Hebeloma mesophaeum Identification Guide

The First Impression

The habitat gives this one away before the mushroom does. Think of a young birch plantation on sand, a restored quarry, a heathland track, a burnt patch two seasons on, or a strip of poplar and willow whips planted along a new road. Ground like that, thin and disturbed and colonised by pioneer trees, is exactly where Hebeloma mesophaeum turns up, and it turns up in numbers.

What you see is a scatter or dense troop of medium-small mushrooms with a distinctly two-toned cap: a dark, warm brown centre grading fairly abruptly into a much paler, almost whitish or pinkish-buff outer zone. From above, a group looks like a set of small targets. The specific epithet records exactly this, meaning dark in the middle. The fruitbodies are firm, low-slung and unglamorous, with pale stems and no bright colours anywhere. Lean in and there is a faint radish-like smell, weaker and more grassy than in some of its relatives.

Reading the Cap

Caps are 2 to 5 cm across, occasionally reaching 6. The young shape is hemispherical to convex, opening to broadly convex or flat, generally with a low, blunt umbo. The surface is smooth and bald over most of the cap, slightly viscid or greasy when wet and matt when dry.

Colour is the primary field character. The disc runs date-brown, reddish-brown or milk-chocolate, and the margin is cream, pale pinkish-buff or greyish-white. The boundary between the two is not razor sharp but it is obvious at a glance, and it persists as the cap ages, though the pale zone darkens somewhat and the whole cap can become more uniformly brown in very old or very dry specimens. When in doubt, look at several fruitbodies in the troop; there will nearly always be young ones showing the contrast clearly.

Now examine the margin with a lens on the youngest cap you can find. You should see the remains of a whitish, fibrillose to cottony veil hanging as small fibrils or forming a delicate fringe. This veil is a defining feature of the species. It is not a cobweb-like cortina of the Cortinarius type, but a thin fibrous partial veil, and it collapses onto the stipe as the cap opens.

Gills, Stipe and Flesh

Gills are crowded, narrow, and adnate or slightly notched at the stipe. They start pale grey-buff or clay, then darken through dull brown to a warm snuff-brown at maturity. The edges are whitish and finely uneven under a lens, and in humid weather they may bead very faintly with droplets, though this is much less pronounced and much less reliable than in Hebeloma crustuliniforme. Dried droplet spots on the edge, if present, are sparse.

The stipe is 3 to 7 cm long and 3 to 8 mm thick, slender in proportion to the cap, cylindrical or slightly widening downward. It is whitish at the apex and darkens brownish from the base upward with age, so an old specimen shows a pale collar over a brown lower stipe. The surface is silky-fibrillose, and somewhere in the upper half there is usually a fibrillose ring zone or a distinct band of veil fibrils that catches falling spores and turns brown, giving a smudged brown girdle. That combination, pale two-toned cap plus fibrillose girdle, is the fastest route to a name.

The stipe apex is finely pruinose. The stipe is stuffed then hollow, and it snaps rather than shredding. There is no membranous ring, no volva and no rooting base; the bottom simply tapers into white mycelium threading through sand or gravel.

Cut a fruitbody vertically. The flesh is whitish in the cap and upper stipe, becoming brownish in the lower stipe and base, and it does not change colour on exposure. The smell is weakly raphanoid or slightly earthy.

Taking and Reading a Spore Print

Set two or three mature caps gills-down on white paper under a cover for three to five hours. The deposit is dull clay-brown to snuff-brown, the same warm, muted, coffee-with-milk tone as its congeners. Check it in daylight against a white background: not rust-orange, not purple-black, not white.

Microscopically the spores are amygdaliform, that is almond-shaped with a slightly drawn-out apex, and they measure roughly 8 to 11 micrometres by 5 to 6, making them noticeably smaller than those of Hebeloma sacchariolens. The surface is only weakly roughened, sometimes appearing nearly smooth at low magnification, and the spores develop reddish tones in Melzer's reagent. Cheilocystidia on the gill edge are cylindrical to narrowly clavate.

Substrate, Season and Ecology

Hebeloma mesophaeum is ectomycorrhizal and behaves as a pioneer. It partners readily with young trees, especially birch, willow, poplar and pine, and it is one of the first ectomycorrhizal fungi to colonise newly planted or newly disturbed sites. Sandy soil, gravel, spoil, mine waste, road embankments, restored land and old fire sites are all typical, as are nursery beds and young shelterbelts. It can also appear in mature woodland where the ground has been churned or scraped.

The season is long. Fruiting begins in late summer and continues right through autumn, and this is one of the species you still find after the first hard frosts have cleared most other agarics. Fruitbodies frequently occur in dense gregarious troops rather than singly, sometimes carpeting a stretch of track edge.

That pioneer ecology is worth understanding because it changes what you expect. Where a mature-woodland Hebeloma might come and go, this species can dominate a young plantation for several years and then fade as the stand closes and other fungi take over the root tips.

Separating It From Look-alikes

Hebeloma crustuliniforme is larger and consistently paler, with a cream to clay cap that shades gently from centre to rim rather than showing a dark disc, no fibrillose veil zone on the stipe, gills that weep conspicuously, and a stronger radish smell. If your specimen has no veil and a strongly beaded gill edge, go there.

Hebeloma sacchariolens has an overpowering sweet scent and much larger spores.

Hebeloma radicosum is far bigger, has a scaly cap, a genuine membranous ring, a long rooting base and an almond smell.

Within the wider group of small brown Hebeloma species, several close relatives exist and separating them fully needs microscopy. In the field, the combination of a sharply two-toned cap, a persistent fibrillose veil zone on the stipe and a pioneer habitat is as far as most people can go, and that is a legitimate stopping point.

Outside the genus, Inocybe species have dry, radially fibrillose or scaly caps, often with a pointed umbo and a splitting margin, and a dull brown print; their caps are matt and fibrous rather than smooth and greasy. Cortinarius species show a genuine cobwebby cortina and give a rusty orange-brown print, which is decisively different from clay-brown. Alnicola and Naucoria species growing with alder are more slender, more uniformly rusty-brown, and give a browner-rust print. Psathyrella species are far more fragile with a whiter, brittler stipe and a much darker print.

Field Checklist

Two-toned cap with dark brown disc and pale buff margin, smooth and greasy when moist; whitish fibrillose veil on the young margin and a brown-dusted fibrillose girdle on the stipe; crowded gills going snuff-brown, weeping only weakly; slender stipe browning from the base up, no ring and no root; unchanging whitish flesh, weak radish smell; clay-brown print with small weakly roughened almond spores; sandy or disturbed ground with young birch, willow, poplar or pine, late summer to hard frost.