Never eat a wild mushroom identified only from a website, a book or an app. Have every find checked in person by an expert, such as a mycological society.
Identification
Agaricus bisporus has a firm, rounded cap that opens to flat with age, 3–15 cm across, white in the white strain and pale to dark brown in brown strains and the wild form, smooth or with fine flattened scales. The gills are free from the stem, pale pink in young caps and turning chocolate brown to blackish brown as the spores mature. The short, thick stem carries a thin, single membranous ring and has no sac-like volva at the base.
The spore print is dark chocolate brown. As the name indicates, most basidia bear only two spores, a feature visible under the microscope. The flesh is white and may turn slightly pinkish when cut; it never turns bright chrome-yellow at the stem base, which is a warning sign for the toxic Agaricus xanthodermus. The smell is mild and mushroomy, never of ink, phenol or iodine.
Habitat and ecology
In the wild the species is a saprotroph of nitrogen-rich, disturbed habitats such as compost heaps, manure, gardens, hedgerows and roadside verges. It occurs in Europe, North America and elsewhere, mostly in summer and autumn. Cultivated strains descend from wild populations domesticated in France from the seventeenth century onwards, when mushrooms were first grown on horse manure, later in caves and quarries around Paris, which gave the name champignon de Paris.
Uses and importance
White button mushrooms, brown cremini and open-capped portobellos are the same species at different strains and stages. They are eaten raw in salads in some cuisines, but cooking improves digestibility and reduces natural compounds such as agaritine, a hydrazine derivative found in Agaricus species. They are used sautéed, grilled, in sauces, soups, stuffed or as a meat substitute.
Mushrooms exposed to UV light form vitamin D2. In the EU, UV-treated Agaricus bisporus with increased vitamin D2 is authorised as a novel food (Commission Implementing Decision (EU) 2017/2355).
Cultivation
Commercial cultivation is an industrial process. Straw is mixed with horse or chicken manure, gypsum and water and composted (Phase I), then pasteurised and conditioned (Phase II), and spawned and colonised (Phase III). The colonised compost is covered with a casing layer of peat or similar material, which is essential to trigger fruiting together with lowering the temperature to about 16–18 °C and reducing CO₂. Harvest comes in flushes. Spent mushroom compost is widely reused as a soil improver. The crop is difficult on a small scale because consistent compost quality and hygiene against pests and diseases are critical.
Safety and edibility
Cultivated button mushrooms are a safe, everyday food when fresh and properly stored; cooking is recommended. Rare allergies occur.
Wild Agaricus are a classic source of dangerous mistakes. Never eat a wild mushroom based on a website or a photo: identification must be confirmed in person by an expert such as a mycological society or a trained mushroom adviser. Young white Amanita phalloides and Amanita virosa are deadly; they have white gills, a white spore print and a volva at the base. The yellow-staining Agaricus xanthodermus causes gastrointestinal poisoning.
If you suspect mushroom poisoning, call 999 in an emergency or NHS 111 for urgent advice straight away, even before symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Deadly white Amanitas (Amanita phalloides, Amanita virosa, Amanita verna) differ in their gills, which stay white, their white spore print and their sac-like volva, which may be hidden in the soil; always dig up the whole base. Agaricus xanthodermus stains chrome-yellow at the stem base when scratched and smells of ink or phenol. Agaricus campestris, the field mushroom, is a related edible of grassland with a fragile ring. Chlorophyllum molybdites, common in warm regions, has greenish spores.
In the UK
In the UK this is the everyday supermarket mushroom, sold as button, closed cup, chestnut and flat or portobello mushrooms, all forms of the same species. It also grows wild in Britain and Ireland on manured or disturbed ground, in gardens, roadside verges and near stables, mainly from summer to autumn.
Wild white Agaricus are easily confused with the yellow stainer, which bruises chrome yellow at the stem base and smells of ink or phenol, and young white amanitas can resemble them too. Pick only what you can identify with certainty and have finds checked by an experienced local recorder or fungus group.
