How to tell Agaricus romagnesii from Yellow stainer

Cap 6-15 cm across; in young mushrooms hemispherical to egg-shaped, later becoming convex to flat. Color brownish-cream to grayish-brown, with distinct small brown or dark brown scales in the center and middle zone. The surface is dry, slightly sticky in wet weather.

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.

Gilled hymenophore — gills free, crowded. When young — pale pink, becoming with age dark brown, chocolate-blackish. Spore print dark brown.

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 8-12 cm tall, 1-2 cm thick, cylindrical, slightly thickened at the base. Color white to cream, with fine brownish fibers below the ring. On the upper part — a white membranous ring, distinct in young mushrooms.

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; when cut it slowly turns pinkish to brownish. Does not turn chrome yellow (the test against the yellow stainer). The smell is pleasantly mushroomy with a subtle almond-anise note.

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.

Pleasantly mushroomy, with a subtle almond-anise note. It doesn't smell of ink or medicine.

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.