KingdomFungiGenusAgaricusSpeciesxanthodermus

Agaricus xanthodermus

T1 · Gastrointestinal irritantPublished 2026-09-14By The Spore DeskSummer / Autumn

The yellow stainer is the species behind most of the mushroom poisonings that are not Chlorophyllum, and it is also the easiest of the T1 species to exclude, because the test takes five seconds and needs no equipment: scrape the base of the stem with a thumbnail. It grows in the situations where people are least suspicious — mown lawns, verges and playing fields.

Block 01 · Record

The base-scrape test

Do not test the cap. Cut or scrape the very bottom of the stem and watch for two to five seconds. Agaricus xanthodermus turns bright chrome yellow at the base and then fades to brown; Agaricus campestris and A. arvensis do not yellow at all. The cap cuticle of A. xanthodermus may also yellow in potassium hydroxide, but an alkali test in the field is unnecessary when the base-scrape works on its own. Run the test on every specimen you intend to keep, not on one representative of the troop.

The odour and the two-tone gills

The second field character is the smell. Cut a cap and smell it immediately: A. xanthodermus produces a phenolic, inky, hospital-like odour that most people find unpleasant and that becomes much stronger when several fruit bodies are kept together. The gills pass from pink through a distinctly purple-brown to deep chocolate, and the spore print is deep chocolate brown. Spores are small for the genus, 5.5-7.5 x 4-5 micrometres, smooth, thick-walled and inamyloid.

Where it grows and when

This is a nitrophilous saprotroph of grass and enriched soil and it tracks management. Verges, lawns, parks, playing fields, garden beds and hedge bottoms are standard; it tolerates heavy trampling and appears in troops, sometimes forming partial rings. In temperate Europe it fruits from June to November, and it is often the first large agaric to appear on turf that has been fed or top-dressed in the preceding season.

The syndrome

The gastrointestinal syndrome produced by A. xanthodermus is short and self-limiting: nausea, vomiting and diarrhoea beginning thirty minutes to two hours after the meal and resolving within a few hours. It causes no organ damage and is rarely dangerous in adults, but it is thoroughly unpleasant, it can occur from a small amount, and it is frequently mistaken for a food hygiene problem rather than a mushroom, which delays the right conclusion for the household.

Points to carry to the bench

Scrape the stem base — chrome yellow within two to five seconds, then fading to brown.
Phenolic, inky odour that intensifies when several specimens are kept together.
Deep chocolate brown spore print; spores small at 5.5-7.5 micrometres and inamyloid.
Grows in mown, trampled or fertilised grass from June to November; T1 syndrome is short and self-limiting.

Morphology
Cap	shape: Hemispherical then convex to plano-convex, 50-150 mm across, occasionally larger; the margin is inrolled at first and can be slightly appendiculate.surface: White to grey-white, silky when young, breaking up into appressed grey-brown squamules that concentrate at the disc and leave a paler fibrillose margin.margin: Inrolled then straight; thin veil remnants may hang from the edge in dry weather.
Gills	attachment: Free, with a clear 2-4 mm gap at the stem apex.spacing: Crowded, 3-4 gills per millimetre, with three series of intermediates.edge: Pale and slightly floccose; the gills pass from pink through purple-brown to chocolate brown as the spores mature.
Spore print	Ellipsoid, smooth, with a small germ pore and a short hilar appendix.Smooth, thick-walled, inamyloid; the spores are noticeably smaller than those of most white-capped Agaricus.
Habitat and season	Saprotrophic in grass and in enriched soil: verges, lawns, parks, playing fields, garden beds, the base of hedges and the edges of arable fields. Tolerates heavy trampling and appears in troops, sometimes in partial rings, often where the turf has been fed or top-dressed.

Block 02 · Chemistry

Reagent	Verdict	Reaction	Weight
KOH5	positive	Cap cuticle flashes chrome yellow within seconds, then fades through orange to brown over about a minute — the reaction is fast enough to be visible while you are still holding the pipette.	2.00
MELZ	inamyloid	Spore wall stays yellow-brown; no blue-black development.	3.00
FESO4	negative	Flesh unchanged or very faintly grey after 60 s; no salmon-pink.	3.00
NH4OH	positive	Cap and flesh turn yellow immediately, matching the KOH result; the two should be run on opposite sides of the same cap so that the duplicate is not mistaken for independent confirmation.	2.00

Injury stain Chrome yellow, and it is the single most useful field character in the species. Scrape the very base of the stem with a thumbnail and the exposed flesh turns bright yellow within two to five seconds, then fades to brown over the following minute. Cut the cap flesh and it may stay white; the reaction is concentrated at the stem base, which is why the test must be done there and not on the cap.

Block 03 · Confusion

Separating character	This record	Confused with
Flesh does not yellow at the base, there is no phenolic odour, and the ring, though fragile, is present on most specimens; the cap is smooth rather than squamulose.	Agaricus xanthodermusT1	Agaricus campestris▲ +1
Cap flavescens-yellow with a distinct cogwheel-shaped double ring on the underside, and an odour of aniseed rather than phenol.	Agaricus xanthodermusT1	Agaricus arvensis▲ +1
The cultivated species: two-spored basidia, no yellowing at the base, and a mild mushroom odour; it is not a risk pair but it is the reason people assume a white lawn agaric is safe.	Agaricus xanthodermusT1	Agaricus bisporus▲ +0

Toxicity ladder

Reproducible vomiting, cramping and diarrhoea with no hepatic, renal or neurological involvement. Onset is short and recovery is complete, but the same species can produce a severe dehydrating episode in children and in people on diuretics or beta blockers.

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