Coprinellus radians

Coprinellus radians belongs to the group of small, quickly collapsing inkcaps that were split off from Coprinus into Coprinellus. Members of the genus typically have caps dusted with granular veil remnants, radially grooved margins, and gills that deliquesce, though usually less dramatically than in Coprinopsis.

What sets this species apart in the field is not the mushroom at all but the mycelium. It produces an ozonium: a dense, felted, rusty-orange to tawny mat of thick-walled hyphae that spreads over the surface of the wood it is colonising. The mat is often far more conspicuous and far longer-lived than the fruit bodies, and can be found on its own for months, looking like a patch of orange fur or dried rust on damp timber.

It is one of the fungi most frequently encountered indoors, appearing on wet joists, skirting boards, damp plaster and cellar timbers where a leak has kept the wood saturated. In that setting the orange mat is often the first thing a building surveyor notices, and the small inkcaps appear later.

Cap

1-4 cm across, at first ovoid to ellipsoid, expanding to conical then flat and finally splitting. The colour is tawny, ochre-brown or date-brown, palest at the margin and darkest over the disc, and the surface is dusted when young with pale buff to whitish granular veil particles that rub away with handling. The margin becomes deeply radially grooved, the grooves following the lines of the gills beneath.

Gills

Adnexed to almost free, crowded, narrow, white at first, then grey-brown, finally blackish, deliquescing modestly rather than dissolving into copious ink.

Stipe

3-10 cm long and only 2-4 mm thick, white to cream, hollow, brittle, finely downy especially toward the base. There is no ring. The base emerges directly from an orange felted mat, and if the fruit body is teased away from the wood, strands of that mat usually come with it.

Ozonium & spore print

The ozonium is the key field character: a persistent rusty-orange, tawny or foxy-brown felted mat of mycelium spreading over the substrate, sometimes covering many square centimetres. The spore print is dark blackish-brown.

It grows on damp, decaying hardwood, including fallen trunks and stumps of broadleaved trees, buried timber, sawn boards, fence posts and, very often, structural timber inside buildings where a persistent leak has kept the wood wet. It also appears on damp plaster and behind panelling where mycelium has spread from an infected joist.

The species is widespread across Europe and North America and occurs in temperate regions generally. Outdoors it fruits mainly from spring through autumn, but in heated buildings with a steady source of moisture it can produce fruit bodies at any time of year, and the ozonium persists indefinitely as long as the wood stays damp.

This is a wood-rotting saprotroph. The mycelium degrades the cellulose and hemicellulose of dead hardwood, producing a soft, pale rot, and the tough ozonium mat acts as a long-lived storage and exploration structure that lets the fungus persist between fruiting episodes and spread across the surface of the substrate.

In buildings the fungus is entirely dependent on the moisture problem that allowed it to establish; the mycelium cannot advance through dry timber. Ecologically it belongs to the community of small saprotrophs that finish the breakdown of wood already softened by larger decay fungi, returning carbon and minerals from fallen timber to the soil.

Coprinellus domesticus, the Firerug Inkcap, is the closest match and also produces an ozonium; separating the two reliably needs microscopy, but C. domesticus usually has a paler, more scurfy whitish veil on a greyer-ochre cap and a somewhat larger fruit body, while C. radians tends to a warmer tawny cap and a brighter orange mat.

Coprinellus micaceus forms dense clumps at the base of hardwood stumps and has a tawny cap sparkling with mica-like glistening granules, but it never produces an orange mycelial mat. Coprinellus xanthothrix is similar in size but its veil granules have dark brownish tips, and Coprinellus disseminatus is far smaller and greyer, fruiting in swarms of hundreds without any ozonium.