Wet Rot Fungus
Coniophora puteana
Also called cellar fungus, wet rot
A thin, closely attached crust that starts creamy white and matures to olive- or umber-brown, its surface roughened with small warts and lumps. It grows on persistently wet dead wood outdoors and on damp timber indoors.
Overview
The Wet Rot Fungus, also widely called the Cellar Fungus, is a resupinate crust fungus in the family Coniophoraceae, part of the order Boletales. Like its relative the Dry Rot Fungus it makes no cap or stem, instead spreading as a flat skin over the surface of the wood it is consuming.
It is far commoner in the wild than Dry Rot Fungus and is one of the standard crusts of wet fallen logs, waterlogged stumps and the undersides of sodden planks. Indoors it appears wherever timber stays genuinely wet — around leaks, in unventilated cellars, behind failed rainwater goods — hence the English names.
Its distinguishing character is the way the fertile surface develops: smooth and cream-coloured while young, it becomes progressively studded with small rounded warts and tubercles and darkens through yellow-olive to rich olive-brown or umber, while the actively growing rim stays pale, thin and slightly fringed.
How to identify it
Fruiting body
A thin, soft, closely adnate film, typically a fraction of a millimetre to about a millimetre thick, spreading in irregular patches from a few centimetres to tens of centimetres across. It peels away from the substrate as a soft skin rather than snapping like a dried crust.
Fertile surface
Initially smooth, waxy and pale cream to yellowish; with age it becomes ochre, olive and finally dark umber-brown, and the surface roughens with low warts, pimples and irregular lumps. The colour change progresses from the older centre outwards, so a mature patch is often dark in the middle with a pale ring at the edge.
Margin and cords
The margin is narrow, whitish to pale yellow, thin and often finely fibrillose or web-like rather than thick and cottony. Fine, thread-like cords are produced, and these are characteristically dark brown to almost black and no more than a millimetre or so thick.
Flesh and spore print
The context is very thin and soft. Mature fruitbodies release abundant olive-brown to yellow-brown spores, and the surrounding wood may be dusted with them.
Where and when it grows
Outdoors it grows on the wet underside of fallen conifer and broadleaf logs, on stumps, on waterlogged fence timber and on buried or partly submerged wood, favouring positions that stay saturated. It is widespread and common through temperate Europe, Asia and North America, and is present in cool parts of the southern hemisphere.
Indoors it colonises timber kept persistently wet by leaking pipes, defective roofing or rising damp: joist ends, sole plates, skirtings and cellar shelving are typical sites. Unlike the Dry Rot Fungus it does not build a substantial water-conducting cord system, so it stays close to the moisture source and dies back quickly if the wood dries. Fruiting occurs whenever conditions allow, peaking in autumn outdoors.
Ecological role
Coniophora puteana is a saprotroph producing a brown rot. It strips cellulose from the wood, leaving a darkened, lightweight residue that cracks along and across the grain into blocks, though the cracking is usually finer and more longitudinal than that of the Dry Rot Fungus.
Spores landing on wet wood germinate readily and the mycelium spreads through the surface layers before forming a fertile film on the exposed face. Because the fungus needs high and continuous moisture, its patches map closely onto the wettest zones of a log or timber, making it a useful indicator of where water is standing.
Similar species
The Dry Rot Fungus (Serpula lacrymans) is separated by its far thicker fleshy fruitbody, rust- to brick-brown fertile surface with deep merulioid folds, broad white cottony margin, thick grey-white cords, and rust-brown spore dust. Coniophora arida and its allies are thinner, paler and stay yellowish-ochre rather than darkening to umber, with a less warted surface, and reliable separation is often microscopic.
Leucogyrophana species have brighter yellow to orange fertile surfaces raised into distinct radiating folds rather than scattered warts. Thin brown Hyphodontia and Botryobasidium crusts differ in being felty, cobwebby or minutely spiny and in never producing the dark brown to black thread-like cords typical of the Wet Rot Fungus.