Dry Rot Fungus Identification Guide

The First Impression and the Setting

You almost never meet Dry Rot Fungus on a woodland walk. You meet it behind a skirting board, under a suspended timber floor, in a cellar, in a poorly ventilated void behind panelling, or in the roof of a building where a leak has been running for a season or two. It is a fungus of worked softwood in still, humid, unventilated air, and by the time a fruit body appears the mycelium has usually been established out of sight for a long time.

There are three quite different things to recognise, and it helps to treat them as three separate identifications that support one another: the sheet-like fruit body, the vegetative mycelium and strands, and the decayed wood itself.

The fruit body is the showiest. It forms a flattened plate or a bracket-like shelf pressed against the timber, anything from a few centimetres to well over half a metre across, sometimes irregular and spreading across a joist, a wall face or the underside of a board. From a distance it reads as a rusty orange-brown pancake with a thick, soft, snow-white or pale grey rim. That colour combination, rust in the middle and cotton-wool white at the edge, is the first impression you learn to trust.

Reading the Spore-Bearing Surface

Get close and look at the fertile face. It is not smooth, not gilled, not toothed, and not straightforwardly pored. It is merulioid: a shallow, irregular, brain-like landscape of folds, wrinkles, ridges and blunt shallow pits that run into one another. The pits are wide and shallow, the walls between them are low, rounded and soft, and the pattern is meandering rather than geometric.

The colour of this surface starts pale ochre or yellowish and becomes rust-brown, cinnamon-brown or foxy-brown as spores are produced and shed. Around the fertile centre runs a sterile margin, a soft, thick, cottony band of white to pale grey mycelium, occasionally with faint lilac or violet-grey tinges, which is where the fruit body is actively growing outward.

Fresh, actively growing specimens often bear droplets of clear or slightly brownish liquid standing on the surface, sometimes enough to bead and run. This weeping is the feature the scientific epithet records, and it is a good confirmation when present, though it dries away in a warm room.

Flesh, Texture and Section

Cut a fruit body across with a knife. The flesh is soft, fleshy-tough, spongy and slightly rubbery when fresh, drying to something like stiff felt or leather. It is whitish to pale buff inside, with the fertile layer as a thin darker band along the outer face. There is no separate cap, no stipe, no ring and no volva, because the whole structure is essentially a spore-bearing sheet with a growing edge.

There is no colour change on cutting in the sense of a bruising reaction; the cut face simply shows the pale interior against the rusty surface. A strong, damp, mushroomy, slightly musty odour is typical of an active outbreak and is often noticed before anything is seen.

The Mycelium and the Strands

This is where the identification is usually made, because the mycelium is far more often encountered than the fruit body. On damp surfaces it forms silky white sheets and fans, or fluffy cotton-wool masses, spreading across masonry, plaster and timber alike. Where it is exposed to light and air the sheets often develop grey, silvery or faintly lilac to yellowish patches, and old sheets become tough, leathery and skin-like, peeling away in sheets.

Within and beneath these sheets run the strands, and they are the decisive character. They are cords of aggregated hyphae, from thread-thickness up to the diameter of a pencil, whitish to grey, that snake across brickwork and mortar and can carry water for metres. Take a dried strand between finger and thumb and try to bend it: in this species the strands become brittle when dry and snap with an audible crack. That brittle snap is the classic separation from the strands of the cellar fungus, which remain flexible.

The Wood Itself

The decay is a brown rot. The fungus removes the cellulose and leaves the brown lignin behind, so the timber darkens to a dull brown, loses weight dramatically, and shrinks. As it shrinks it cracks both along and across the grain, breaking the wood into rectangular blocks, the so-called cubical cracking. Push a screwdriver into affected timber and it crumbles to a dry, friable, brownish powder. Softwood structural timber is the usual victim. Finding cuboidal cracking, a white or grey mycelial sheet and a rusty merulioid plate together is as complete a field diagnosis as this species allows.

The Spore Deposit

You do not need to lay anything on card here, because the species prints itself all over the room. An actively fruiting outbreak sheds spores in such quantity that a fine rusty-brown to yellow-brown dust settles on every horizontal surface nearby: floorboards, ledges, the tops of pipes, cobwebs.

For a controlled deposit, hold a sheet of white paper an inch beneath the fertile face of a fresh specimen for a few hours in still air; the deposit is a warm rust or yellow-brown. Microscopically the spores are ellipsoid to slightly bean-shaped, smooth, thick-walled, pale yellow-brown, and comparatively large at roughly eight to twelve by five to six micrometres.

Season and Conditions

There is no true season indoors. The requirements are moisture in the timber, still air and moderate warmth, and where those persist the fungus fruits at any time of year, though flushes are commonest in autumn and after prolonged damp.

Separating It From Its Look-alikes

Coniophora puteana, the cellar fungus, is the perennial confusion because it occupies the same buildings. Its fruit body is a thin, flattish, olive-brown to dark brown skin with a narrow creamy margin, and its fertile surface is warty, bumpy and irregular rather than folded into wide shallow merulioid pits. Its strands are thinner, darker brown to almost black, and stay flexible when dry rather than snapping. Its spore deposit is darker olive-brown.

Serpula himantioides is the closest relative and looks similar in colour, but it is thinner, more closely resupinate, more often greyish or darker brown, has a much less developed cottony margin, and is largely a species of dead wood in natural habitats rather than of buildings.

Phlebia tremellosa, on natural wood outdoors, has a gelatinous, translucent, rubbery texture, a white to greyish hairy upper surface where it turns out as a bracket, and a pinkish to orange folded fertile face; it is soft and jelly-like where Dry Rot Fungus is fleshy and felted.

The white pored resupinates such as Fibroporia and Antrodia species have genuine tubes with distinct circular or angular mouths, and their fertile surfaces stay white or cream rather than turning rust-brown.

A Working Field Sequence

Note the setting: indoors, softwood, still damp air. Look for rusty-brown merulioid folds with a thick white cottony margin. Check for weeping droplets. Trace the white or grey mycelial sheets and test a dried strand for brittleness. Confirm brown cubical cracking in the timber. Look for fine ochre dust on nearby surfaces.