Agaricus arvensis

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

Cap:

Diameter: 7-14 cm.

Shape: Hemispherical, then convex to broadly convex.

Texture: Dry, initially covered with fine flaky scales, becoming smooth to fibrillose-squamulous-appressed.

Color: White, sometimes yellowish with age or when crumpled, turning tawny.

Margin: Long curved, acute, often appendiculated with velar remains at the beginning.

Gills:

Free and up to 8 mm wide.

Initially white, becoming grayish brown, then blackish brown at the end.

Tight, with entire awns.

Slightly paler than the faces.

Stem:

Length: 4-8(10) x 1-2.5 cm.

Shape: Subequal, potbelly to bulbous, often stocky.

Texture: Smooth or with discontinuous velar remains toward the apex.

Surface: Numerous fine white scales, often pointed or obtuse, and arranged in complete circles or spirals toward the base.

Color: White background.

Partial Veil:

Membranous, leaving a superior or subapical ring on the stipe.

Pendulous, ample, thick, and fragile.

Double, toothed.

Initially smooth above, typically with white cottony patches that turn yellowish, orange, or buff and break into a star or ship's wheel below.

Flesh:

Thickness: Up to 20 mm.

Texture: Firm.

Color: White, unchanging or slowly yellowing when crushed.

Smell and Flavor:

Pleasant smell, resembling almonds or anise when fresh.

Flavor pleasant at first, sweet like almonds, but it can become nauseous or musty with age.

Spore Characteristics:

Spore color: Dark brown, purple brown to blackish brown.

Spores: Ellipsoid, smooth, with thickened walls, dark brown, (5.3)6-6.5(7.5) x (4.1)4.6-5.1(5.4)[6.8] µm on average.

Growth and Ecology:

Growth Mode: Gregarious or subcespitous, often in circles.

Ecology: Saprotrophic, typically found on rich soil in open grassy areas, lawns, fields, pastures, forest edges, and sometimes clearings.

Fruiting Period:

June to October.

Frequency:

Common.

Edibility:

Edible of choice but with caution. It may cause stomach upset in some people. Agaricus species often have a build-up of heavy metals, such as cadmium and copper.

Chemical Reactions:

The cap turns yellow with KOH.

Remarks

This agaric is characterized by its large size, its whitish, buff to yellowish cap, smooth or sometimes cracked into small scales or plates, often yellowish when crumpled, its free, tight blades, pale then grayish and chocolate brown to blackish brown, its base stocky, smooth or with cottony scales towards the base, its double ring, distinctly cracked and bordered below, its flesh unchanging or slowly yellowing when crushed, its smell of almond or anise at first, of mold or urine with age, and its growth in open grassy environments.
Its yellowing and aniseed odor can be fickle characteristics.
It can be very similar to A. crocodilinus when found in the forest, however this is not the case. The spores testify, being larger, 8.5-13 x 5.5-7.5 µm.
The latter has a stronger size, a hat tends more strongly to crack and does not yellow when cut or crumpled.
A. silvicola often has more slender basidiomes, a ± double and ± toothed ring, smaller spores, 5-6 x 3-4 µm, and grows in mainly coniferous forests.
A. fissuratus has spores 7.6 x 5.4 µm on average, Q = 1.41.
White amanitas, poisonous-deadly, have a white spore and are associated with forests.

References

1, MycoMatch (MatchMaker) Mushrooms of the Pacific Northwest
2. Version 2.4.1. 2023.
3. Davis, R. Michael; Sommer, Robert; Menge, John A. (2012). Field Guide to Mushrooms of Western North America. Berkeley: University of California Press. p. 220.
4. Dogan, A., Dalar, A., Sadullahoglu, C., Battal, A., Uzun, Y., Celik, I., & Demirel, K. (2018). Investigation of the protective effects of horse mushroom (Agaricus arvensis Schaeff.) against carbon tetrachloride-induced oxidative stress in rats. Molecular Biology Reports, 45(5), 787–797.