Agaricus bisporus (J.E. Lange) Imbach - Commercial (Button) Mushroom

Distribution - Taxonomic History - Etymology - Identification - Culinary Notes - Reference Sources

Agaricus bisporus - Commercial Button Mushroom

Taxonomy

Phylum: Basidiomycota

Class: Agaricomycetes

Order: Agaricales

Family: Agaricaceae

Agaricus bisporus is not particularly common in the wild, where like the Field Mushroom and the Horse Mushroom it springs up in fields and appears after rain. The specific name bisporus refers to the fact that each of the basidia bear just two spores - most Agaricus species have four-spored basidia. (You will need a powerful microscope if you want to see these features on the gill faces of agaricoid fungi.)

Distribution

An uncommon to rare mushroom in the wild, in Britain and Ireland Agaricus bisporus is found nearly always in grassland. This species occurs also in mainland Europe and elsewhere in the world including parts of North America. The specimens shown on this page are cultivated 'Chestnut Mushrooms'.

Taxonomic history

Although known for many years beforehand, this mushroom was recognised as a species distinct from the Field Mushroom Agaricus campestris by the Danish mycologist Jakob Emanuel Lange (1864 - 1941) in 1926; it was transferred to the genus Agaricus by German mycologist Emil J Imbach (1897 - 1970) twenty years later.

Synonyms of Agaricus bisporus are many and include Psalliota hortensis (Cooke) J.E. Lange,  Psalliota hortensis f. bispora J.E. Lange, Psalliota bispora (J.E. Lange) F.H. Møller & Jul. Schäff.,  Agaricus campestris var. bisporus Kligman,  Agaricus bisporus var. bisporus (J.E. Lange) Imbach, Agaricus hortensis (Cooke) Pilát, and Agaricus bisporus var. albidus (J.E. Lange) Singer.

Etymology

The specific epithet bisporus means having two spores - there are two spores on each basidium, whereas other Agaricus species (and indeed the majority of the Basidiomycota) have four-spored basidia.

Identification guide

Cap of Agaricus bisporus

Cap

Initially hemispherical, becoming convex and eventually flattening. At maturity, the cap diameter is usually between 5 and 10cm.

Gills of Agaricus bisporus

Gills

The crowded gills are narrow and free; initially pink they turn reddish and then chocolate brown as the fruitbody matures.

Stem

At 3 to 6cm tall and 1.5 to 2cm.

Below the insubstantial membranous double ring, the surface of the stem is flaky.

Spores of Agaricus bisporus

Spores

Oviod to subglobose, smooth, 4-7.5 x 4-5.5µm.

Spore print

Deep chocolate brown.

Basidia of Agaricus bisporus

Basidia

Two-spored - hence the specific name bisporus.

Odour/taste

They have a 'mushroomy' odour, of course...

Habitat

Saprobic, in permanent pastures and other grassy places.

Season

Late spring to autumn.

Similar species

Agaricus campestris, the Field Mushroom, is very similar but has four-spored basidia.

Culinary Notes

It surely goes without saying that this is a good edible mushroom and one of the tiny minority of edible fungi that are considered safe to eat raw. The very pale-capped mushrooms sold in most supermarkets are a derived form of Agaricus bisporus, which in the wild has a brown cap as shown in the main picture, above. When fully expanded, the caps are sometimes referred to as Portobello Mushrooms (sometimes spelt Portobella).

Reference Sources

Pat O'Reilly (2011) Fascinated by Fungi; First Nature

BMS List of English Names for Fungi

The genus Agaricus in Britain, 3rd Edition, self-published, Geoffrey Kibby 2011

Dictionary of the Fungi; Paul M. Kirk, Paul F. Cannon, David W. Minter and J. A. Stalpers; CABI, 2008

Taxonomic history and synonym information on these pages is drawn from many sources but in particular from the British Mycological Society's GB Checklist of Fungi and (for basidiomycetes) on Kew's Checklist of the British & Irish Basidiomycota.