How to tell Button mushroom from Yellow stainer

Cap 5–12 cm across, globular or hemispherical in young mushrooms, then flat-convex. Surface smooth or slightly scaly, from white to grayish brown depending on the strain. Margin rolled under in young mushrooms.

Cap 5-15 cm across; in young mushrooms firmly hemispherical to squared-off (a characteristic feature — the "square" young cap), later becoming convex to flat. Color white to cream, sometimes grayish-brown in the center. The most characteristic sign — touching the cap or its margin leaves bright chrome-yellow spots, especially noticeable in young mushrooms. The surface is smooth, slightly sticky in wet weather.

Gills free, crowded, pink or pale at first, then darkening to purplish brown and almost black. Gill color is a reliable sign of freshness: pink means a fresh mushroom, dark brown means a mature one.

Gilled hymenophore — gills free, crowded. When young — pale pinkish-gray, becoming with age dark brown, chocolate-blackish. Spore print dark brown. The gill color is typical of Agaricus — it doesn't help tell the yellow stainer apart.

Stem 3–8 cm tall, 1–2 cm thick, even or slightly tapering downward, firm, solid. White. A thin membranous ring in the upper or middle part. No volva.

Stem 5-12 cm tall, 1-2 cm thick, cylindrical, with a pronounced bulbous base (a difference from other Agaricus species, whose stems are more even). Color white. The most important diagnostic sign: when cut, the flesh at the base of the stem and in the bulb is bright chrome yellow, often almost neon, and the reaction is fast (10-30 seconds). On the upper part — a white membranous ring, distinct in young mushrooms.

Flesh firm, white, turning slightly pinkish when cut. Does not yellow. Taste mild, neutral in cultivated forms, more pronounced in wild specimens.

Flesh firm, white in the cap and upper stem, bright yellow at the base of the stem even when undamaged. When cut, THE BASE OF THE STEM TURNS BRIGHT CHROME YELLOW — the main diagnostic sign. The cap yellows too, but more weakly. The smell is distinct and unpleasant — ink, phenol, medicinal ointment, creosote.

Cultivated forms have a moderate, mushroomy, unobtrusive smell. Wild specimens have a more distinct, slightly anise-almond aroma. Frying intensifies the aroma many times over.

Distinct and unpleasant, with notes of ink, phenol, medicinal ointment, creosote, printer's ink. It comes out when you rub the flesh between your fingers, especially at the base of the stem. This smell is the most reliable sign: no edible Agaricus smells like this.