Agaricus

Agaricus is a large genus of fungi that includes both edible and poisonous mushrooms. It contains over 400 recognized species worldwide, with possibly as many again that are disputed or newly discovered. The genus includes the common button mushroom (Agaricus bisporus) and the field mushroom (Agaricus campestris), which are the main cultivated mushrooms in the West.

Taxonomy

The name Agaricus may come from ancient Sarmatia Europaea, where a people called the Agari, a promontory named Agarum, and a river called Agarus were located on the northern shore of the Sea of Azov, near modern Berdiansk in Ukraine. For many years, members of this genus were called Psalliota, a name still seen in older mushroom books. Proposals to officially conserve either Agaricus or Psalliota have been considered unnecessary.

Carl Linnaeus' original name for the genus is considered devalidated because it was not linked to Tournefort's name; the proper author citation is "L. per Fr., 1821." Linnaeus placed both Agaricus Dill. and Amanita Dill. in synonymy, but this was actually a replacement for Amanita Dill., which would require Agaricus quercinus, not Agaricus campestris, as the type. Elias Magnus Fries used Agaricus roughly in Linnaeus' sense, causing issues with Amanita. Agaricus campestris was later excluded from Agaricus by Karsten and was placed in Lepiota at the time Donk wrote, who noted that a type conservation might be needed.

The alternate name Psalliota comes from the Greek psalion (ψάλιον), meaning "ring," and was first published by Fries in 1821 as tribus Psalliota. The type species is Agaricus campestris (widely accepted, except by Earle, who proposed Agaricus cretaceus). Paul Kummer was the first to elevate the tribe to a genus; Quélet merely excluded Stropharia. Since Psalliota contained the type of Agaricus, separating it should have forced the rest of the genus to be renamed, but that did not happen.

Phylogeny

Phylogenetic analysis has improved understanding of this taxonomically difficult genus, though much work remains. Before such analyses, Rolf Singer divided Agaricus into 42 species in five sections, based on tissue reactions to air or chemicals and on subtle morphological differences. Restriction fragment length polymorphism analysis showed this classification needed revision.

As of 2018, the genus is divided into six subgenera and more than 20 sections. Subgenus Agaricus includes the type species Agaricus campestris, a popular edible mushroom common across the Holarctic temperate zone and introduced elsewhere. This is one of the older lineages, containing species typically found in open grassland, such as Agaricus cupreobrunneus, and at least one undescribed species.

Their caps are whitish to pale reddish-brown and smooth to slightly fibrous; the flesh is soft, whitish, and does not change color when bruised or treated with KOH or Schäffer's test (aniline and nitric acid). Agaricus annae may belong here, as might Agaricus porphyrocephalus, but the latter's flesh blushes red when bruised and smells like rotten fish when old—traits usually associated with subgenus Pseudochitonia, section Chitonioides. The Agaricus bresadolanus/radicatus/romagnesii group (possibly one or several species) is sometimes placed here but may belong to subgenus Spissicaules.

Uses

Subgenus Flavoagaricus includes section Arvenses, traditionally containing about 20 large species similar to the horse mushroom Agaricus arvensis. Today, additional species are recognized, including Agaricus aestivalis, Agaricus augustus, Agaricus caroli, Agaricus chionodermus, Agaricus deserticola (formerly Longula texensis), Agaricus fissuratus, Agaricus inapertus (formerly Endoptychum depressum), Agaricus macrocarpus, Agaricus nivescens, Agaricus osecanus, Agaricus silvicola, and the doubtfully distinct Agaricus essettei, Agaricus urinascens, and disputed taxa such as Agaricus abruptibulbus, Agaricus albertii, Agaricus altipes, Agaricus albolutescens, Agaricus brunneolus, Agaricus excellens, and Agaricus macrosporus. It also includes Agaricus subrufescens, which began to be widely grown and traded under various obsolete and newly invented names in the early 21st century, as well as the Floridan Agaricus blazei, with which the Brazilian Agaricus subrufescens was often confused. These species have versatile heterothallic life cycles, are found in often arid habitats, and typically have smooth white to scaly light brown caps.

Their flesh turns distinctly yellow to pinkish when bruised, especially on the cap, while the stalk end may remain white; KOH causes a marked yellow stain. A sweetish smell of almond extract or marzipan (due to benzaldehyde and related compounds) distinguishes them from section Xanthodermatei, as does a bright dark-orange to brownish-red color in Schäffer's test. Many are highly regarded as food and even considered medically beneficial, but some accumulate cadmium and other toxic chemicals from the environment, so they may not always be safe to eat.