The Yellow Staining Mushroom, Agaricus xanthoderma =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.
Xanthoderma means "yellow-skinned". Agaricus xanthoderma has a cap which is generally 6-10 cm in diameter, though it 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.
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.