Agaricus xanthodermus - Yellow Stainer

Taxonomy

Phylum: Basidiomycota

Class: Agaricomycetes

Order: Agaricales

Agaricus xanthodermus, the Yellow Stainer, can cause serious stomach upsets and so should not be eaten. It can be confused with other members of the Agaricaceae.

No other common white mushroom turns such a bright chrome yellow so rapidly when handled, although a few of the edible Agaricus species do bruise slightly yellow, so it is important to check all characteristics, including odour, when making an identification.

Identification guide

	Cap Initially globose but with a flat top, the cap expands to become broadly convex without an in-rolled margin. Initially pure white and matt, later greying, caps grow to between 5 and 15cm at maturity, and they quickly turn crome yellow when scratched or bruised.
	Gills In this picture the partial veil is just beginning to rupture, revealing free, off-white gills that soon turn pink and later become grey-brown as the fruitbody reaches maturity. Stem The stem diameter is 1 to 2cm with a smooth, silky surface and a bulbous base. A large white stem ring persists through to maturity. When cut the stem flesh quickly turns from white to chrome yellow, most markedly at the stem base.
Spore print	Very dark brown.
	Spores Eliptic or ovoid 4.5 to 6.5 x 3.5 to 5.5 µm.
Odour/taste	Taste not distinctive, but this mushroom is poisonous. Noticeable odour of ink, phenol or iodine, especially when the flesh is bruised or cut.
Habitat	Most commonly under hedgerows and at the edges of gardens and other areas of disturbed soil.
Season	June to October.
Occurrence	Frequent.
Similar species	Agaricus arvensis is superficially very similar but bruises more slowly and only slightly yellow; its flesh smells of aniseed rather than of iodine. Agaricus campestris is similar in appearance but does not turn yellow when cut or bruised.