Tuberous Polypore

The Tuberous Polypore is unusual among the stalked polypores in that it frequently develops from a sclerotium, a hard, rounded, dark-crusted mass in which fungal mycelium binds together soil particles, sand and small stones. This structure gave rise to the old name stone fungus and to a long European tradition of digging up and keeping such lumps, which will produce fresh fruit bodies when watered. Not every collection has one, however, and fruit bodies also arise directly from buried branches and roots.

Taxonomically it sits in the family Polyporaceae and is often listed under the segregate genus Cerioporus. In gross form it is a scaled-up version of the Lentinus arcularius body plan: a scaly cap with a fringed margin, a central to off-centre stipe, and a layer of large angular white pores that run down the stem. Everything, though, is heftier, with thicker flesh and a broader, more robust cap.

It is widespread in Europe and temperate Asia and occurs across much of North America, generally on well-drained ground in deciduous or mixed woodland, and it is one of the few polypores that a walker is likely to find apparently growing straight out of the soil.

Cap

The cap is 5 to 15 cm across and occasionally larger, convex at first, then flattening and often becoming shallowly funnel-shaped. The ground colour is cream to ochre-tan or pale leather-brown, overlaid with appressed reddish-brown to dark brown fibrillose scales that are largest and densest toward the centre and thin out toward the rim. The margin is thin and carries a distinct fringe of coarse hairs, especially in young fruit bodies.

Pores

The pore surface is white to cream, discolouring pale buff with age. Pores are large and angular, roughly 0.5 to 2 per millimetre, often somewhat irregular or elongated, and they run conspicuously down the stipe. Tubes are 2 to 6 mm deep, deeper than in the smaller stalked polypores.

Stipe

The stipe is stout, 2 to 8 cm long and 1 to 2.5 cm thick, central to distinctly eccentric. It is whitish to cream in the upper part and becomes brownish and finely scurfy or scaly toward the base, without the abrupt black boot seen in some relatives.

Flesh and spore print

The flesh is white, thick and firm, tough and elastic when fresh and drying hard and corky. It does not change colour on cutting. The spore deposit is white.

Sclerotium

When present, the sclerotium is a hard, irregular, blackish-crusted underground body, ranging from the size of a walnut to considerably larger, made of white mycelium densely cemented with soil and stone fragments. Digging carefully below the stipe base will reveal it if it is there.

Fruit bodies appear on buried or partly buried deciduous wood, including roots, branch fragments and old stumps, and consequently often look as though they are growing directly out of bare soil, leaf litter or a mossy bank. Recorded substrates include beech, oak, hazel, ash, elm, lime, maple and various fruit trees. Occasional collections come from wood lying on the surface, where the stipe is shorter and the cap broader.

The season runs from late spring through summer into early autumn, with the main flush in warm weather following rain. It is widespread across Europe, particularly on calcareous and well-drained soils, occurs through temperate Asia, and is scattered across North America. Typical habitats are open deciduous woodland, woodland edges, scrub, hedge banks and old parkland.

Polyporus tuberaster is a saprotroph that causes a white rot, degrading lignin along with cellulose so that colonised wood turns pale, soft and stringy. Its speciality is buried and partly buried wood, a resource pool that is protected from drying but also poorer in oxygen than exposed logs.

The sclerotium is the ecological signature of the species. By packing mycelium together with soil and mineral grains into a dense, crusted mass, the fungus creates a durable storage organ that survives drought and cold and holds enough reserves to push out a substantial fruit body when conditions turn favourable. This lets it fruit opportunistically after rain rather than depending on continuous favourable weather, and allows a single colony to persist in one spot for many seasons while it works through the buried wood beneath.

Lentinus arcularius shares the scaly cap, hairy fringed margin and large angular decurrent pores, but is far smaller and thinner, with a slender stem, and never produces a sclerotium. Cerioporus squamosus is much larger still, with a broad kidney-shaped cap, big flattened concentric brown scales, a stubby lateral stipe with a blackened base, and a habit of growing high on standing trunks and stumps rather than from the ground.

Polyporus radicatus also appears to grow from soil, but instead of a sclerotium it has a long tapering rooting extension of the stipe running deep into the ground, a blackish scurfy stem base, and a duller olive-brown to yellow-brown cap with much finer pores. Picipes badius differs in its smooth glossy chestnut cap, minute pores and sharply black lower stem.