Wet Rot Fungus Identification Guide

Learning to See a Crust Fungus

Nothing about this species stands up. It has no cap, no stipe and no gills; it grows as a flat film pressed against its substrate, and the whole identification depends on close inspection of a surface. That is a different discipline from working through a gilled mushroom, and it rewards a hand lens, a torch held at a low raking angle, and a willingness to get your face close to a damp log.

The typical encounter is under a fallen conifer trunk in wet woodland. You roll the log, or lift a slab of loose bark, and the underside carries an irregular patch of skin-like growth spreading across the wood, anywhere from a few centimetres to half a metre or more, often coalescing from several separate patches into one continuous sheet. The second common setting is indoors or under structures: unheated cellars, crawl spaces, the undersides of floorboards, sleepers, fence posts and joinery in persistently damp conditions.

Reading the Surface

Colour and its gradient across the patch are the primary characters.

The actively growing margin is white to pale cream, thin, cottony to finely fringed, and clearly demarcated. Behind it the fruit body thickens and darkens through pale ochre and yellowish-tan to olive-brown, umber and finally a dark chocolate or blackish-brown in the oldest central areas. A well-developed patch therefore shows a colour sequence from a white rim inward to a dark core, which reads almost like a contour map of the colony's growth.

Texture next. The surface is not smooth and it is not poroid. It is finely to coarsely tuberculate: covered in low warts, pimples, bumps and irregular nodules, sometimes becoming almost lumpy where it is thickest, with shallow irregular depressions between them. Under a lens the warts are rounded and irregularly scattered, not arranged into pores, tubes, teeth or a folded network of ridges. Getting this right is the single most useful observation in the whole identification.

Consistency depends on moisture. Fresh material is soft, waxy and slightly rubbery, and it can be peeled off the wood in continuous sheets or flakes with the tip of a knife. Dried material becomes brittle, hard and papery, cracks into plates, and lifts away in chips.

The Strands

Follow the fruit body out beyond its margin and look for mycelial cords, or rhizomorphs, running over the wood surface, into bark crevices and across adjacent surfaces. In this species they are relatively fine, branching, thread-like to slender-cord-like, and they change colour with age from whitish or pale yellowish through ochre to dark brown or almost black. They lie flat and fan out in a branching network.

Their proportions matter. These are slim strands, typically well under a couple of millimetres thick, and old ones are brittle and snap rather than flexing. They usually stay on or immediately beneath the wood surface rather than travelling long distances across non-woody materials.

The Decay Pattern

The wood itself is part of the identification. This species produces a brown rot: it breaks down the cellulose and leaves the brown lignin behind. Decayed wood therefore turns dark brown, shrinks, and cracks both along and across the grain, breaking into a pattern of roughly rectangular blocks, described as cubical checking. Advanced material crumbles between finger and thumb into brown powder and small cubes.

Look for that cubical checking in the timber around and beneath a suspected patch. Brown, cracked, cube-forming wood under a warty olive-brown crust with a white fringing margin is a very strong combination.

Flesh and Section

Peel a piece away and examine it in section with a lens. The fruit body is thin, usually a fraction of a millimetre to two or three millimetres thick, consisting of a soft subiculum of loosely woven hyphae against the wood and a smooth-to-warty spore-bearing layer on the outer face. There is no distinct crust, no gelatinous stratum, no separable tube layer and no colour change on cutting; the material is simply paler within than on its exposed surface. It has no strong odour beyond a general damp, earthy, cellar-like smell.

Spore Deposit

You cannot invert a resupinate fungus onto paper, so the trick is to sample what falls naturally. If the patch is on the underside of a log, slide a sheet of white paper or a glass slide beneath it and leave it in place for a day in still, humid conditions. Alternatively, look for a natural deposit: a mature, actively sporulating patch dusts everything below it, so check the surfaces beneath a colony for a fine powdery film.

The deposit is yellow-brown to olive-brown, sometimes described as tobacco or clay brown. Under the microscope the spores are ellipsoid to almond-shaped, smooth, distinctly thick-walled and pale yellow-brown, which is unusual among the pale-spored crust fungi and is a strong confirmation.

Substrate, Season and Ecology

The species is a saprotroph on dead wood and is strongly associated with conifers, though it also colonises hardwoods. Outdoors it fruits on fallen trunks and large branches, on the undersides and shaded faces where humidity stays high, on stumps, and on worked timber such as posts, sleepers, decking and fencing that sits in contact with damp ground. Indoors it is one of the classic colonisers of persistently wet structural timber, where the requirement is simply free water: it needs consistently high moisture and cannot spread through dry material.

Fruiting is possible whenever conditions are damp enough, so patches can be found year-round, but they are most conspicuous in autumn and winter when humidity is sustained.

Separating It from Look-alikes

Dry rot fungus (Serpula lacrymans) is the crucial comparison. Its fruit body is thicker and fleshier, with a spore-bearing surface thrown into folds, wrinkles and shallow irregular pores rather than scattered warts, and its colour is a distinctive rusty ochre to cinnamon with a broad white to lilac-tinged margin. Its spore deposit is rusty red-brown, not olive-brown, and it is heavy enough to coat surroundings visibly. Its mycelium forms thick, greyish, rope-like strands, several millimetres across, which remain flexible when dry and can travel across masonry and plaster, whereas the strands here are fine, brittle and stay close to the wood.

Serpula himantioides is thinner than dry rot but still has a folded, merulioid surface and warmer ochre-brown tones rather than an evenly warted olive-brown skin.

Other Coniophora species are close relatives and can require microscopy, but several are thinner, smoother and paler, without the strong olive-brown maturity or the well-developed pimpled texture.

Poroid crusts (Antrodia, Amyloporia, Ceriporiopsis) are ruled out at once by their true pores, and most are white to cream rather than olive-brown.

Toothed crusts (Hyphodontia, Steccherinum) carry distinct spines or teeth on the surface.

Phlebia and Merulius relatives have radially folded, wrinkled or gill-like ridged surfaces and are usually orange, pink or violet-tinged.

Algal films and slime moulds on damp wood can look similar from a distance but cannot be peeled away as a coherent sheet and lack the fringed white growing margin.