Star-spored Fibrecap | Inocybe asterospora Identification Guide

First impression: a cracked brown cone on a clay bank

The setting is broadleaf woodland on heavy or lime-rich ground: beech and oak on clay, hazel along a path bank, lime and hornbeam in old parkland. The fruit bodies are scattered on bare soil or thin litter, often right at the edge of a track where the ground has been cut back. What you see is a small tawny-brown mushroom, 2 to 5 cm across, with a sharply pointed centre and a cap that has split radially from the margin inward so that pale flesh shows between the fibre bundles. That torn, straw-thatched look combined with a pointed umbo is the fibrecap signature, and it puts you in the right genus within seconds. Getting to the species means looking at three things in order: the cap surface, the stipe base, and the spores.

Step 1: the cap

The cap starts conical and expands to broadly bell-shaped or flat with a strong, persistent, often acute umbo. Colour is ochre-brown to date-brown or rusty brown, with the umbo usually darker and smoother than the rest. The surface is dry, never sticky, and is built from coarse radial fibrils that separate as the cap expands, producing the strongly rimose (split) surface that is central to the identification. In older specimens the margin is torn into ragged strips and may lift and curl.

Step 2: the gills

The gills are adnexed to nearly free, crowded, and follow the standard fibrecap colour progression: pale grey-buff when young, then clay-brown, then tobacco or snuff-brown as spores mature. Put a lens along the gill edge. It is finely fringed and noticeably whiter than the gill face, a small feature that is easy to see and worth recording in your notes.

Step 3: the stipe and the bulb, which is the key macro character

This is where the species is actually decided in the field. The stipe is 4 to 7 cm long and 4 to 8 mm thick, cylindrical, whitish through pinkish-buff to pale brown. Two features matter.

First, it is pruinose over its entire length. Hold it against a dark background in raking light and look for a fine white mealiness running from the apex all the way down to the base, not just in the top few millimetres. This corresponds to caulocystidia present the whole way down.

Second, and decisively, the base swells into an abrupt, flattened bulb with a distinct rim, as though the stipe were standing on a small saucer. The technical term is a marginate bulb, and the transition from stipe to bulb is a sharp shoulder rather than a gradual taper. Dig the fruit body up rather than pulling it; a snapped stipe destroys exactly the feature you need.

Step 4: flesh, colour change and smell

Section the fruit body from cap centre to bulb. The flesh is white to pale buff, sometimes dingy in age, and it does not change colour on cutting. There is no reddening, no blueing and no milk. The smell is strongly spermatic and earthy, released fully when the flesh is crushed between finger and thumb; it is characteristic of the genus rather than of this species alone, but its presence supports the generic placement.

Step 5: the spore print

Cut the cap free, place it gills down on white paper, cover it, and leave it for four to eight hours. The print is a dull clay-brown to snuff-brown, sometimes described as tobacco. Compare it deliberately against the alternatives: a bright rusty-ochre print points to Cortinarius or Galerina, a pink print to Entoloma, a white print to a great many other genera, and a black print to Coprinoid or Psathyrella relatives. Only a dull brown print keeps you in the fibrecaps.

Step 6: the spores, and the name

Mount a scrape of the print in water or dilute potassium hydroxide. Inocybe asterospora has spores 9 to 13 by 8 to 10 micrometres, strongly nodulose with roughly eight to twelve prominent conical to blunt knobs distributed over the surface. Because they are close to equal in all dimensions, they look star-shaped whichever way they roll under the coverslip, which is exactly what the epithet records. While you are at the microscope, check the gill faces and edges for thick-walled cystidia with crystal-capped apices, and check the stipe base for the same structures; finding cystidia right down to the bulb confirms in the laboratory what the pruinose surface suggested in the field.

Habitat, partners and season

This is an ectomycorrhizal fungus, associating with beech, oak, hazel, lime and occasionally birch. It shows a marked preference for heavy clay and calcareous soils, which is why path banks, woodland edges, cut slopes and parkland under mature broadleaves are the most productive places to search. The season runs from midsummer to late autumn, with fruit bodies appearing singly or in small scattered groups rather than troops.

Separating it from look-alikes on morphology

Inocybe napipes. Also has a marginate bulb, so it is the most serious confusion. Its cap, however, is smoothly fibrillose and only slightly cracked rather than deeply rimose, it is darker chestnut-brown, and it grows in damp acid ground with birch and conifers, often near Sphagnum. Its spore nodules are fewer and blunter.

The Inocybe rimosa group (including I. fastigiata). Very similar in shape and just as strongly split, but the stipe is pruinose only at the extreme apex and the base is simply thickened, never abruptly rimmed. Its spores are smooth and bean-shaped, which is an absolute separation.

Inocybe praetervisa. Shares both the marginate bulb and knobbly spores, but is paler yellow-ochre, with a less deeply cracked cap and spores whose nodules are fewer and more irregular in size.

Inocybe margaritispora. Nodulose spores again, but the knobs are blunt and sparse, the cap is paler with a scaly-cracked rather than fibrous-split surface, and the bulb is at most weakly rimmed.

Inocybe lanuginosa and relatives. Woolly or shaggy-scaly caps, dark brown, no bulb at all.

Hebeloma species. Larger, with smooth greasy caps, droplets on the gill edge when humid, and a clay-brown print but smooth to finely roughened amygdaliform spores.

Cortinarius species. Young caps have a cobwebby veil between cap margin and stipe, and mature prints are rusty rather than dull brown.

Field sequence

1. Small brown mushroom on clay under broadleaf trees.
2. Cap conical with a pointed umbo, radially split to show pale flesh.
3. Gill edge finely fringed and whitish.
4. Stipe finely mealy from apex to base.
5. Base an abrupt, rimmed, saucer-like bulb.
6. Print dull snuff-brown; spores knobbly and star-like in every view.