Frosty Fibrecap Identification Guide

First Impressions and the Setting

Frosty Fibrecap is a mushroom of shaded woodland paths and thin litter, and the classic setting is beech or mixed broadleaf woodland on lime-rich soil, especially along compacted verges, the edges of tracks and the trampled ground under old trees. It also appears with hornbeam, hazel, lime and occasionally conifers, always on soil and always near living trees. Autumn is the season, from late summer through to the first hard frosts, with fruit bodies scattered or in small troops, sometimes in loose lines along a path edge.

The first impression is of a dark, dry, upright mushroom with a pointed cap and a fibrous, streaky surface, standing three to eight centimetres tall in poor litter. It looks businesslike rather than showy: rich chestnut to date-brown, silky-fibrous, with an obviously conical profile. The detail that stops you is the frosting, patches of whitish veil material clinging to the top of the cap like scraps of dried paint, which is what the common name records.

Reading the Cap

Cap diameter is three to seven centimetres, occasionally a little more. The shape develops from conical or bluntly bell-shaped through broadly convex to flattened, but it almost always retains a pronounced, sometimes acutely pointed central umbo. The cap never becomes fully flat and never becomes depressed.

Work outward from the umbo. The ground colour is a warm dark brown, chestnut, date-brown or tobacco-brown, darkest at the centre and slightly paler towards the edge. The surface is dry and matt, never viscid, and it is finely and conspicuously fibrillose, with silky fibres running radially from the umbo to the margin so that the cap looks streaked or combed. As the cap expands those fibres separate and the margin frequently splits radially, sometimes deeply, producing a torn, star-like outline in old specimens.

Now look for the frosting. Scattered over the disc and upper half of the cap are irregular, flattened, whitish to cream patches and flecks, the dried remnants of a universal veil. They are appressed rather than erect, are easily washed off by heavy rain and are often much more obvious in young fruit bodies. Their presence on a dark chestnut fibrillose cap is the fastest route to this species; their absence in a weathered specimen simply means you have to rely on the other characters.

The margin is initially incurved and may carry a few pale fibrous fringes, later spreading, wavy and split.

The Gills

Gills are narrowly attached to the stipe, adnexed to almost free, and moderately crowded, fairly broad and ventricose.

Colour progression matters here and is worth checking on several fruit bodies of different ages. Very young gills are pale, whitish to greyish-cream. They then pass through pale clay to a dull greyish-brown or olive-brown, and finally settle at a dull earth-brown or snuff-brown at maturity. The important negative is that they never become bright rust, cinnamon, pink or black.

Under a hand lens the gill edge appears finely white-fringed and paler than the face, a fine pale line running the length of the gill. Damaged gills discolour dully brownish rather than reddening.

Stipe, Base and Flesh

The stipe is four to nine centimetres long and five to twelve millimetres thick, cylindrical and equal or slightly thickened downwards, ending in a rounded to slightly swollen base. Two negatives are diagnostic: there is no abrupt, sharply rimmed marginate bulb, and there is no ring, only occasionally a faint fibrous zone where the veil detached.

Surface colour is whitish to pale buff at the apex, becoming pale brownish and finally darker brown towards the base, especially where handled. The whole length is finely longitudinally fibrillose, silky and dry. Check the apex with a lens for a fine mealy coating; in this species pruinosity is limited to the top of the stipe rather than running the whole length, which is a useful distinction from a number of relatives.

Split the fruit body lengthwise. The flesh is white to pale cream in the cap, darkening slightly to brownish in the stipe base, firm and fibrous, and it stays essentially unchanged on exposure to air, with no reddening, blueing or blackening. The odour is strong, sharp and unmistakably spermatic or faintly earthy-acrid, and checking it is part of confirming the genus rather than the species.

Taking and Reading a Spore Print

Sever the stipe flush with the gills, set the cap gill-side down on half-black, half-white paper, cover with a bowl and leave for three to six hours or overnight.

The print is dull brown: snuff-brown, clay-brown or tobacco, with a slightly greyish or olive cast, and it lacks the bright rust or cinnamon tint of a webcap and the dark chocolate tone of a fieldcap. It reads clearly on the white half of the paper. Under a microscope the spores of this species are smooth, almond-shaped to ellipsoid with a blunt apex, roughly nine to eleven micrometres long, and there are no bumps, nodules or angular projections. The gill-edge cells are thin-walled and club-shaped, without the thick-walled, crystal-tipped cystidia found in many other fibrecaps. Together, smooth spores and thin-walled edge cells are strong confirmation.

Substrate, Season and Ecology

Frosty Fibrecap is ectomycorrhizal, forming root sheaths on broadleaf trees, particularly beech, and it must therefore be growing in soil under or near living hosts. It shows a marked preference for base-rich and calcareous ground and a striking tolerance of compaction, which is why path edges, track verges and thin, worn litter are the most productive places to search. Fruiting runs from late summer into late autumn, and fruit bodies are typically scattered rather than clustered.

Separating It from Look-Alikes

Split Fibrecap (Inocybe rimosa and its allies) is the closest general match in outline but is paler, straw-yellow to ochre-brown rather than dark chestnut, splits even more dramatically at the margin, and never carries the whitish veil patches on the disc.

Inocybe corydalina has a comparable conical fibrillose cap but shows greenish or blue-green tones at the umbo and has a differently scented, sweeter odour.

Inocybe cookei is paler, ochre-yellow, and has a distinctly marginate, abruptly rimmed bulb at the stipe base, which the Frosty Fibrecap lacks.

Inocybe asterospora looks superficially similar in colour but also has a sharply marginate bulb and, under the microscope, strongly star-shaped nodulose spores, quite unlike the smooth almond spores here.

Hebeloma species on the same ground are paler and often greasy or viscid-capped, with sinuate gills that frequently weep droplets and edges that stain brown, plus a differently structured, more clay-coloured spore deposit.

Cortinarius species are excluded by the cobwebby cortina between margin and stipe and by their rusty-brown print.

The reliable field bundle for this species is therefore: dark chestnut, radially fibrillose, umbonate and often split cap; whitish appressed veil patches on the disc; dull brown adnexed gills with a pale fringed edge; a fibrillose stipe pruinose only at the apex, with no rimmed bulb and no ring; unchanging pale flesh; a strong sharp odour; and a dull snuff-brown print from beech woodland paths in autumn.