Hebeloma radicosum

Hebeloma radicosum is by far the most spectacular member of its genus, and one of the most remarkable of all woodland agarics in its ecology. Above ground it is a stout, pale tan mushroom with a scaly, tacky cap and a distinct membranous ring. Below ground it produces a long tapering pseudorhiza, a root-like extension of the stipe that may descend twenty centimetres or more through the soil.

The pseudorhiza is not merely an anchor. It leads down to the latrine chamber of a burrowing mammal, usually a mole and sometimes a shrew or vole, where accumulated urine and droppings create a pocket of soil extraordinarily rich in ammonia and other nitrogen compounds. The fungus colonises this pocket and sends its fruitbody to the surface along the root-like cord, making it one of the classic ammonia fungi.

The species also smells strongly of bitter almonds or marzipan, another unusual trait in a genus better known for radish odours. Between the ring, the rooting base, the size and the almond scent, it is one of the few Hebeloma species that can be named with confidence without a microscope.

Cap

Large for the genus, 4–15 cm across, convex then flattening with a broad low umbo. The colour is pale clay, buff or warm tan, sometimes with an ochre centre. The surface is slightly viscid when moist and characteristically dotted with small darker brown appressed scales or fibrillose patches, particularly toward the centre, giving a mottled appearance uncommon in other Hebeloma.

Gills

Sinuate, crowded, whitish to pale grey-buff when young and maturing to a dull clay or snuff-brown. The edges are whitish and finely fringed, and may bead faintly in humid conditions as in other members of the genus.

Stipe

Stout and long, 8–20 cm above ground and 10–25 mm thick, with a persistent membranous ring in the upper part. Above the ring the stipe is whitish and smooth to mealy; below it is covered in shaggy brownish scales arranged in rough zones. The base tapers gradually into a long, pointed, root-like pseudorhiza that runs deep into the soil.

Flesh and spore print

Flesh is white and firm, unchanging when cut. The odour is strong and sweetly almond-like, recalling marzipan or bitter almonds. The spore print is snuff-brown, and the spores are almond-shaped and warted as in other members of the genus.

This species grows in deciduous and mixed woodland, most often under beech, oak, hornbeam and hazel, and occasionally with conifers. It shows a preference for well-drained soils in mature woodland, particularly where the ground is soft enough for burrowing mammals to tunnel. Fruitbodies appear singly or in small groups, always in the same spot in successive seasons while the underground nitrogen source persists.

The season is autumn, typically from early September to November in temperate Europe. It is widespread but rather local across Europe and temperate Asia, and reported from parts of North America. Its apparent scarcity partly reflects the specialised buried habitat required for it to fruit.

Hebeloma radicosum is an ectomycorrhizal partner of broadleaf trees, but it is also a well-studied ammonia fungus, one of a small group whose fruiting is triggered by a localised pulse of nitrogen in the soil. Mole and shrew latrines, and occasionally decomposing animal remains, generate exactly such a pulse. The fungus colonises the enriched pocket, mineralises the nitrogen-rich material and channels resources upward through the pseudorhiza to build a comparatively large fruitbody.

Because of this association, the position of a fruitbody effectively maps a buried mammal chamber, and researchers have used the species to locate and study mole tunnel systems. The fungus links two very different processes: the recycling of concentrated animal waste and the delivery of mineral nutrients to living tree roots, sitting at the junction between decomposer and mycorrhizal roles.

Other Hebeloma species can be excluded at once by the combination of a persistent ring and a deeply rooting stipe base, which no common relative possesses. Hebeloma sinapizans is comparably large but ringless, smooth-capped and radish-scented, with a stipe that ends bluntly rather than tapering into a root.

Pholiota species have scaly caps and rings but drop rusty-brown spore prints and grow on wood rather than soil. The Rooting Shank, Hymenopellis radicata, also has a long pseudorhiza but is entirely different in colour and structure, with a slimy honey-brown cap, widely spaced white gills, no ring and a white spore print.