Hebeloma mesophaeum

Hebeloma mesophaeum is a small to medium, drab but distinctly two-toned member of the genus. The epithet means dark in the middle and describes the cap precisely: a chestnut to dark hazel-brown disc grades outward to a much paler clay or buff margin, a contrast that is easiest to see on young moist specimens.

The second key character is the veil. Unlike most Hebeloma species, which have essentially bare stipes, this one has a well-developed cortina-like veil that leaves white fibrils hanging from the cap margin in young fruitbodies and forms a faint fibrillose ring zone on the stipe, often darkened by trapped spores in maturity.

Ecologically it is a pioneer, one of the most reliable mycorrhizal fungi to appear on newly disturbed, burnt or planted ground. It fruits abundantly and often in huge numbers around young birches, willows and pines on made ground, road verges, restored quarries and forestry plantings, sometimes carpeting an area with hundreds of small brown caps.

Cap

2–5 cm across, convex to broadly bell-shaped and finally almost flat, often retaining a low umbo. The surface is smooth and slightly greasy when moist, with a dark chestnut to hazel-brown centre and a distinctly paler clay, buff or greyish margin. Young caps carry whitish veil fibrils along the margin, which soon disappear.

Gills

Sinuate to adnexed, moderately crowded, pale grey or whitish-clay at first and maturing to a dull snuff-brown. The edges are whitish and finely fringed, and may bead with tiny droplets in very humid conditions, drying to faint brown spotting.

Stipe

3–7 cm tall and 3–8 mm thick, cylindrical and slender, whitish and finely mealy toward the apex and brownish and fibrillose toward the base. A faint fibrillose ring zone marks where the veil detached, and this zone often becomes brown as spores accumulate on it. There is no membranous ring.

Flesh and spore print

Flesh is whitish in the cap and browning in the stipe base, unchanging when cut. The odour is weak and faintly radish-like. The spore print is snuff-brown, and the spores are almond-shaped and finely warted as in the rest of the genus.

This is above all a fungus of disturbed and recently colonised ground. It fruits on sandy and gravelly soils, road verges, restored quarry floors, burnt sites, forestry plantings, garden landscaping and made ground, wherever young trees have been established. Birch, willow, poplar and pine are its most frequent partners, though it associates with many other hosts.

Fruiting runs from late summer to late autumn, often continuing well after most woodland fungi have finished, and can be extremely prolific, with hundreds of fruitbodies over a small area of bare ground. It is widespread and common across Europe, North America and temperate Asia, and follows planted trees into urban and industrial landscapes.

Hebeloma mesophaeum is ectomycorrhizal, forming root sheaths with birch, willow, pine and a range of other trees. It belongs to the group of early-stage mycorrhizal fungi that colonise the roots of seedlings and saplings on poor, disturbed substrates, where they play a disproportionate role in nutrient uptake and in helping young trees survive drought and low fertility.

Because it colonises so readily and fruits so heavily, it is often the dominant mycorrhizal species in the first years of a new plantation or on reclaimed industrial land, gradually giving way to a more diverse community of later-stage fungi as the canopy closes and litter accumulates. It also fruits well after fire, taking advantage of the flush of mineral nutrients and reduced competition on burnt ground.

Hebeloma crustuliniforme is larger, uniformly pale clay-buff with no dark centre, has a stronger radish odour and no veil zone on the stipe. Hebeloma sinapizans is larger and stouter still, again ringless and uniformly coloured.

Cortinarius species share the cobwebby veil but produce rusty-brown rather than snuff-brown spore prints, which stain the veil zone a distinctly rusty colour. Fibrecaps such as Inocybe lacera occupy the same disturbed sandy ground but have dry, torn, fibrous-scaly caps rather than the smooth, slightly greasy two-tone cap seen here. Pholiotina and Tubaria species are much more fragile with thinner flesh and different gill attachment.