Agaricus xanthodermus

Agaricus xanthodermus, the Yellow Stainer, is a toxic gilled mushroom of the family Agaricaceae that closely mimics several edible field mushrooms — making it one of the most frequently mistaken poisonous species. It has a smooth white-to-greyish cap 5–15 cm across, crowded free gills that pass from white through dingy pink to chocolate brown, and a ringed stem whose base flushes bright yellow within seconds when cut. The whole mushroom carries an unpleasant phenolic (carbolic) odor that strengthens on cooking. It grows in grass — lawns, pastures, parks, and roadsides — often in fairy rings from late spring to autumn across Europe and eastern and central North America.

History

Agaricus xanthodermus was formally described by the French pharmacist Léon Gaston Genevier in 1876. The epithet xanthodermus combines the Greek xanthos ("yellow") and derma ("skin"), referring to the diagnostic yellow staining of its surface and flesh. Because the genus Agaricus contains the world's most familiar edible mushrooms — the cultivated button mushroom and the wild field and horse mushrooms — the Yellow Stainer has long been a notorious cause of accidental poisonings among foragers who substitute it for safe lookalikes. It is grouped with related phenolic-smelling species in what mycologists call the "xanthodermus group."

Overview

The Yellow Stainer is a saprotrophic decomposer of grassland soils. It is not edible and is not cultivated. Identification rests on two consistent cues that separate it from edible Agaricus species: cut or bruise the flesh — particularly the base of the stem — and the toxic species turns chrome-yellow almost instantly, then often fades to brown; and check the smell, which is medicinal, metallic, or phenolic rather than the pleasant nutty or almond-like aroma of edible field and horse mushrooms. Its toxins are phenolic compounds that cause gastrointestinal illness; it also concentrates heavy metals. Dangerous confusions include the edible Agaricus bisporus, A. arvensis (Horse Mushroom), and A. campestris (Field Mushroom), so the yellow-stain and odor tests should always be applied before any wild Agaricus is considered for the table.