Dry Rot Fungus

The Dry Rot Fungus is a resupinate (crust-forming) member of the family Serpulaceae, which sits — perhaps surprisingly — in the order Boletales alongside the fleshy boletes. Instead of a cap and stipe it produces a flat, pancake-like fruiting sheet pressed against its substrate, with the spore-bearing surface facing outward.

It is one of the most recognisable wood-decay fungi in the northern temperate world, largely because of where it grows: it is overwhelmingly encountered on worked conifer timber inside cool, damp, poorly ventilated buildings rather than in forests. The species epithet lacrymans means "weeping", referring to the clear droplets that often stand on the surface of an actively growing fruitbody.

What makes it distinctive is the combination of features that no other crust fungus quite matches: a soft, thick, brick- to rust-coloured fertile surface thrown into shallow folds and pits, a broad snow-white to lilac-tinged cottony growing edge, extensive silvery-white mycelial sheets, and thick, greyish, rope-like strands (rhizomorphs) that become brittle when dry.

Fruiting body

Forms flat, plate- or bracket-like patches from a few centimetres to well over a metre across, often merging into large irregular sheets. Fresh growth is fleshy and soft; old material dries to a papery, brittle crust.

Fertile surface

The outward-facing spore surface is ochre to rust-brown or brick-red, thrown into low, blunt, maze-like wrinkles and shallow pits (a merulioid surface) rather than true tubes or gills. Clear watery droplets frequently sit in the depressions on actively expanding fruitbodies.

Margin and mycelium

The advancing edge is a thick, soft, cottony white to pale grey band, sometimes flushed lilac or pale yellow. Behind and beneath the fruitbody, silvery-white to grey mycelial felts spread over timber, plaster and brickwork, and give rise to greyish white cords up to several millimetres thick.

Flesh and spore print

The context is soft, spongy and pale. Spores are shed in vast quantity and settle as a conspicuous rust-brown to ochre-brown dust on nearby surfaces — one of the most reliable field clues, since few crust fungi coat their surroundings so heavily.

Almost always found on seasoned coniferous timber in cool, humid, still-air situations: cellars, voids under floors, behind panelling, and in roof spaces where moisture has been present. It tolerates lower substrate moisture than most wood-decay fungi because its cords conduct water from wetter areas, allowing it to spread onto drier timber and to grow across non-woody materials such as brick, mortar and plaster.

In the wild it is genuinely rare, with the best-documented natural populations on fallen conifer wood and rocky slopes in the Himalaya and parts of temperate Asia; it also turns up occasionally on dead conifer wood in northern Europe and North America. Fruiting is not tied to a season indoors and can occur at any time of year wherever conditions stay damp and cool.

This is a saprotroph that causes a brown rot: it breaks down the cellulose and hemicellulose of the wood while leaving the brown lignin behind, so decayed timber darkens, shrinks and cracks into a characteristic pattern of cubical blocks that crumble to powder between the fingers.

The life cycle runs from a wind-dispersed spore that germinates on damp wood, through a spreading mycelium that organises itself into water-conducting cords, to the flat fruitbody that regenerates the spore load. The cord system is the key ecological trait: it lets a single individual bridge inert materials and exploit timber some distance from the original moisture source.

The commonest confusion is with the Wet Rot Fungus (Coniophora puteana), which forms a much thinner, more closely attached film that is cream at first and matures olive-brown to almost black, is studded with small warts rather than deep folds, and has thin dark brown to blackish cords instead of thick pale ones; its spore dust is olive-brown, not rust-brown. Serpula himantioides is the outdoor counterpart, forming thinner, more leathery, less strongly folded fruitbodies on dead conifer wood with a narrow, poorly developed margin.

Yellow-toned crusts of Leucogyrophana species share the wrinkled fertile surface but are smaller, brighter yellow to orange, and lack the broad white cottony edge and heavy cord system. Bracket-forming crusts such as Phlebia species differ in their gelatinous, brightly coloured, radially wrinkled surfaces and complete absence of thick rhizomorphs.