Agaricus is a genus of mushroom-forming fungi containing both edible and poisonous species, with over 400 members worldwideBas C. (1991). A short introduction to the ecology, taxonomy and nomenclature of the genus Agaricus, 21–24. In L.J.L.D. Van Griensven (ed.), Genetics and breeding of Agaricus. Pudoc, Wageningen, The Netherlands.Capelli A. (1984). Agaricus. L.: Fr. (Psalliota Fr.). Liberia editrice Bella Giovanna, Saronno, Italy and possibly again as many disputed or newly discovered species. The genus includes the common ("button") mushroom (A. bisporus) and the field mushroom (A. campestris), the dominant cultivated mushrooms of the West.
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).
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.
Donk reports Carl 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 Elias Magnus 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.Donk, M.A. (1962). "The generic names proposed for Agaricaceae". Beihefte zur Nova Hedwigia 5: 1–320. ISSN 0078-2238
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:Bon, Marcel (1987) The Mushrooms and Toadstools of Britain and North Western Europe. Hodder and Stoughton, (paperback) (hardback)
Subgenus Agaricus Section Agaricus This is the group around the type species of the genus, the popular edible A. campestris which is common across the Holarctic temperate zone, and has been introduced to some other regions. One of the more ancient lineages of the genus, it contains species typically found in open grassland such as A. cupreobrunneus, and it also includes at least one undescribed species. Their cap surface is whitish to pale reddish-brown and smooth to slightly fibrous, the flesh usually without characteristic smell, fairly soft, whitish, and remaining so after injury, application of KOH, or Schäffer's test (aniline and HNO3). A. annae may also belong here, as might A. porphyrocephalus, but the flesh of the latter blushes red when bruised or cut, and it has an unpleasant smell of rotten fish when old; these traits are generally associated with subgenus Pseudochitonia, in particular section Chitonioides. The A. bresadolanus/radicatus/romagnesii group which may be one or several species is sometimes placed here, but may be quite distinct and belong to subgenus Spissicaules.
Subgenus Flavoagaricus Section Arvenses Konrad & Maubl. (sometimes named Arvensis) Traditionally contained about 20 rather large species similar to the horse mushroom A. arvensis in six subgroups. Today, several additional species are recognized – in particular in the A. arvensis species complexand placed here, such as A. aestivalis, A. augustus, A. caroli, A. chionodermus, A. deserticola (formerly Longula texensis), A. fissuratus, A. inapertus (formerly Endoptychum depressum), A. macrocarpus, A. nivescens, A. osecanus, A. silvicola and the doubtfully distinct A. essettei, A. urinascens, and the disputed taxa A. abruptibulbus, A. albertii, A. altipes, A. albolutescens, A. brunneolus, A. excellens and A. macrosporus. It also includes A. subrufescens which started to be widely grown and traded under various obsolete and newly-invented names in the early 21st century, as well as the Floridan A. blazei with which the Brazilian A. subrufescens was often confused in the past. They have versatile heterothallic life cycles, are found in a variety of often rather arid habitats, and typically have a smooth white to scaly light brown cap. The flesh, when bruised, usually turns distinctly yellow to pinkish in particular on the cap, while the end of the stalk may remain white; a marked yellow stain is caused by applying KOH. Their sweetish smell of almond extract or marzipan due to benzaldehyde and derived compounds distinguishes them from the section Xanthodermatei, as does a bright dark-orange to brownish-red coloration in Schäffer's test. Many members of this subgenus are highly regarded as food, and even medically beneficial, but at least some are known to accumulate cadmium and other highly toxic chemicals from the environment, and may not always be safe to eat.
Subgenus Minores A group of buff-white to reddish-brown species. Often delicate and slender, the typical members of this subgenus do not resemble the larger Agaricus species at a casual glance, but have the same telltale chocolate-brown gills at spore maturity. Their flesh has a barely noticeable to pronounced sweetish smell, typically almond-like, turns yellowish to brownish-red when cut or bruised at least in the lower stalk, yellow to orange with KOH, and orange to red in Schäffer's test. Species such as A. aridicola (formerly known as Gyrophragmium dunalii), A. colpeteii, A. columellatus (formerly Araneosa columellata), A. diminutivus, A. dulcidulus, A. lamelliperditus, A. luteomaculatus, A. porphyrizon, A. semotus and A. xantholepis are included here, but delimitation to and indeed distinctness from subgenus Flavoagaricus is a long-standing controversy. Unlike these however, subgenus Minores contains no choice edible species, and may even include some slightly poisonous ones; most are simply too small to make collecting them for food worthwhile, and their edibility is unknown. Section Leucocarpi Includes A. leucocarpus. Section Minores Includes A. comtulus and A. huijsmanii. Section Pantropicales Includes A. candidolutescens and an undescribed relative.
