Purple Brittlegill Identification Guide

The Setting and the First Impression

Midsummer to late autumn, a mature oak or beech wood on acid to neutral soil, or the grassy margin of a parkland stand. Among the litter sits a solid, medium-sized mushroom with a strikingly dark cap — deep wine-purple fading toward the rim, almost black over the centre — on a firm white stalk. In many British and northern European oak woods this is the commonest dark purple brittlegill on the ground, and once you have handled a few the combination of very dark disc, pale gills and hard white stem becomes instantly recognisable.

Before anything else, confirm the genus. Snap the stipe: it should break cleanly and abruptly, like a stick of chalk, rather than tearing into fibres. Rub a thumb across the gills: they should shatter and crumble rather than bending. Check that there is no ring, no volva, no latex and no cobwebby veil. Only then work the species characters.

Step One: The Cap

The cap runs 4 to 9 cm across, occasionally to 11, convex when young, then flat, and finally shallowly depressed at the centre. The colour scheme is the species' calling card: deep purple-red to wine-purple over most of the surface, always darkest at the disc, where it is commonly blackish-purple or nearly black. Ageing and weathered caps develop pale straw, ochre or buff bleached patches, sometimes extensive, so a mixed troop can look inconsistent until you notice that every cap still holds the dark centre.

The surface is slightly viscid when wet and dull, smooth and faintly matt when dry. Test the cuticle peel from the margin inward with a fingernail: it lifts roughly halfway to two-thirds of the way to the disc, coming away in a soft strip. The margin stays even and smooth for a long time, developing only faint grooves in fully expanded old caps.

Step Two: The Gills

The gills are white to very pale cream, adnexed and moderately crowded, and their pallor against so dark a cap is the visual contrast that names the search image. They are rigid and brittle, snapping under sideways pressure.

With age they pick up rusty-brown spotting, especially along the edges and where slugs and insects have worked. Some gills fork close to the stipe, and interveining develops between the gills near the stem in older material. Under a lens the gill edge is even and concolorous — not serrated, which matters for one of the look-alikes below.

Step Three: Stipe, Base and Flesh

The stipe is 3 to 6 cm long and 1 to 2.5 cm thick, cylindrical or tapering slightly downward, white, firm, and with the chalky, slightly wrinkled surface typical of the genus. There is no ring and no bulb.

The important behaviour is discolouration. As the fruitbody ages, and particularly at the base and where handled, the stipe dulls to grey and then greyish-brown. Slice it lengthwise: the interior becomes spongy and cavernous with age, and the pith greys noticeably. That greying stalk on an otherwise white stem, under broadleaves, is one of the most dependable supporting characters for the species.

The flesh is white, firm, and either unchanging or slowly turning dingy grey on exposure; there is no reddening, no blueing and no latex. The odour is faint and unremarkable, sometimes slightly fruity-sour in very old specimens.

Iron salts (FeSO4) applied to the stipe surface give the usual salmon-pink reaction of the genus, while a drop of ammonia on the gills produces no rose flush — a negative that is worth recording because it is exactly where the pine-wood purple species differs.

Step Four: Taking and Reading a Spore Print

In this genus the spore print is a primary character, and a pale print needs careful handling. Choose a fully expanded, mature cap, cut the stipe flush so the gills lie flat, and set the cap gill-side down on white paper or on a glass slide. Cover it to keep humidity high and leave it six to twelve hours or overnight.

A thin deposit reads paler than the truth, so scrape the deposit into a compact ridge with a razor blade before assessing it in daylight against white. Better still, print a known cream-spored species alongside; the eye ranks two samples far more reliably than it scores one.

Russula atropurpurea gives a white to very pale cream print, near the top of the pale range at roughly Ia to IIa on the Romagnesi scale. That near-white result is the single most useful discriminator against the darker-printed purple species. Microscopically the spores are broadly ellipsoid with low to moderate warts joined by a few connecting lines, and the ornamentation blackens in Melzer's reagent.

Substrate, Season and Ecology

The species is ectomycorrhizal, associating chiefly with oak and beech, and also with birch and occasionally with conifers where they grow mixed in. It prefers acid to neutral soils and appears on woodland floors, wood banks, hedgerow lines and the fringes of mature parkland, usually within the root spread of its partner tree.

Fruiting runs from July into November, peaking in September and October, with fruitbodies scattered or in small groups. It is common and widespread across Europe and has been reported from broadleaved woodland elsewhere in the north temperate zone. Note that the taxonomy of this complex has been revised, and much European material is now treated by many authors under the name Russula krombholzii; older literature and modern accounts do not always agree, which is worth flagging in any record you keep.

Separating It From Its Closest Look-Alikes

Russula sardonia is the classic confusion at a distance because it is equally dark purple. It grows with pine on sandy acid soils, its gills mature to a clear primrose-yellow, its stipe is flushed violet rather than white, and ammonia on its gills produces a rose-pink flush. Four independent differences, all easy to check.

Russula queletii is another conifer species with a dark wine-purple cap, but its stipe is strongly violet-tinted and its gills are cream rather than white; it also tends to a more conical young cap.

Russula fragilis is much smaller and more delicate, its cap mixing purple with olive and green tones, and — decisively — its gill edges are finely serrated when examined under a lens. Its print is white and it occurs in mixed and damp broadleaved woodland.

Russula nobilis (also treated as R. mairei) is a small, very firm beechwood species with a scarlet to bright red cap that often keeps a narrow white marginal band, white gills, and a distinctly harder texture.

Russula xerampelina and its relatives include dark wine-red forms, but their stipes discolour ochre-brown throughout with age rather than greying, and their spore prints are a deep ochre. Print colour ends the comparison quickly.

Russula cyanoxantha can carry purple tones, but its gills are soft, greasy and flexible under a thumb rather than brittle, and its print is white.

Russula caerulea is a pine associate with a dark purple, distinctly umbonate cap and an ochre print.

A Working Field Sequence

Oak or beech overhead on acid ground; a dark purple cap with a near-black centre and bleached ochre patches in age; a cuticle peeling half to two-thirds; brittle white to pale cream gills with even edges; a chalky white stipe that greys with age, especially at the base; white flesh going dingy grey but never red or blue; no ammonia reaction; and a white to very pale cream print. Run that sequence and the species holds up consistently.