Agaricus xanthodermus

The Yellow Staining Mushroom, Agaricus xanthodermus, is a mushroom of the genus Agaricus, which displays a strong yellow coloration at the base of the stem when cut. It is poisonous for most people, causing gastrointestinal upsets but is eaten by some without apparent negative effect.

Name
xanthodermus means "yellow-skinned", and this is the name used by many authorities, such as Index Fungorum. However the form Agaricus xanthoderma is also often seen, and in fact this is more correct in classical terms; here -derma (δέρμα) is a Greek neuter noun, not an adjective which would agree in gender with Agaricus.

Description
The cap is generally 6-10 cm in diameter, though can reach 15cm. It is initially convex, with some young specimens having a squarish shape, though flattening with age. It is whitish, with light brown tints towards the center. The cap is dry and smooth, but can be scaly when old. The gills of this mushroom progress from pale-pink to a chocolate color. The spore print is brown. Its white stipe is bulbous with a skirt-like ring. Microscopically, there are club-shaped cheilocystidia. The spores are 6-7×3-4µ.

The main identifying feature is an immediate bright yellow coloration on cutting through the base of the stem, or scraping the flesh; later the affected area fades to a dull brown. Numerous edible Agaricus species, such as A. augustus, A. arvensis and A. silvicola, go yellow to a greater or lesser extent, but they do not display such an intense reaction.

Agaricus xanthodermus has an unpleasant characteristic smell, which is phenolic (i.e. like ink or carbolic soap). The smell is especially strong at the base of the stem. On cooking the smell becomes very noticeable, and in some cases this has deterred people from eating it inadvertently.

Distribution and habitat
This mushroom is very common and widely distributed in North America and Europe, and is also found in North Africa, and has been introduced into Australia. It occurs in woods, lawns, gardens and hedgerows in autumn. This is a saprobic species.

Related species
A. xanthodermus belongs to a group of related species (the "Xanthodermatei") which likewise discolour bright yellow and have a phenolic smell. They include A. praeclaresquamosus (formerly A. placomyces) which has dark grey scales, and A. pilatianus which does not have a bulbous stem.