Cap 3-8 cm broad, hemispheric to convex, expanding to plano-convex; margin at first incurved, then decurved; surface dry, disc glabrous to matted tomentose, fibrillose to squamulose toward the margin, cap fibrils hazel-brown to medium-brown over a cream-buff ground color; cuticle slowly bruising reddish, yellowing with KOH; context up to 5 mm thick, soft, off-white, bruising irregularly pinkish-vinaceous where cut; odor indistinct; taste mild.
Gills free, close, moderately broad, at first buff-brown, in age chocolate brown.
Stipe 4-9 cm long, 0.5-2.5 cm thick, equal to slightly enlarged at the base, stuffed at maturity; cortex discoloring pinkish-orange when cut; apex surface white, patchy fibrillose over a dull-buff ground color, elsewhere pallid, glabrous to minutely fibrillose-striate, brownish fibrils at the base forming inconspicuous partial rings, stipe base becoming brown to rusty-brown from handling; veil membranous, white, yielding a flaring or pendulous, superior, thin ring, the margin sometimes light brown.
Spores 4.5-5.0 x 3.0-3.5 µm, elliptical, inequilateral in profile, moderately thick-walled, lacking a germ pore.
Scattered to gregarious in mixed hardwood/conifer woods; fruiting early in the mushroom season.
Unknown, but probably edible.
Agaricus arorae is a modest-sized, forest-dwelling species (usually with hardwoods) which exhibits distinctive color changes. The context reddens when cut while the cap cuticle yellows in KOH. No other Agaricus in California shows this combination of color changes. In the field it is recognized by a brownish, fibrillose/squamulose cap, which stains red when bruised, and a stipe with inconspicuous rings at the base. Agaricus arorae was first described from Santa Cruz Co. but since has been found in San Mateo and Alameda counties. Other sylvan Agaricus species that resemble A. arorae include A. hondensis, A. subrutilescens, A. fusco-fibrillosus, and A. fuscovelatus. Agaricus hondensis has a lighter brown cap, a thick white annulus, bulbous stipe base, yellows in KOH, but lacks a red-staining reaction. Agaricus subrutilescens has a cap with dark-brown fibrils/scales, a stipe covered with conspicuous floccose-cottony scales, and does not stain red, although the gills of young specimens may redden when bruised. Agaricus fuscofibrillosus, another fibrillose to squamulose species, can be separated by color reactions and habitat. It reddens when bruised, but does not yellow with KOH, and occurs only with Monterey Cypress a habitat not known for A. arorae. Finally, another red-staining species, Agaricus fuscovelatus, is found only with conifers, usually pines. It has a conic-shaped cap when young, and a distinctive coco-colored veil. It also does not yellow with KOH.