Agaricus arvensis

Agaricus arvensis, commonly known as the horse mushroom, is a mushroom-forming fungus of the genus Agaricus.

Taxonomy
It was described as Agaricus arvensis by Jacob Christian Schaeffer in 1774, and given numerous binomial descriptions since. Its specific name arvensis means 'of the field'.

Description


The cap is 7-20 cm, 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. There is a large spreading ring, 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 5-12 cm long and 1–3 cm wide. The spores are brown and smooth. The odor is similar to that of almond extract or marzipan, due to the presence of benzaldehyde. It belongs to a group of Agaricus which tend to stain yellow on bruising.

Similar species
When young, this fungus is often confused with species of the deadly genus Amanita.
 * Agaricus osecanus is rare, and is without the almond smell.
 * Agaricus xanthodermus, the yellow stainer, can cause stomach upsets.

Distribution and habitat
It is one of the largest white Agaricus species in Britain (where it appears during the months of July–November), 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 sometimes found associated with spruce.

Conservation
This mushroom is considered common and widespread, and is not a conservation concern.

Edibility
This is a choice edible species which has been much prized by farmers for generations, being regarded as one of the most delicious of all edible fungi. Despite this, the fruit bodies of this and other yellow-staining Agaricus species often have a build-up of heavy metals, such as cadmium and copper. However, this mushroom can resemble deadly mushrooms in the genus Amanita, such as A. verna.