Geography and distribution
The horse mushroom is widespread in temperate regions throughout much of the world. It is frequent in most of Europe and North America, also found in parts of Asia and frequent in Australia and New Zealand. It occurs as far north as Greenland. It is occasional to frequent throughout the British Isles.
Agaricus arvensis can be recognised by the large, whitish cap up to 15 cm across, usually yellowing on the surface, the well-developed ring with cog-wheel like scales on the underside, the presence of dark spores when mature, and a pleasant aniseed-like odour.
Close up of Agaricus arvensis (Photo: Malcolm Storey, 2004)
It is important to distinguish the horse mushroom from Agaricus xanthoderma (commonly known as the yellow-stainer), which occurs frequently in grassy habitats and can look very similar. It also has a white or whitish cap and quite large fruitbodies up to about 10 cm across. It must be carefully distinguished as it is a somewhat toxic species which may cause severe stomach upsets in some people. It differs from the horse mushroom in having an unpleasant odour (quite unlike the aniseed smell of A. arvensis), in having greyish gills, and, most distinctly, in having the flesh in the base of the stem turn rapidly bright yellow when cut or broken. Agaricus arvensis, although bruising or staining yellowish on the surface of the cap and stem, lacks this distinctive reaction.
Agaricus arvensis belongs to Section Arvenses, which includes species with whitish fruitbodies which bruise or stain yellowish (rather than reddish as seen, for example, in field mushroom, Agaricus campestris), and have a ring with cog wheel-like scales on the underside. The ring results from rupture of the well-developed partial veil as the fruitbody expands. This veil covers and protects the gills when young so that immature specimens, as in many Agaricus species, appear closed. Most of the Arvenses group are good edible species, though it is recommended that fruitbodies are properly cooked and not eaten raw.
There are several other white species of Agaricus, some typical of grassland habitats, others found in woodland. They may be difficult to distinguish from A. arvensis without microscopic examination but, apart from the yellow-stainer, they are edible or at least non-toxic species. Agaricus ossecanus (A. nivescens), also found in grassy places, is widespread but much less frequent than A. arvensis. It is very similar, differing mainly in slightly smaller spores and a tapering stem base. Agaricus silvicola, a woodland fungus, is a similar white species which also has yellow bruising of the cap and stem. However, it has fruitbodies which are smaller and more slender than those of the horse mushroom, and has a distinct bulbous base to the stem. Agaricus urinascens (A. macrosporus), found in grassy places and in open woodland, has large, robust fruitbodies up to 20 cm across, with a rather floccose (covered with tufts of soft hair) -scaly cap and a noticeably unpleasant odour especially when old. It also has much larger spores than those of A. arvensis and is hence easy to identify under a microscope.
Threats and conservation
No specific threats are known, and the species has not been considered of conservation concern. However, there is some evidence for decline of the species in Britain.
A good edible species, wild collected in parts of Europe, the Americas and China, and sought after in some areas. The species can be cultivated, and has been grown, for example, on horse manure composts, though commercial cultivation is very limited. It is apparently comparatively resistant to mushroom phorids (flies of genus Megaselia), which are important pests in other Agaricus species. Antioxidant activity has been investigated for this and some other Agaricus species, and in China it is claimed to have anti-cancer properties and has been used to cure lumbago (lower back pain) and pain in tendons and veins.
A good edible species, though fruitbodies should be collected in good, fresh condition and not from polluted sites such as roadside verges. Fruitbodies of this and other yellow-staining Agaricus species are liable to build-up of cadmium and copper, with possible health concerns, though it appears that these metals are not much absorbed in the gut. Care should be taken to avoid confusion with the superficially similar but somewhat toxic yellow stainer (Agaricus xanthoderma).
Horse mushroom in situ (Photo: Malcolm Storey, 2004)
Horse mushroom at Kew
Agaricus arvensis can be found in grassy places in the Gardens and may be fairly common in some years. Other Agaricus species may also be found, including the yellow stainer (Agaricus xanthoderma, see above) which can closely resemble the horse mushroom.
Preserved specimens of Agaricus arvensis from throughout its range are maintained in the Kew Mycology Herbarium and, although not accessible to the general public, are available for study by research workers worldwide.
The species can be cultivated, and has been grown, for example, on horse manure composts, though commercial cultivation is currently very limited.