Dark Fieldcap

Dark Fieldcap (Agrocybe erebia) is the darkest and most distinctly ringed of the common European fieldcaps. Where most Agrocybe species are pale tan or straw-coloured mushrooms of lawns, verges and woodchip, this one is a creature of shaded, humus-rich woodland floors, with a deep date-brown to sepia cap that looks almost varnished in wet weather.

The genus Agrocybe sits in the family Strophariaceae (older books place it in the Bolbitiaceae) and is defined by its dull brown spore deposit, its adnate to slightly decurrent gills, and its fleshy, terrestrial habit. A. erebia is unusual within the genus for combining a genuinely dark cap with a well-formed membranous ring on the stipe — a combination that makes it recognisable in the field once the eye is trained.

The specific epithet erebia derives from Erebus, the classical personification of darkness, and refers to the mushroom's shadowy colouring. It is a modest, unshowy fungus, but its habit of appearing in troops along damp woodland tracks in late autumn, long after many other species have finished, gives it a distinct seasonal signature.

Cap

The cap is 3–8 cm across, at first convex or bluntly bell-shaped and later flattening, often with a low broad umbo. Colour is a deep date-brown, sepia or chestnut, paler and more clay-coloured towards the margin and fading noticeably as it dries. When moist the surface is slightly viscid and shining; as it dries it develops a characteristic radially wrinkled or minutely rugulose texture, especially towards the centre. The margin may be faintly striate and can carry hanging remnants of the veil in young specimens.

Gills

The gills are adnate to shortly decurrent — running a short way down the stipe — and are moderately crowded. They begin pale greyish or clay-buff and darken with maturity to a dull cigar-brown or umber as the spores ripen. The gill edges remain paler and are finely fringed when examined with a lens.

Stipe

The stipe is 4–9 cm tall and 4–10 mm thick, fairly slender, more or less equal or slightly swollen at the base, and whitish to pale brown, darkening downwards with age. Its most useful feature is the membranous, persistent ring set in the upper half; the upper surface of this ring collects falling spores and becomes stained dark brown. Above the ring the stipe is finely grooved or striate; below it is fibrillose.

Flesh and spore print

The flesh is whitish to pale brown, thin in the cap, and does not change colour appreciably when cut or bruised. The spore print is a dark brown, distinctly deeper in tone than the tobacco-brown of most other fieldcaps, and spores are smooth and ellipsoid with a small germ pore.

Dark Fieldcap grows on the ground in damp, nutrient-rich deciduous woodland, favouring beech, ash and hazel on calcareous or base-rich soils. It has a strong association with disturbed, compacted ground — woodland rides, path edges, ditch banks and the margins of clearings — where leaf litter is deep and moisture is retained. It fruits singly, in small groups or in loose troops.

The season runs from late summer well into late autumn, and the species often persists after the first frosts. Its distribution is broadly European and temperate, with records across Britain, Ireland and much of continental Europe, extending into parts of northern Asia and reported sporadically in North America.

Agrocybe erebia is saprotrophic, drawing its nutrition from decaying leaf litter, buried woody fragments and humus rather than forming a partnership with living trees. Its mycelium permeates the upper soil horizon and the litter layer, breaking down cellulose and lignin-rich debris and returning nutrients to the woodland floor.

Because it depends on accumulated organic matter, it is most productive where litter is undisturbed and the soil stays moist, which explains its preference for shaded, path-side ground in mature woodland. Fruiting is triggered by autumn rain following warm spells, and the dark spores released from the gills are trapped readily on the sticky, wet ground beneath, dispersing further on air currents and on the coats of passing animals.

The other common fieldcaps are all paler. Spring Fieldcap (Agrocybe praecox) has a tan to cream cap, appears in late spring and early summer rather than autumn, and has a thinner, more fugacious ring. Wrinkled Fieldcap (Agrocybe rivulosa) is a pale buff, strongly wrinkled species of fresh woodchip mulch with a large flaring ring. Common Fieldcap (Agrocybe pediades) is a small ochre-yellow grassland mushroom without any ring at all.

Ringed brown woodland mushrooms in Pholiota can suggest it, but those grow in tufts directly on wood and usually have scaly or slimy caps. Some Hypholoma and Psathyrella species share the dark cap tones, but Hypholoma has purple-brown spores and Psathyrella is brittle, hollow-stemmed and yields a blackish deposit rather than the warm dark brown of A. erebia.