A very large, white to pale ochre European grassland Agaricus. Its common name, Macro Mushroom, refers to its spores, the largest of any Agaricus found in Britain. Young fruit bodies smell faintly of almonds, but the odour becomes increasingly ammoniacal (urine-like) with age, which gives the species its epithet. It grows in permanent pastures, grassy verges and woodland clearings.
Edible and good when cooked; First Nature suggests using it like portobello mushrooms. Separate from A. arvensis (smaller spores) and from deadly white Amanitas, which keep white gills, drop a white spore print and have a volva. The poisonous Yellow Stainer (A. xanthodermus) turns chrome yellow when the stem base is cut and smells of ink or phenol.
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Habitat
Saprobic in small groups in permanent pastures, grassy roadside verges, woodland edges and grassy woodland clearings. Season early July to late October in Britain and Ireland.
Distribution
Europe, from northern Scandinavia to the Mediterranean, including Britain and Ireland. The sources consulted do not report it from North America.
Characteristics
Cap: 10-25 cm (reported up to 30 cm), globose then plano-convex to flat, white becoming pale ochre, radially fibrillose to scaly at maturity, bruising slowly yellowish toward the margin. Gills: free, crowded, pale greyish pink becoming dark brown to purplish brown, edges paler. Stem: 7-12 cm long, 2-3.5 cm thick, with a pendent membranous ring with a cog-wheel pattern beneath; smooth above the ring, with white to brownish snakeskin-like scales below. Flesh: white, the stem flesh turning slightly brown when cut, especially at the base. Odour: faint almond when young, ammoniacal with age. Spore print: brown. Spores: ellipsoid, smooth, 8.5-12.3 x 5.2-6.9 um (First Nature).
