Yellow Stainer

White cap with a flattened, boxy top

5–15cm across, smooth and white to greyish, often ageing with fine brown flecks. Young caps are distinctly square-shouldered, as though the dome has been pressed flat, before opening out. The shape is visible across a lawn and is worth noticing before you pick anything up.

Bright chrome yellow where rubbed

Scratch the cap edge firmly with a fingernail. A vivid, almost artificial yellow appears within seconds, quite unlike the slow buff or ochre flush some other white mushrooms show. It fades to dull brown after a few minutes, so judge it on first appearance.

Intense yellow at the stem base when cut

Slice the stem lengthways right through the swollen base and look at the bottom few millimetres. This is where the reaction is strongest and fastest, and it is the decisive test on this species. Faint or slow yellowing higher up the stem alone does not settle the question either way.

Smells of ink or disinfectant

Crush the cut base and smell it. There is a chemical, phenolic note like old-fashioned antiseptic or a hospital corridor, sometimes faint when raw and unmistakable once heat is applied. A clean mushroomy or almond smell is a different species.

Gills free of the stem, whitish to pink to dark brown

The gills never quite meet the stem, leaving a narrow gap. They start pale greyish-white, pass through dull pink and darken to chocolate brown with age. There is a ring on the upper stem, and no sac or bag at the base.

Thick, pendulous, double ring on the stem

The ring hangs like a skirt from the upper stem and is layered, often with soft scaly patches on its underside. Field mushroom's ring is thin, single and often reduced to fragments by the time it is picked.

Abruptly swollen stem base

The stem is usually white and smooth with a bulbous, sometimes flattened base sitting in the soil. There is no sac-like volva around it, and slicing that base lengthways is the quickest way to trigger the yellow reaction.

Spore print dark chocolate brown

Lay a mature cap gill-side down on paper for an hour or two. A dark brown print confirms an Agaricus; a white print points to something else entirely, including the white-gilled Amanita species that never brown with age.

Yellowing fades to dingy brown

The chrome yellow appears almost at once and then dulls to a brownish yellow over a minute or two, so a specimen examined only after it has been in the bag a while can look unremarkable. Rub a fresh cap edge in the field rather than at home.