Agaricus arvensis (horse mushroom)

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.