How to tell Agaricus subperonatus from Yellow stainer

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.

The cap of Agaricus subperonatus is 5 to 12 centimeters across. When young it is hemispherical, opening with age to convex and flat. The surface is covered with numerous pressed-down fibrous scales of brown to reddish-brown on a paler background, especially dense in the center. The scaly brown cap gives the mushroom a stout look.

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.

The hymenophore is gilled. The gills are free, not attached to the stem, and very crowded. Their color changes with age: in young mushrooms they are pale pink or grayish-pink, then darken to chocolate-brown and almost black. The spore print is dark brown. Pinkish young gills distinguish this mushroom from the white-gilled poisonous amanitas.

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.

The stem is cylindrical, fairly thick, white, 4 to 9 centimeters tall. Its defining feature is a dense ring, below which the stem is covered with bands of small brown scales that look like a pulled-on "stocking" or shoe — hence the name. When cut, the flesh may turn faintly red. There is no volva at the base.

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.

The flesh is white, fairly dense, and when cut slowly turns red or brown, especially at the base of the stem. The taste is mild, pleasant and mushroomy, typical of cultivated button mushrooms. Reddening (rather than bright yellowing) is a useful feature that distinguishes this species from the poisonous yellow stainer.

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.

The smell is pleasant and mushroomy, with no sharp notes. If, however, the mushroom smells of ink or carbolic, and the flesh turns bright yellow when cut, it is the poisonous yellow stainer, and you must not take it.