Agaricus is a genus of mushrooms containing both edible and poisonous species, with possibly over 300 members worldwide. The genus includes the common mushroom and the fieldmushroom, the dominant cultivated mushrooms of the West. 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
For many years, members of the genus Agaricus were given the generic namePsalliota, 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. Several origins of Agaricus have been proposed. It possibly originates from ancient Sarmatia Europaea, where people Agari, promontory Agarum and a river Agarus were known. Note also Greek ἀγαρικόν, "a sort of tree fungus" Dok reports Linnaeus' name is devalidated 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, 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 as trib. Psalliota. The type is Agaricus campestris. Paul Kummer 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.
Phylogenetics
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.
Agaricus xantholepis Echigoshirayukidake is also thought to be in Agaricus, either as a subspecies of Agaricus blazei'' or a new species.
Edibility and toxicity
The genus contains the most widely consumed and best-known mushroom today, A. bisporus, with A. campestris also being well known. A. porphyrocephalus is a choice edible, and some others are edible as well. The most notable inedible species is the yellow-staining mushroom, A. xanthodermus. One species reported from Africa, A. aurantioviolaceus, is reportedly deadly poisonous. 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'".
