, to a hard tuber like mass of its own making. Either way, the practical rule in the field is to follow the stem down.

The general impression is of a mushroom shaped polypore: a circular, centrally depressed to shallowly funnel shaped cap on a stout central stem, tough rather than fleshy, in warm ochre, tan and cream with darker fibrous scales scattered over it.

Reading the cap

The cap is typically 4 to 12 cm across, occasionally larger, circular in outline when it grows unobstructed, and depressed at the centre so that mature specimens look like shallow funnels. The ground colour is cream, buff or ochre tan, and over it lie small, flattened, brownish fibrous scales, most concentrated over the centre and often arranged in radiating lines that fade towards the edge. The surface is dry.

The margin is thin and initially incurved, and in young material it is fringed with fine hairs that you should look for with a lens before they wear away. In old caps the rim becomes wavy and may split radially.

The pore surface

Turn it over. The undersurface is white to cream, ageing pale buff, and it is decurrent: the tube layer runs down onto the top of the stem rather than stopping cleanly at the cap edge. That decurrent pore layer is a strong genus signal for the stalked polypores.

Pores are angular and comparatively large, roughly one to two per millimetre, becoming elongated and almost maze like where they run down the stem. The tubes themselves are short and white. There is no colour change on rubbing, and the surface does not bruise to a different colour.

Stipe, base and the buried tuber

The stipe is 3 to 8 cm long, central or slightly off centre, solid, tough and whitish, often becoming brownish and scurfy towards the base. It narrows downwards and continues as a taproot like extension into the soil or into a buried stick.

Now do the digging that most people skip. Excavate carefully around the stem base with a knife or trowel. In a proportion of collections you will uncover a sclerotium: an irregular, hard, blackish brown mass, anything from a few centimetres to twenty or more across, made of fungal tissue cemented with soil particles and small stones, and marbled white inside when cut. Finding one is conclusive, and it is the feature the species is named for. Note that fruit bodies growing directly on a rotting log may have no sclerotium at all, so the absence of one does not rule the species out. The presence of one rules everything else out.

Flesh and colour change

Cut the cap in half. The flesh is white, thick over the centre and thin at the margin, tough, elastic and fibrous, and it resists a blunt blade. It does not change colour on cutting or bruising, and it dries hard and pale.

That toughness is structural: the flesh contains skeletal and binding hyphae in addition to ordinary generative hyphae, which is why the fruit body holds its shape for weeks and why it feels quite unlike a soft gilled mushroom of similar size.

Taking and reading the spore print

Polypores print more slowly than gilled fungi, so allow plenty of time. Cut the cap free of the stem, rest it pore side down on dark card, or half on white and half on dark, and cover it with a box containing a scrap of damp tissue so that it does not dry out and shut down. Leave it overnight, ideally twelve hours. Young caps whose pore mouths are still bright and open give far better results than weathered ones.

The deposit is white and shows best against the dark half of the card. Under the microscope the spores are smooth, colourless, and cylindrical to oblong ellipsoid with a slightly flattened side, and they do not stain blue grey in iodine.

Substrate, ecology and season

This is a white rot fungus of dead hardwood. Its usual substrate is buried or partly buried deciduous branches, logs and root fragments, with hazel, ash, beech, lime, elm and maple among the recorded hosts, and it also fruits openly on fallen wood. The sclerotium is best understood as a nutrient reserve and survival structure: mycelium binds soil and stone into a durable mass from which fruit bodies can be launched in a favourable season.

The season is early, chiefly late spring through summer and into early autumn, which sets it apart from the great mass of woodland fungi. Fruit bodies are usually solitary or in small groups, and they persist for weeks, gradually becoming tougher and more faded.

Separating the closest look-alikes

Dryad's Saddle, Polyporus squamosus, is the species most often confused with it. It is far larger, often 20 to 60 cm, fan or kidney shaped rather than circular, with a lateral or strongly eccentric stem that has a black velvety base, much bigger and flatter dark scales, and very large angular pores. It grows in tiers on standing trunks and large fallen logs, frequently well above head height, not from soil.

Spring Polypore, Polyporus arcularius, is smaller at 2 to 6 cm, has a distinctly ciliate margin fringed with coarse hairs, and above all has very large, hexagonal, radially elongated pores that are obvious to the naked eye. It grows on exposed hardwood sticks and has no sclerotium.

Winter Polypore, Polyporus brumalis, has a grey brown, finely hairy to nearly smooth cap without conspicuous scales, small round pores of two to three per millimetre, and a slender stem, and it fruits in late autumn to spring.

Bay Polypore, Polyporus badius, has a smooth, shining, chestnut to almost blackish cap with no scales at all, thin flesh, minute pores and a distinctly black stem base.

Tiger Sawgill and other sawgill relatives can grow in similar situations with scaly caps and central stems, but they have true gills with jagged edges beneath rather than pores.

Tiger's Eye, Coltricia perennis, grows from soil on heathy paths and is a stalked polypore too, but its cap is thin, velvety and concentrically zoned in rusty brown, and both its pores and its flesh are brown rather than white.

Finally, the fleshy Albatrellus polypores are stalked and pale but have soft, brittle flesh, unscaled caps and a strong association with conifers, so texture and habitat separate them at once.