Subgenus Minoriopsis Somewhat reminiscent of subgenus Minores and like it closely related to subgenus Flavoagaricus, it contains species such as A. martinicensis and A. rufoaurantiacus.
Subgenus Pseudochitonia This highly diverse clade of mid-sized to largish species makes up much the bulk of the genus' extant diversity, and this subgenus contains numerous as of yet undescribed species. It includes both the most prized edible as well as the most notoriously poisonous Agaricus, and some of its sections are in overall appearance more similar to the more distantly related Agaricus proper and Flavoagaricus than to their own closest relatives. Some species in this subgenus, such as A. goossensiae and A. rodmanii, are not yet robustly assigned to one of the sections. Section Bohusia Includes A. bohusii which resembles one of the dark-capped Flavoagaricus or Xanthodermatei but does not stain yellow with the standard (10%) KOH testing solution. It is a woodland species, edible when young, but when mature and easily distinguished from similar species it may be slightly poisonous. Other members of this section include A. crassisquamosus, A. haematinus, and A. pseudolangei. Section Brunneopicti A section notable for containing a considerable number of undescribed species in addition to A. bingensis, A. brunneopictus, A. brunneosquamulosus, A. chiangmaiensis, A. duplocingulatus, A. megacystidiatus, A. niveogranulatus, A. sordidocarpus, A. subsaharianus, and A. toluenolens. Section Chitonioides Contains species such as A. bernardii and the doubtfully distinct A. bernardiiformis, A. gennadii, A. nevoi, A. pequinii, A. pilosporus and A. rollanii, which strongly resemble the members of section Duploannulatae and are as widely distributed. However, their flesh tends to discolor more strongly red when bruised or cut, with the discoloration slowly getting stronger. Their smell is usually also more pronounced umami-like, in some even intensely so. Some are edible and indeed considered especially well-tasting, while the unusual A. maleolens which may also belong here has an overpowering aroma which renders it inedible except perhaps in small amounts as a vegan fish sauce substitute. Section Crassispori Related to section Xanthodermatei as traditionally circumscribed, it includes such species as A. campestroides, A. lamellidistans, and A. variicystis. Section Cymbiformes He, Chuankid, Hyde, Cheewangkoon & Zhao A section proposed in 2018, it is closely related to the traditional section Xanthodermatei. The type species A. angusticystidiatus from Thailand is a smallish beige Agaricus with characteristic boat-shaped basidiospores. It has a strong unpleasant smell like members of section Xanthodermatei, but unlike these, its flesh does not change color when bruised, but turns dark reddish-brown when cut, and neither application of KOH nor Schäffer's test elicit a change in color. Section Duploannulatae (also known as section Bivelares or Hortenses) Traditionally often included in section Agaricus as subsection Bitorques, it seems to belong to a much younger radiation. It unites robust species, usually with a thick, almost fleshy ring, which inhabit diverse but often nutrient-rich locations. Some are well-known edibles; as they are frequently found along roads and in similar polluted places, they may not be safe to eat if collected from the wild. Their flesh is rather firm, white, with no characteristic smell, in some species turning markedly reddish when bruised or cut (though this may soon fade again), and generally changing color barely if at all after application of KOH or Schäffer's test. Based on DNA analysis of ITS1, ITS2, and 5.8S sequences, the studied species of this section could be divided into six distinct clades, four of which correspond to well-known species from the temperate Northern Hemisphere: A. bisporus, A. bitorquis (and the doubtfully distinct A. edulis), A. cupressicola and A. vaporarius. The other two clades comprise the A. devoniensis (including A. subperonatus) and A. subfloccosus (including A. agrinferus) species complexes. Additional members of this section not included in that study are A. cappellianus, A. cupressophilus, A. subsubensis, A. taeniatus, A. tlaxcalensis, and at least one undescribed species. The cultivated mushrooms traded as A. sinodeliciosus also belong here, though their relationship to the A. devoniensis complex and A. vaporarius is unclear. Section Flocculenti