Agaricus is a genus of mushroom-forming fungi 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 bisporus) and the field mushroom (A. campestris), the dominant cultivated mushrooms of the West.

Description

Members of Agaricus are characterized by having a fleshy cap or pileus, from the underside of which grow a number of radiating plates or gills, on which are produced the naked spores. They are distinguished from other members of their family, Agaricaceae, by their chocolate-brown spores. Members of Agaricus also have a stem or stipe, which elevates it above the object on which the mushroom grows, or substrate, and a partial veil, which protects the developing gills and later forms a ring or annulus on the stalk.

Taxonomy

Several origins of genus name Agaricus have been proposed. It possibly originates from ancient Sarmatia Europaea, where people Agari, promontory Agarum and a river Agarus were known (all located on the northern shore of Sea of Azov, probably, near modern Berdiansk in Ukraine). Note also Greek ἀγαρικόν, agarikón, "a sort of tree fungus" (There has been an Agaricon Adans. genus, treated by Donk in Persoonia 1:180.) For many years, members of the genus Agaricus were given the generic name Psalliota, and this can still be seen in older books on mushrooms. All proposals to conserve Agaricus against Psalliota or vice versa have so far been considered superfluous. Dok reports Linnaeus' name is devalidated (so the proper author citation apparently is "L. per Fr., 1821") because Agaricus was not linked to Tournefort's name. Linnaeus places both Agaricus Dill. and Amanita Dill. in synonymy, but truly a replacement for Amanita Dill., which would require A. quercinus, not A. campestris be the type. This question is compounded because Fries himself used Agaricus roughly in Linnaeus' sense (which leads to issues with Amanita), and A. campestris was eventually excluded from Agaricus by Karsten and was apparently in Lepiota at the time Donk wrote this, commenting that a type conservation might become necessary. The alternate name for the genus, Psalliota, derived from the Greek psalion/ψάλιον, "ring", was first published by Fries (1821) as trib. Psalliota. The type is Agaricus campestris (widely accepted, except by Earle, who proposed A. cretaceus). Paul Kummer (not Quélet, who merely excluded Stropharia) was the first to elevate the tribe to a genus. Psalliota was the tribe containing the type of Agaricus, so when separated, it should have caused the rest of the genus to be renamed, but this is not what happened.

Phylogeny

The use of phylogenetic analysis to determine evolutionary relationships amongst Agaricus species has increased the understanding of this taxonomically difficult genus, although much work remains to be done to fully delineate infrageneric relationships. Prior to these analyses, the genus Agaricus, as circumscribed by Rolf Singer, was divided into 42 species grouped into five sections based on reactions of mushroom tissue to air or various chemical reagents, as well as subtle differences in mushroom morphology. Restriction fragment length polymorphism analysis demonstrated this classification scheme needed revision.

Subdivisions

As of 2018, this genus is divided into 6 subgenera and more than 20 sections: Subgenus Agaricus Subgenus Flavoagaricus Subgenus Minores Subgenus Minoriopsis Subgenus Pseudochitonia Subgenus Spissicaules

Selected species

The fungal genus Agaricus as late as 2008 was believed to contain about 200 species worldwide but since then, molecular phylogenetic studies have revalidated several disputed species, as well as resolved some species complexes, and aided in discovery and description of a wide range of mostly tropical species that were formerly unknown to science. As of 2020, the genus is believed to contain no fewer than 400 species, and possibly many more. The medicinal mushroom known in Japan as Echigoshirayukidake (越後白雪茸) was initially also thought to be an Agaricus, either a subspecies of Agaricus "blazei" (i.e. A. subrufescens), or a new species. It was eventually identified as sclerotium of the crust-forming bark fungus Ceraceomyces tessulatus, which is not particularly closely related to Agaricus. Several secotioid (puffball-like) fungi have in recent times be recognized as highly aberrant members of ' Agaricus, and are now included here. These typically inhabit deserts where few fungi – and even fewer of the familiar cap-and-stalk mushroom shape – grow. Another desert species, A. zelleri, was erroneously placed in the present genus and is now known as Gyrophragmium californicum. In addition, the scientific names Agaricus and – even more so – Psalliota were historically often used as a "wastebasket taxon" for any and all similar mushrooms, regardless of their actual relationships. Species either confirmed or suspected to belong into this genus include:

Toxicity

A notable group of poisonous Agaricus is the clade around the yellow-staining mushroom, A. xanthodermus. One species reported from Africa, A. aurantioviolaceus, is reportedly deadly poisonous. Far more dangerous is the fact that Agaricus, when still young and most valuable for eating, are easily confused with several deadly species of Amanita (in particular the species collectively called "destroying angels", as well as the white form of the appropriately-named "death cap" Amanita phalloides), as well as some other highly poisonous fungi. An easy way to recognize Amanita is the gills, which remain whitish at all times in that genus. In Agaricus, by contrast, the gills are only initially white, turning dull pink as they mature, and eventually the typical chocolate-brown as the spores are released. Even so, Agaricus should generally be avoided by inexperienced collectors, since other harmful species are not as easily recognized, and clearly recognizable mature Agaricus are often too soft and maggot-infested for eating. When collecting Agaricus for food, it is important to identify every individual specimen with certainty, since one Amanita fungus of the most poisonous species is sufficient to kill an adult human – even the shed spores of a discarded specimen are suspected to cause life-threatening poisoning. Confusing poisonous Amanita with an edible Agaricus is the most frequent cause of fatal mushroom poisonings world-wide. Reacting to some distributors marketing dried agaricus or agaricus extract to cancer patients, it has been identified by the U.S. Food and Drug Administration as a "fake cancer 'cure. The species most often sold as such quack cures is A. subrufescens, which is often referred to by the erroneous name "Agaricus Blazei" and advertised by fanciful trade names such as "God's mushroom" or "mushroom of life", but can cause allergic reactions and even liver damage if consumed in excessive amounts.

Uses

The genus contains the most widely consumed and best-known mushroom today, A. bisporus, with A. arvensis, A. campestris and A. subrufescens also being well-known and highly regarded. A. porphyrocephalus is a choice edible when young, and many others are edible as well, namely members of sections Agaricus, Arvense, Duploannulatae and Sanguinolenti.

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