Blackening Brittlegill

Blackening Brittlegill (Russula nigricans) is the most substantial and longest-lasting of the common brittlegills. Where most Russula fruiting bodies collapse within days, this one hardens as it blackens and can stand on the woodland floor like a lump of charcoal for the rest of the season, which makes it one of the easiest species in the genus to recognise even out of its prime.

It belongs to the section Compactae, a group of thick-fleshed brittlegills that blacken or brown on ageing and that generally have hard, dense flesh very different from the light, crumbly texture of the coloured Russula species. Within that group R. nigricans is defined by its remarkably distant, thick gills — the widest gill spacing of any common brittlegill — and by a two-stage colour change in cut flesh.

The blackened old fruiting bodies play a further ecological role as hosts for the parasitic Asterophora mushrooms, which fruit directly on the decaying caps, so a find of black R. nigricans husks in late autumn is worth examining for small pale mushrooms growing on top of them.

Cap

5–20 cm across, sometimes larger, thick-fleshed and heavy. Convex with an inrolled margin at first, expanding to flat and finally broadly funnel-shaped. The surface is dirty white to grey-buff when young, darkening progressively to brown, then sooty umber and eventually blue-black. It is dry, slightly felty, and frequently caked with soil and leaf debris because the young cap pushes up through the litter. The cuticle barely peels.

Gills

The key feature: gills are very thick, brittle, and exceptionally widely spaced, with conspicuous gaps between them and short intermediate gills interspersed. They are pale cream to dingy straw, adnate to slightly decurrent, and darken with the rest of the fruiting body. In section the gill trama is coarse and easily crumbled.

Stipe

3–8 cm tall and 1.5–4 cm thick, very stout, cylindrical to slightly tapering, whitish then browning and finally blackening from the base upward. It is solid and hard rather than hollow, with no ring or volva, and is often shorter than the cap is wide.

Flesh & spore print

The flesh is white, hard and dense. Cut or bruised, it first flushes salmon to brick-red over several minutes, then passes through grey and finally to black over an hour or more — this red-then-black sequence is diagnostic. The odour is faint and slightly fruity. The spore print is white; spores are broadly ellipsoid with low warts joined by fine ridges.

Blackening Brittlegill grows on the ground in broadleaved and mixed woodland, most often with beech and oak but also with birch, sweet chestnut and, less commonly, with pine and spruce. It favours well-drained soils and is equally at home on acid sands and on calcareous clays, frequently appearing on bare ground, path edges and slopes where the litter is thin.

It is widespread and common throughout Europe, temperate Asia and North America. Fruiting runs from mid-summer through to the first frosts, and the blackened remains persist far longer — old, hard, charcoal-like specimens are still recognisable in mid-winter, and are often the only trace of the previous season's fruiting.

The species is ectomycorrhizal, forming sheathed root tips with beech, oak and other broadleaved trees and supplying them with water and soil minerals in return for carbohydrates. Its dense, long-lived fruiting bodies invest heavily in structural tissue, which is why they endure so much longer than typical brittlegills and why they can go on shedding spores over an extended period.

Decaying fruiting bodies of R. nigricans are the standard substrate for two small parasitic agarics: Asterophora parasitica, with a greyish silky cap and thick grey gills, and Asterophora lycoperdoides, whose cap surface breaks down into a powdery brown mass of asexual chlamydospores. Finding these fungi is itself a reliable indicator that the blackened host below is R. nigricans or a close relative in the same section.

Russula densifolia is the closest match and also reddens then blackens, but its gills are narrow and crowded rather than fat and distant, and it tends to a somewhat more slender build. Russula adusta darkens more slowly and unevenly to a dull wine-brown or grey-brown rather than a true black, has moderately crowded gills, and its cut flesh greys without a strong red stage.

Russula albonigra goes almost directly from white to black with little or no intervening red flush, and has crowded, slightly decurrent gills. Lactarius vellereus and L. piperatus can look superficially similar in size and pale colour on the same woodland floors, but they exude a white latex from cut gills, which no Russula ever does. Gill spacing plus the exact sequence of colour change in cut flesh will separate the Compactae group in the field.