Dark Fieldcap Identification Guide

The Setting You Find It In

Despite the "fieldcap" in its name, the Dark Fieldcap is a mushroom of shaded, damp ground rather than open pasture. Picture a late-autumn walk along a woodland ride or the soft, rutted edge of a forestry track cut through ash, beech and hazel on heavy clay or chalk. The ground there is bare or thinly mossy, churned a little by boots and tyres, and plastered with wet fallen leaves. Scattered troops of dull brown caps push up along the trodden margin, sometimes following the line of a buried twig or the lip of a ditch. That combination of shaded broadleaf woodland, base-rich soil, lightly disturbed damp ground and a late date is half the identification before you have even crouched down.

First Impression

At a glance this is a solid, medium-sized brown mushroom with a greasy sheen and a conspicuous pale collar partway up the stem. It is not thin and brittle like a Psathyrella, and not warm rusty-orange like a Pholiota. The cap reads as genuinely dark, a wet date-brown with an olive cast, and the ring stands out against it as a light band. A ringed brown mushroom growing out of woodland soil rather than out of wood, with mature gills that stay dull brown instead of turning foxy, should put Agrocybe erebia near the top of your list.

Reading the Cap

Take a cap 2 to 6 cm across. Young ones are strongly convex, almost hemispherical, with the margin curved inward and often hung with a few pale veil shreds. With age the cap flattens, sometimes keeping a low broad umbo, and the surface develops fine radial wrinkles or a rugulose, slightly brain-like puckering towards the centre. Run a finger over it: when moist it is distinctly greasy to sub-viscid, and grit and leaf fragments stick to it.

The species is strongly hygrophanous, so one troop can look like two species. Wet caps are blackish date-brown; drying caps fade from the centre outwards to clay, ochre or tan, usually leaving a darker damp band around the rim. Always judge the colour from the wettest cap in the group, and note the two-tone effect rather than treating it as a problem.

The Gills

Turn a cap over. The gills are moderately crowded, fairly broad, and attached to the stem, running from adnate to shortly decurrent with a distinct little tooth. That decurrent tooth is worth registering, because it gives the underside a subtly funnelled set that many brown-spored genera lack. Colour runs pale clay or greyish buff in youth, then dull brown, tobacco-brown or cigar-brown at maturity. They never take on the bright rust of a Galerina or the blackish-purple mottling of a Hypholoma. Under a hand lens the gill edge is finely whitish and slightly uneven.

The Ring and Stipe

The stipe is 4 to 8 cm tall and 4 to 8 mm thick, more or less equal, occasionally slightly swollen or tapering into a short rooting point. The single most useful character is the ring: a genuine membranous ring set high on the stem, often loose and movable in age, with a finely grooved or striate upper surface where the developing gills pressed against it. Above the ring the stem is whitish and finely mealy; below it the surface is longitudinally fibrillose and grows browner towards the base, which is usually soil-stained and knitted with white mycelial threads. In old, weathered fruitbodies the ring collapses into a persistent brown-dusted zone once fallen spores have coloured it.

Flesh, Odour and Colour Change

Split a fruitbody lengthwise from cap to base. The flesh is whitish to pale brown, firm rather than watery in the cap, more fibrous and darker down the stipe. There is no dramatic reaction to cutting or bruising: no blueing, no reddening, no rapid blackening. That stability is itself informative, because several similar brown-capped genera do react. The smell of crushed fresh flesh is markedly mealy or farinaceous, a flour-and-cucumber note easiest to catch by tearing a cap and cupping it in your hands for a few seconds.

Taking a Spore Print

The print settles the genus faster than anything else. Cut the stipe flush with the gills, set the cap gill-side down on a sheet that is half white and half black, cover it with an upturned cup to slow drying, and leave it three to six hours somewhere cool. A good deposit is a dull, slightly milky brown, best described as snuff-brown, cigar-brown or date-brown. Set it beside a rusty print from a Galerina or Pholiota and the difference is obvious: those are warmer and more orange. Under the microscope the spores are smooth, ellipsoid, roughly 10 to 14 by 5 to 7 micrometres, thick-walled and slightly truncate at one end, with only an indistinct germ pore.

Substrate and Season

Agrocybe erebia is a soil-dwelling saprotroph that recycles leaf litter, fine woody debris and buried twigs rather than colonising standing logs. It shows a firm preference for base-rich, often calcareous ground under broadleaved trees, and a strong habit of appearing along paths, ride edges, ditch banks and other lightly disturbed places where the litter layer is thin. Fruiting is autumnal, usually from September into November in temperate regions, sometimes running on until the first hard frosts. Fruitbodies come up gregariously, occasionally two or three fused at the base, but never in the dense tufts typical of wood-rotting species.

Separating the Look-alikes on Morphology Alone

Agrocybe praecox is the commonest confusion. It shares the membranous ring and dull brown print but fruits in spring and early summer, is much paler in cream, buff or tan tones, has a drier cap that often cracks, and usually shows a rooting stipe base with white cords.

Agrocybe dura is paler again, a mushroom of grassland, verges and gardens with a whitish cap that cracks into plates in dry weather.

Agrocybe pediades and its small relatives are ringless, yellow-ochre, dry-capped grassland species, far smaller and never with the striate ring.

Kuehneromyces mutabilis and the Pholiota species grow in tufts directly from wood, carry scaly or shaggy fibrils on the stipe below the ring, and drop a clearly rusty print.

Hypholoma species are tufted on wood, with yellowish or olive-tinted young gills ageing to purple-brown and a purple-brown to blackish print.

Cortinarius species have a cobwebby cortina instead of a grooved membranous ring, and their prints are rusty.

Psathyrella species are thin-fleshed and brittle, with a strongly striate cap margin and a dark grey-brown to blackish print; the cap flesh snaps rather than bends.

Field Checklist

Dark, greasy, hygrophanous cap that wrinkles with age; attached gills with a short decurrent tooth ripening dull brown; a grooved membranous ring set high on a whitish stem that browns downwards; a mealy smell; flesh that does not change colour; a snuff-brown spore print; and growth from base-rich woodland soil along a path in autumn. Score all seven and you are standing over a Dark Fieldcap.