''Agaricus xanthodermus'', commonly known as the yellow-staining mushroom or simply the yellow-stainer, is a

mushroom A mushroom or toadstool is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground, on soil, or on its food source. ''Toadstool'' generally denotes one poisonous to humans. The standard for the name "mushroom" is ...

of the genus ''

Agaricus ''Agaricus'' is a genus of mushrooms containing both edible and poisonous species, with over 400 members worldwide and possibly again as many disputed or newly-discovered species. The genus includes the common ("button") mushroom ('' Agaricus bi ...

'', which displays a strong yellow colouration at the base of the stem when cut. It is

poisonous Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...

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. Genevier described the ''

'' mushrooms commonly eaten (perhaps sometimes inadvisedly) in the region of

Nantes Nantes (, , ; Gallo: or ; ) is a city in Loire-Atlantique on the Loire, from the Atlantic coast. The city is the sixth largest in France, with a population of 314,138 in Nantes proper and a metropolitan area of nearly 1 million inhabita ...

, 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''. The epithet ''xanthodermus'' is derived from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...

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. 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.

Description

2011-10-13 Agaricus xanthodermus Genev 181971

The

cap A cap is a flat headgear, usually with a visor. Caps have crowns that fit very close to the head. They made their first appearance as early as 3200 BC. Caps typically have a visor, or no brim at all. They are popular in casual and informal se ...

ranges from in diameter. 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 A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

of this mushroom progress from pale-pink to a chocolate color. Its white stipe measures tall and 1–3 wide, and is bulbous with a skirt-like ring. Microscopically, the cheilocystidia are club-shaped. The

spores In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

are brown, elliptical, and smooth, measuring 6–7 × 3–4

µm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

. 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. Karbol-Champignion2

''Agaricus xanthodermus'' has an unpleasant characteristic smell, which is phenolic, reminiscent of ink or

carbolic soap Carbolic soap, sometimes referred to as red soap, is a mildly antiseptic soap containing carbolic acid (phenol) and/or cresylic acid (cresol), both of which are phenols derived from either coal tar or petroleum sources. History In 1834, German ...

. The smell is especially strong at the base of the stem.

Distribution and habitat

This mushroom is very common and widely distributed in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

, West Asia including eastern

Anatolia Anatolia, tr, Anadolu Yarımadası), and the Anatolian plateau, also known as Asia Minor, is a large peninsula in Western Asia and the westernmost protrusion of the Asian continent. It constitutes the major part of modern-day Turkey. The re ...

and Iran, North Africa, and

southern Africa Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number o ...

. It has been introduced into

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

. It occurs in woods, lawns, gardens and hedgerows in autumn. This is a

saprobic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...

species.

Toxicity

Although it appears that some people can eat this mushroom without ill effects, it contains toxins which can cause serious gastric problems. It is indigestible and gives rise to symptoms of sweating, flushing, and severe stomach cramps. Of those who gather ''Agaricus''-style mushrooms, about 50% of the cases of poisoning are from this species. On cooking, the smell becomes very noticeable, and this may deter people from eating it inadvertently.

Related species

''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. ''

Agaricus californicus ''Agaricus californicus'', commonly known as the California agaricus, is a poisonous mushroom in the section ''Xanthodermati'' of the genus ''Agaricus''. It is mildly poisonous, causing gastrointestinal upset in many individuals. The etiology ...

'' is also similar in appearance.

References