Glue Crust Identification Guide

How you actually find it

Glue Crust is not a mushroom you spot from a path. You find it by handling dead wood. The classic encounter is in an old hazel coppice or a hedge with plenty of hazel in it: you tug at a dead branch that is hanging in the canopy, expecting it to drop, and it refuses to move. When you look closely, the dead branch is welded to a living stem, or to another dead branch, by a hard brown sheet that has grown across the contact point and set like resin. That is the fungus, and the habit is so consistent that it effectively names itself.

Once you know to look, the second encounter is easier. Turn over any attached dead hazel branch, especially on the underside where it is shaded and stays damp, and look for a thin crust spreading along the bark. It is flat against the wood - resupinate is the technical word - with no cap, no gills, no pores and no stem of any kind. It is simply a skin of fungus lying on the branch.

Reading the crust surface

Start with colour. Fresh, actively growing patches are pale grey to lilac-grey or a soft buff, and they look slightly velvety or dull rather than shiny. As the crust ages and dries it darkens through clay-brown and cocoa to a deep umber, and this is when the second key feature appears: the surface cracks. It breaks into small irregular plates and blocks separated by fine fissures, so the whole sheet looks like dried mud or crazed glaze, with paler wood or paler subsurface showing in the cracks. The specific name refers to that corrugated, cracked appearance.

Run a fingertip over a dry patch. It is hard, slightly rough, and firmly attached; it does not peel off in sheets and it does not feel waxy or gelatinous. Try lifting an edge with a knife point. The margin is usually thin, paler than the centre, sometimes finely fibrillose or slightly fringed, and it fades out gradually into the bark rather than ending in a raised rim.

The bristles - the genus feature

Every member of Hymenochaete carries setae: thick-walled, dark brown, pointed bristles embedded in the spore-bearing layer and projecting slightly above it. They are what makes the surface look velvety-matt to the naked eye and slightly bristly under a strong lens.

In the field, use a 20x lens on a fresh, uncracked patch held at a low angle to the light. You are looking for a fine dark stubble, like the nap on suede. Under a microscope the case is conclusive: the setae are conspicuously thick-walled, brown, spear-shaped and typically 40-90 micrometres long, standing among the basidia. No corticioid fungus outside this group and its relatives will show them. If you scrape a flake of crust and mount it, they are the first thing you see.

A drop of potassium hydroxide on the crust darkens the brown tissue markedly, close to black, which is a quick confirmation of the pigmented flesh type. The flesh itself is extremely thin - well under a millimetre - and uniformly brown in section.

Capturing spores

Crust fungi rarely give a satisfying print because the fruitbody is stuck to a branch, but it can be done. Cut a section of colonised bark, tape or prop it hymenium-downwards a few millimetres above a glass slide or a piece of dark card, cover the assembly with a box to keep humidity up, and leave it overnight in a cool place. A sparse whitish deposit forms if the fungus is actively sporulating.

Microscopically the spores are small, smooth, colourless, and cylindrical to slightly curved, roughly 4.5-6 x 2-2.5 micrometres. They are unremarkable on their own; the identification rests on the setae plus the macroscopic habit, with the spores serving mainly to confirm that the material is mature and correctly placed.

Substrate, season and ecology

Glue Crust is overwhelmingly a hazel fungus. It also occurs on ash, oak, willow, birch and other broadleaves, but hazel is where you should search first, and hazel-rich coppice, scrub and hedgerow are its strongholds. It colonises dead attached branches rather than fallen logs on the ground, which is central to understanding it: by staying up in the canopy it avoids the intense competition of the soil surface.

The gluing habit is a genuine ecological strategy rather than an accident. Mycelium growing out from a colonised dead branch contacts a neighbouring branch or the living stem, produces a dense pad of tissue at the junction, and cements the two together. The dead branch is then held aloft, drier and less accessible to soil-borne competitors, and the fungus gains a bridge to new wood. Hazel coppice managed on a long rotation can carry dozens of such welded junctions, and the tangles of suspended dead branches in an untended hazel stand are largely its work.

Being a perennial-tending crust, it is present year-round. Fresh growth and sporulation are best in autumn and through mild wet winter spells; in summer the crusts are dry, dark and heavily cracked but still identifiable.

Separating the look-alikes

• Hymenochaete rubiginosa shares the setae but forms rust-brown, leathery brackets with a reflexed upper surface, usually in tiered rows on oak stumps and posts. It is a bracket, not a flat sheet, and it does not glue branches together.
• Hymenochaete cinnamomea is a flatter cinnamon-brown crust that often turns up a narrow reflexed edge, favours alder and birch, and has a more fibrous, less cracked surface.
• Peniophora species, several of which also live on hazel, are waxy and often pinkish, lilac or greyish, they swell noticeably when wet and shrink and crack when dry, and they have no setae at all. A quick lens check for bristles separates them every time.
• Vuilleminia comedens grows under the bark of dead attached oak and hazel twigs, pushing the bark back in strips; it is pale, waxy-gelatinous, and again setae-free.
• Stereum species have a distinct free-growing upper surface with a hairy or zoned cap edge, and a smooth cream to ochre underside.
• Phanerochaete and Hyphodontia species form cream to ochre crusts with smooth, granular or toothed surfaces and lack the brown pigmented flesh and bristles.

Field checklist

Thin resupinate sheet on the underside of dead attached branches; grey-lilac when fresh, dark brown and cracked into plates when old; velvety matt surface with brown setae under a lens; extremely thin brown flesh darkening in potassium hydroxide; hazel the commonest host; and above all, branches cemented together at their contact points. That last habit is worth more than any single microscopic character in the field.