Yellow Stainer Identification Guide

First impression and where you meet it

Agaricus xanthodermus is a mushroom of managed and edge habitats rather than deep woodland. You meet it on a park lawn, along a hedge bank, on a grassy roadside verge, in a churchyard, at the margin of a playing field or pushing up through a garden border, often in troops, arcs or full fairy rings. It is common from midsummer into late autumn in temperate regions and has spread widely with urban planting.

The first impression is of a clean, solid white mushroom, larger than a Field Mushroom and noticeably squarer. Young caps of this species have a distinctive profile: the top is flattened and the shoulders are abrupt, so the button looks like a marshmallow or a small white drum rather than a dome. That boxy silhouette, seen from the side across a lawn, is worth training your eye on, because it flags the species before you have touched anything.

Cap

Five to fifteen centimetres across, occasionally larger. Starting as the flat-topped cylinder described above, it expands through broadly convex to nearly flat, sometimes remaining slightly depressed or truncate at the centre. The surface is white to greyish white, dry, smooth to very finely silky, and in dry weather it may crack into fine flat scales or take on a dingy grey-brown tone toward the centre. The margin is inrolled at first and may carry a few hanging veil fragments after the ring tears.

Run a thumbnail firmly across the cap margin. In this species the rubbed area flushes chrome yellow within seconds. The reaction fades over several minutes to a dull ochre or brownish tone, so watch it as you make it rather than checking back later.

Gills and spore print

The gills are free from the stem, crowded and narrow, and they run through a predictable colour sequence with age: pale greyish white or very faintly pinkish grey in the button, then dull pink, then a rich chocolate brown, and finally a dark blackish brown at full maturity. Note the starting colour carefully. This species begins pale grey-pink rather than the bright clean rose-pink of the true Field Mushroom, and that difference in the button stage is a genuinely useful character.

Take a print from a fully opened cap. Cut the stem flush, set the cap gills-down on white paper under a bowl for two to four hours, and expect a dark chocolate to blackish brown deposit. That dark print, combined with free gills and a ring, establishes Agaricus and eliminates the white-spored genera outright.

Stem, ring and base

The stem is six to fifteen centimetres tall, white, smooth above the ring and often finely fibrillose below, and it is usually hollow or loosely stuffed at maturity. It widens downward to a distinctly swollen, rounded, sometimes abruptly rimmed basal bulb. The ring is large, membranous and pendulous, hanging like a skirt from the upper stem, and its underside typically carries thickened patches arranged in a ragged cogwheel pattern.

The base test, which decides the identification

Never pull an Agaricus out of the ground by the cap. Dig it, brush the soil off the bulb, and cut the very base of the stem in half lengthwise with a sharp blade. In Agaricus xanthodermus the exposed flesh in the bulb turns an immediate, vivid chrome or egg yellow, strongest in the lowest centimetre and often forming a bright band across the cut. The colour is fully developed within ten to thirty seconds and then slowly ages to a dirty yellow-brown over the following minutes.

Three practical notes. First, do the cut on fresh, firm material; old, waterlogged or frost-hit fruit bodies react weakly. Second, the reaction is concentrated in the base, so a cut halfway up the stem may show only a faint tint. Third, other Agaricus species yellow too, but slowly, faintly and in a warmer ochre tone, and none of them produce that instant saturated chrome flash in the bulb.

Odour

Smell is a real diagnostic character here. Cut the base, cup the mushroom in your hands to warm it, and sniff. The odour is sharply chemical, variously described as inky, phenolic or like carbolic, quite unlike the warm mushroomy smell of the Field Mushroom or the sweet aniseed of the Horse Mushroom. The smell intensifies dramatically with warmth and with cutting the base, and it is the single character most likely to convince a doubtful observer.

Flesh and other details

The flesh is white and firm, thick in the cap centre, and shows no significant colour change on cutting except at the extreme stem base. There is no reddening. Under the microscope the spores are ellipsoid, smooth, thick-walled, brown in mass, and roughly 5 to 6.5 by 3 to 4.5 micrometres, with a small germ pore visible in good preparations.

Separating the look-alikes on structure alone

• Agaricus campestris, Field Mushroom. Gills are a clear bright pink from very early on, the ring is small, simple and often fugitive, the stem tapers slightly downward with no swollen bulb, and cutting the base gives no yellow flash. The odour is plainly mushroomy.
• Agaricus arvensis, Horse Mushroom. Larger and more elegant, the cap yellows slowly and warmly where rubbed rather than flashing chrome, the ring is large with a pronounced cogwheel underside, the base is tapered or club-shaped rather than sharply bulbous, and the odour is sweet aniseed or almond.
• Agaricus augustus, The Prince. Cap covered in concentric tawny-brown fibrillose scales on a paler ground, yellows where rubbed, and again smells of aniseed rather than ink.
• Agaricus bitorquis. An urban, path-cracking species with a firm low cap, a double sheathing ring, and no yellow reaction anywhere.
• Agaricus moelleri and relatives in the same section. These share the instant yellow base reaction and the inky odour but differ in the cap, which is covered in fine grey to blackish fibrillose scales over a paler ground rather than being plain white.
• Leucoagaricus leucothites. Superficially a white Agaricus but the gills stay white to very pale pink and never go chocolate, and the spore print is white.
• White Amanita species. Free gills and a ring, but a white spore print, a sac-like or rimmed volva at the base, and no yellowing.

Putting it together

A white, flat-shouldered lawn mushroom with free gills that go grey-pink then chocolate, a dark brown spore print, a bulbous base that flashes chrome yellow the instant it is cut, and a sharp inky smell is Agaricus xanthodermus. Any two of those characters could mislead; all four together are conclusive.