Cap 3.0-7.0 (10) cm broad, obtuse-conic, expanding to broadly-convex, eventually nearly plane, usually with a low umbo; margin decurved, at times wavy and upturned; surface covered by a thin, white, tomentose veil concentrated at the disc, patchy elsewhere, sometimes disappearing entirely with age, overlying a hazel-brown, fibrillose surface; context white, firm, unchanging when cut, up to 5 mm thick at the disc, 1-2 mm at the margin; odor mild and taste mild.
Gills at first cream-buff, dull-brown at maturity, adnexed to notched with a decurrent tooth, 5.0-6.0 mm broad, edges fringed (use hand lens), lighter than the faces; lamellulae in three to four series.
Stipe 4.0-9.0 cm long, 1.0-1.5 cm thick, slender, straight, occasionally with a basal bend, equal to narrowed slightly toward the base, solid, fleshy-fibrous; surface white, the apex fufuraceous in youth, longitudinally striate below with scattered, loose brownish fibrils, darkening where handled; white mycelium at the base; partial veil absent.
Spores 8.0-10.5 x 5.0-6.0 µm, smooth, moderately thick-walled, elliptical in face-view, similar in profile but inequilateral, i.e. with a curved and flat side, sometimes nearly bean-shaped; hilar appendage inconspicuous; spores dingy-brown in deposit; pleurocystidia lacking.
Solitary to scattered under Quercus agrifolia (coast liveoak) and Lithocarpus densiflorus (tanbark oak); may also occur with oaks in the Sierra foothills; fruiting shortly after the fall rains; common.
Unknown; to be avoided; many Inocybe species contain the toxin muscarine.
While not unique to Inocybe brunnescens, an important field charcter is its cap with a thin, whitish veil patch which masks a hazel-brown, fibrillose surface. Other differentiating features include a white, more or less equal stipe, the lack of a distinctive odor, and a preference for fruiting under oak. Inocybe albodisca is similar, possessing a two-toned cap, but has a bulbous stipe base, green corn odor, and nodulose spores. Less likely to be confused but also found with oaks are Inocybe adaequata and Inocybe sororia. Inocybe adaequata is distinguished by a brown fibrillose, vinaceous-tinged cap and stipe, the cap lacking a veilar patch. Inocybe sororia, which can occur with conifers as well as oaks, has a straw-colored, appressed fibrillose cap with a thin veilar patch when young, and an odor of green corn.