Sebacina incrustans Identification Guide

The first impression

Most people walk past this one, or assume it is something spilled. You are on a shaded hedgebank, a woodland edge, a roadside verge under beech or hazel, or a mossy slope in parkland, and a patch of ground looks as though it has been dipped in candle wax. A whitish to cream crust, anywhere from a few centimetres to more than 30 across, spreads over the ground and rides up and over the bases of grass stems, moss shoots, small herbaceous stems, dead leaf stalks and fallen twiglets, welding them into a single lumpy mass.

The crucial impression is that the fungus is not sitting on a substrate so much as engulfing everything it meets. That habit of climbing over and incrusting live and dead vegetation is what the specific name records, and it is the most useful field character in the whole account.

Reading the growth form

Look at the outline and the profile. The patch is resupinate, meaning it is spread flat rather than forming a cap and stalk, but it is far from a smooth film. It builds up into nodular, lobed masses with rounded knobs, blunt short outgrowths and irregular ridges, often 1 to 4 mm thick and locally thicker where several stems have been enveloped together. Edges are indefinite and fade into the moss and litter rather than ending in a defined rim.

Colour is white to cream when fresh, ageing greyish, buff or dingy ochre, and drying darker, harder and horn-like. Wet weather makes it look softer and paler; drought shrinks it against the stems it has coated.

The fertile surface, and a decisive negative

The whole outer face of the crust is the fertile hymenium. Put a lens on it and check what is not there. There are no pores, no gills, no teeth or spines, no ridges arranged in any pattern, and no visible fruiting cups. The surface is smooth to slightly warty and uneven, matt to faintly shining, and the unevenness is a consequence of the shapes it has grown over, not of any organised fertile structure.

This negative is worth stating clearly in a record, because it removes the toothed and poroid crusts in one line.

Texture, which you must test by touch

Press the surface gently with a fingertip. Fresh material is waxy to soft-gelatinous with a texture often compared to firm tallow: it yields slightly, keeps its shape, and does not wobble. It is not sticky, and it is not the translucent, quivering jelly of the true jelly fungi. If you pinch a lobe off, it holds together as a coherent waxy fragment rather than smearing or collapsing into slime.

Dried material becomes hard, brittle and much darker, and it will revive partially after rain, so check texture on genuinely fresh material.

Sectioning

Cut a nodule cleanly with a razor blade or a sharp knife and look at the cut face with a lens. The fungal tissue is homogeneous, whitish and waxy, usually half a millimetre to a millimetre and a half thick, and it forms a continuous coat. Inside you will see the enclosed plant material, a grass stem base, a moss shoot or a twig, still recognisable and structurally intact.

That internal detail matters. The fungus wraps and coats; it does not rot the stem into mush, and there is no cavity where the plant tissue used to be. If your section shows decayed wood fibres and no distinct fungal layer, you are looking at a wood-rotting crust instead.

Spores

A print is possible but faint. Break off a fragment, rest it hymenium-down over a glass slide inside a covered box, and leave it overnight; the deposit is white and sparse. Microscopy is where this species is confirmed properly.

Under the microscope the basidia are the giveaway: they are longitudinally septate, divided into four cells by vertical walls, which is the defining feature of this group of heterobasidiomycetes and is entirely different from the club-shaped four-spored basidia of ordinary crust fungi. The hyphae are thin-walled, hyaline and clamped, and the spores are large, smooth, hyaline and broadly ellipsoid, roughly 14 to 20 by 9 to 12 micrometres, often germinating by repetition to produce secondary spores.

Habitat, season and ecology

Search shaded, humid, base-rich ground: hedgebanks, woodland edges and rides, roadside verges, old parkland and mossy slopes, especially under beech, hazel and oak, and often on calcareous or otherwise base-rich soils. The main season is summer to late autumn, and patches can persist for weeks, drying and reviving with the weather.

Ecologically this is not a wood rotter. The Sebacinales include many ectomycorrhizal partners of trees, and this species forms mycorrhizal associations with nearby broadleaves while spreading its fruiting crust over the soil surface and low vegetation. That explains why it turns up on the ground under trees rather than on logs, and why patches recur in the same spot year after year. Nomenclatural note: it is now often placed in the genus Helvellosebacina, so records may appear under either name.

Separating it from look-alikes on morphology alone

• Sebacina epigaea: forms flatter, greyish to lilac-tinged patches on bare soil that lift away as waxy-brittle sheets, with far less tendency to swallow stems and without the nodular lobed build-up.
• White Brain, Exidia thuretiana: grows on dead attached hardwood twigs and branches only, never over grass and moss on the ground; it is translucent, wobbly and truly gelatinous, and it collapses to a thin film in dry weather.
• Corticioid crusts such as Peniophora, Hyphodontia and Athelia: these lie flat on wood or bark, are membranous, papery or cobwebby rather than waxy-thick, and they do not build nodular masses over living vegetation. Athelia is white but thin and cottony and peels away easily.
• Tulasnella and Ceratobasidium: extremely thin waxy films, often pinkish or greyish, effectively without thickness and without lobes, though they share the odd basidium type.
• Tremella species: gelatinous, brain-like or lobed, growing on wood and usually parasitic on other fungi, translucent rather than opaque waxy white.
• Mucilago crustacea, a slime mould, is the most convincing look-alike of all: a white crusty mass spreading over grass stems in exactly the same situations. Separate it by texture and section. It begins as a wet, flowing plasmodium, dries to a chalky, fragile, calcareous crust that crumbles at a touch, and its interior becomes a dark powdery spore mass. Sebacina incrustans stays waxy-firm, whitish and homogeneous inside with no internal powder.

A working order

Note the habit first, a crust engulfing grass and moss on the ground. Confirm waxy, tallow-like texture rather than jelly or chalk. Cut a nodule and check for a homogeneous whitish layer over intact plant tissue. Record the trees overhead, since the mycorrhizal partnership explains the location. Then, if you want a firm determination, take a fragment home and look for the longitudinally divided basidia, which settle the matter conclusively.