Agaricus arvensis, commonly known as the horse mushroom, is a mushroom-forming fungus of the genus Agaricus.
It was described as Agaricus arvensis by Jacob Christian Schaeffer in 1774, and given numerous binomial descriptions since. Its specific epithet arvensis means "of the field".
The cap is across, whitish, smooth, and dry; it stains yellow, particularly when young. The gills are pale pink to white at first, later passing through grey and brown to become dull chocolate. A large, spreading ring has white above, but sometimes with yellowish scales underneath. Viewed from below, on a closed-cap specimen, the twin-layered ring has a well-developed "cogwheel" pattern around the stipe. This is the lower part of the double ring. The stalk is long and 1âÂÂ3 cm wide.
The spores are brown and smooth, producing a dark brown spore print. Its odor is similar to that of almond extract or marzipan, due to the presence of benzaldehyde. It tends to stain yellow on bruising.
In addition to its similarity to other related species, this fungus can be confused with deadly species of Amanita.
Agaricus osecanus is rare and lacks the almond smell. Agaricus xanthodermus, the yellow stainer, can cause stomach upsets.
It is one of the largest white Agaricus species in Britain (where it appears during JulyâÂÂNovember), West Asia (Iran), and North America.
Frequently found near stables, as well as in meadows, it may form fairy rings. The mushroom is often found growing with nettles (a plant that also likes nutrient-rich soil). It is associated with cypress on the North American West Coast and spruce to the east.
This mushroom is considered common and widespread, thus is not a conservation concern.
This is a choice edible species. However, Agaricus mushrooms along with many other species bioaccumulate heavy metals when growing in contaminated substrates. Additionally, this mushroom can resemble deadly Amanita mushrooms, such as A. verna.