Inocybe erubescens

Inocybe erubescens, long known as Inocybe patouillardii, is one of the largest and most robust members of the fibrecap genus Inocybe. Fibrecaps are named for the radially arranged silky fibrils that clothe their caps, which frequently split at the margin as the cap expands, and this species shows that structure clearly on a comparatively broad, fleshy cap.

The distinguishing behaviour is its reddening. Fresh fruitbodies are whitish to straw-coloured, but handling, bruising, insect damage and simple ageing bring out a brick-red to reddish-orange stain that spreads through cap, gills, stem and flesh. Old specimens can be almost wholly reddened, so a single troop may show white buttons alongside rust-red veterans.

It is also unusual in its timing: while most fibrecaps fruit in late summer and autumn, I. erubescens is a spring and early-summer species of chalky broadleaf woodland, appearing alongside the first flush of warm-season fungi under beech and lime.

Cap

Relatively large for the genus at 4–8 cm across, conical or bell-shaped at first, then expanding to broadly convex with a persistent low umbo. The surface is dry and covered with fine appressed radial fibrils; as the cap expands the margin often cracks and splits radially. The colour begins whitish to pale straw or ivory and develops brick-red, salmon or rusty-red staining and streaks with age or handling.

Gills

Adnexed to almost free, crowded and rather narrow, whitish or pale pinkish-buff at first, then dull olive-brown to clay-brown as spores mature, and reddening where bruised. The edges are minutely fringed and paler. There is no true ring, only faint fibrillose veil traces near the stem apex in young specimens.

Stipe

Stout for a fibrecap, 4–9 cm tall and 1–2 cm thick, solid, cylindrical to slightly thickened at the base but without a marginate bulb. It is whitish and longitudinally fibrillose, reddening from the base upwards with age and handling.

Flesh & spore print

The flesh is white and firm, staining reddish where cut or bruised, with a slightly fruity to earthy smell. The spore print is dull clay- to snuff-brown, never rusty or dark. Microscopically the spores are smooth and bean-shaped, without nodules or angles, a character shared with the Inocybe rimosa group.

Inocybe erubescens grows on the ground in broadleaved woodland on calcareous soils, associating chiefly with beech and lime and also with hornbeam, hazel and oak. It favours warm, well-drained chalk and limestone sites, and often appears along woodland paths, in open glades, on grassy woodland edges and in parkland under mature trees.

Its season is distinctly early — spring into early summer, mainly from around May to July — which sets it apart from the largely autumnal remainder of the genus. The species is widespread across central and southern Europe on suitable soils, becoming more local and coastal or southern towards the north of its range.

This fibrecap is ectomycorrhizal, forming symbiotic root sheaths with beech, lime and other broadleaves. As in other Inocybe, the fungal mantle and its emanating hyphae greatly extend the tree's absorptive reach, delivering nitrogen, phosphorus and water in exchange for photosynthetic carbon. The genus as a whole is a major component of ectomycorrhizal communities on calcareous and disturbed soils.

Spring fruiting is comparatively rare among ectomycorrhizal agarics and is thought to reflect the fungus's response to soil warming and the flush of root activity as broadleaf trees leaf out. The reddening reaction results from oxidation of pigment precursors in the flesh once tissue is damaged and exposed to air, the same broad type of enzymatic process that browns cut fruit.

Inocybe rimosa and related straw-coloured fibrecaps share the radially fibrillose, splitting cap but are more slender, distinctly yellow-brown to straw, and do not develop the brick-red staining. Inocybe geophylla is much smaller and pure white or lilac with a silky, non-splitting cap.

Calocybe gambosa, the St George's Mushroom, fruits in the same spring window on chalky ground but has crowded white gills that stay white, a white spore print, a strongly mealy smell and a smooth, non-fibrillose cap with no reddening. Agaricus species have free gills that pass from pink to chocolate-brown, a dark brown spore print and a distinct membranous ring, none of which apply here. The combination of radial cap fibrils, brick-red staining and a dull clay-brown spore print is decisive.