Agaricus xanthodermus

Agaricus xanthodermus, commonly known as the yellow-staining agaricus,<ref>Template:Cite book</ref> yellow-staining mushroom or yellow-stainer,<ref name=Phillips/> is a mushroom of the genus Agaricus, which displays a strong yellow colouration at the base of the stem when cut. It has a phenolic smell. It is poisonous for most people, causing gastrointestinal upset, but can be eaten by some without apparent negative effect.

Taxonomy

This species was first officially defined under the name Agaricus xanthodermus in 1876 by Léon Gaston Genevier, in a letter published in the bulletin of the French Botanical Society.<ref name=SBF/><ref name=IndexFungorum/> Genevier described the Agaricus mushrooms commonly eaten (perhaps sometimes inadvisedly) in the region of Nantes, and attempted to clarify the distinctions between them. He proposed a detailed reclassification into 5 species, including this new one. Apparently up until that time, these yellow-staining mushrooms were considered to be just varieties of other species which are edible: A. arvensis, A. edulis, and A. silvicola.<ref name=Genevier/>

The epithet xanthodermus is derived from the Ancient Greek words for "yellow" and "skin", which were then given a Latin adjective ending. This is the official name, but the form Agaricus xanthoderma is also often seen.<ref name="Bon"/><ref name="Courtecuisse"/><ref name=Moser/> In the latter name, -derma "skin" is a neuter noun which does not have to agree in gender with Agaricus, and so this form is legal according to the rules of botanical nomenclature.<ref name="Nomenclature"/>

Description

The cap ranges from Template:Convert in diameter.<ref name=":0">Template:Cite book</ref> 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 centre. 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. Its white stipe measures Template:Cvt tall and 1–3 wide,<ref name=":0" /> and is bulbous with a skirt-like ring. Microscopically, the cheilocystidia are club-shaped. The spores are brown, elliptical, and smooth,<ref name=":0" /> measuring 6–7 × 3–4 μm.

Agaricus xanthodermus has an unpleasant characteristic smell, which is phenolic, reminiscent of ink or carbolic soap. The smell is especially strong at the base of the stem. On cooking, the smell becomes very noticeable, and this may deter people from eating it inadvertently.<ref name=Jordan/>

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Related species

The main identifying feature is an immediate bright yellow colouration 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, turn yellow to a greater or lesser extent, but they do not display such an intense reaction.

Agaricus xanthodermus belongs to a group of related species (the "Xanthodermati") which likewise discolour bright yellow and have a phenolic smell. They include A. praeclaresquamosus (formerly A. placomyces) which has dark grey scales, A. moelleri, and A. pilatianus, which does not have a bulbous stem.<ref name="Courtecuisse"/>

Agaricus californicus is also similar in appearance.<ref name=":0" />

Distribution and habitat

This mushroom is very common and widely distributed in North America, Europe, West Asia including eastern Anatolia<ref name=Dem04/> and Iran,<ref name=Asef2010/> North Africa,<ref name="Courtecuisse"/> and southern Africa.<ref name=Westhuizen/> It has been introduced into Australia. It occurs in woods, lawns, gardens and hedgerows in autumn. This is a saprobic species.

Toxicity

Although it appears that some people can eat this mushroom without ill effects,<ref name=Phillips/><ref name=Genevier/><ref name=Jordan/> it contains toxins which can cause serious gastric problems.<ref name="Miller 2006">Template:Cite book</ref> It is indigestible and gives rise to symptoms of sweating, flushing, and severe stomach cramps.<ref name=Phillips/><ref name=Bon/><ref name=Jordan/>

Of those who gather Agaricus-style mushrooms, about 50% of the cases of poisoning are from this species.Template:Citation needed

See also

• List of Agaricus species

References

• Collins Gem Guide: Mushrooms and Toadstools, Stefan Buckzacki (1982).
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