Inky Mushroom Identification Guide

First impression and setting

The find usually happens on a path edge under beech or hornbeam, or in an old park with mature broadleaved trees on rich, often chalky ground. What catches the eye is a medium-sized white mushroom that reads as grey from a distance — the cap looks dusty or smudged, darkest in the middle, and the overall tone is cold rather than creamy.

Note the habit before you touch it. This species comes up from soil and leaf litter, singly, in scattered troops, or occasionally in arcs and rings, and it favours the humus-rich strip between path and tree rather than deep undisturbed woodland. Late summer into autumn is the window.

Working through the cap

Diameter is typically 4–10 cm. The shape passes from hemispherical, through convex, to nearly flat with a low broad umbo or, in some collections, a slightly depressed disc.

The surface is the first real character. The white or pale ground colour is covered with fine, flattened, fibrillose scales in grey-brown to blackish-grey. At the centre they merge into an almost solid dark disc; towards the margin they break up and thin out, so the edge stays whitish. The scales lie flat and radial rather than standing up. Under a lens they look like torn fibres, not granules.

Check the margin for veil fragments, which may hang as small white flaps in young specimens, and note whether the cap edge extends slightly beyond the gills.

The spore-bearing surface

Turn the cap over. The gills are free — they do not touch the stipe — crowded, narrow, and evenly spaced, with several ranks of shorter gills interleaved.

Colour changes with maturity and is worth staging: pale greyish-pink, almost whitish-grey, in buttons still covered by the veil; clean pink as the veil breaks; then progressively darker through purplish-brown to a deep chocolate or blackish-brown at full maturity. The gill edge often stays paler and looks minutely fringed under a lens. Gills that are white and stay white belong to a different genus.

Stipe, ring and base

The stipe is 5–10 cm long and 1–1.5 cm thick, white, smooth above the ring and faintly fibrillose below, and hollow or narrowly stuffed when split.

The ring sits high on the stipe, is membranous, pendulous and persistent, and often carries pale scaly patches or a slight cog-wheel pattern on the underside. It is a substantial structure, not a fleeting cobweb.

The base is diagnostic. Dig, do not pull — the whole point is to recover an intact base. It swells into a distinct bulb, and in good material the bulb is marginate: it ends in an abrupt rim or shoulder, as if the base had been pressed flat onto a plate. There is no volva, no cup, and no sac of tissue.

Colour change on cutting

Slice the mushroom in half vertically, base included, and watch the base for thirty seconds. In this species the flesh of the bulb flushes an immediate, bright chrome yellow, strongest right at the base and fading over the next few minutes to a dingy ochre or brownish tone. Scratching the cap surface near the margin with a nail gives a weaker, slower yellow.

At the same time, smell the cut base. The odour is strong, chemical and phenolic — ink-like, reminiscent of a hospital corridor or an old dark room — and it becomes far more obvious if the cut specimen is enclosed in a tub for a few minutes or gently warmed in a closed hand.

Grey fibrillose cap scales, a marginate bulb, an instant chrome-yellow base and a phenolic odour together place the collection in Agaricus section Xanthodermatei and, within that group, on A. moelleri.

Taking a spore print

Because the print is dark, use white paper. Cut the stipe off level with the gills, set the cap gill-down, cover it with a bowl to stop draughts drying it out, and give it two to six hours.

The deposit should be a rich dark chocolate brown. It is not rusty-orange, which would indicate Cortinarius and relatives; not purplish-black, which suggests Coprinopsis and allies; and certainly not white, which would move you to Amanita, Lepiota or the tricholomatoid genera. Under the microscope, spores are small, smooth, thick-walled and ellipsoid, roughly 4.5–6 by 3–4 micrometres, with an inconspicuous germ pore.

Habitat, substrate and season

The species is saprotrophic, living on decaying leaf litter and humus rather than on tree roots or wood. It appears in and beside deciduous woodland — beech, oak and hornbeam are typical — as well as in parks, avenues, churchyards, hedge bottoms and shrubbery, often where soil is base-rich or chalky. It is a European species that has been carried elsewhere with landscape planting.

Fruiting runs from mid-summer through autumn, following rain, and a fruiting patch is usually persistent from year to year.

Separating the closest look-alikes on morphology alone

Agaricus xanthodermus shares the yellowing base, the phenolic odour and the marginate bulb, but its cap is smooth and silky white, often cracking into flat plates in dry weather, and never carries dense grey fibrillose scaling. The cap surface alone separates them.

Agaricus phaeolepidotus has warmer brown scales with less contrast against the ground colour, a base that yellows more weakly and more slowly, and a bulb that is less sharply rimmed.

Agaricus placomyces, the closely related North American species, is very similar in scale pattern; on morphology it is separated on finer scale colour and stature, and geography does most of the work.

Agaricus sylvaticus and A. langei also have brown fibrillose scales, but their flesh turns pink to blood-red on cutting rather than yellow, the base does not flush chrome yellow, and the odour is pleasant and mushroomy.

Agaricus impudicus looks close in scale pattern but shows no yellow reaction anywhere and lacks the phenolic odour.

Agaricus arvensis and A. silvicola do yellow, but the reaction is a slow ochre limited mainly to the cap surface, their bulbs are not sharply marginate, and their odour is sweetish and aniseed-like.

Lepiota and Macrolepiota species have white spore prints and gills that stay white, and often a ring that slides freely on the stipe.

Amanita species have white gills, a white print, and a sac-like volva or ring of scales at the base rather than a marginate bulb. Whenever the base looks bulbous, take a print before deciding anything else.