Purple Brittlegill

Purple Brittlegill (Russula atropurpurea) is one of the most frequent dark-capped brittlegills of temperate broadleaf woodland. Its name means "blackish-purple", and it describes the cap well: a rich wine- or beetroot-purple that concentrates to near-black in the centre and often washes out to pinkish, ochre or straw-coloured blotches toward the margin as it ages.

It is a moderately firm, medium-sized species with a pale cream spore print, sitting among the mid-range brittlegills that are neither as robust as the Compactae nor as fragile as the red Sickener group. Older literature applies the name Russula undulata to the same fungus, and the two names are still met interchangeably.

Because purple and wine-red are among the most common cap colours in Russula, this species is a useful anchor for learning the group: it teaches the value of checking the stipe for colour changes and the spore print for exact shade, since colour alone will never resolve a dark brittlegill.

Cap

4–10 cm across, convex then flat and finally slightly depressed. The colour is a deep purple to blackish-crimson, typically darkest, almost black, at the centre and lighter toward the margin, where it often develops pale ochre, straw or pinkish patches as the pigment leaches out. The surface is smooth and slightly sticky in wet weather, drying matt. The cuticle peels about halfway to the centre, and the margin becomes faintly grooved in old specimens.

Gills

White when young, ageing to pale cream, moderately crowded, brittle, adnexed to nearly free, sometimes with a few forks near the stipe. They discolour dull grey-brown where bruised and become spotted in old fruiting bodies.

Stipe

3–7 cm tall, 1–2 cm thick, cylindrical, firm, white at first but ageing to a dirty grey-white, and often distinctly greying at the base — this greying, as opposed to browning, is a useful character. There is no ring and no volva. The interior becomes stuffed then hollow and the stipe snaps cleanly.

Flesh & spore print

The flesh is white, firm, greying slowly in age but not browning strongly and not reddening. The odour is faint, occasionally slightly apple-like. The spore print is white to pale cream; spores are broadly ellipsoid with warts joined into a partial network. Ferrous sulphate on the stipe flesh gives a salmon-pink reaction.

Purple Brittlegill grows on the ground in broadleaved and mixed woodland, most abundantly with oak and beech but also with birch, sweet chestnut and hornbeam, and it occurs less frequently with pine and spruce. It is tolerant of a wide range of soils, from acid sands through loams to mildly calcareous ground, and is common in parkland, hedgerows and wooded commons as well as in closed woodland.

It is widespread and common throughout Europe and into temperate Asia, with related dark-purple taxa in North America. The fruiting season runs from late summer through autumn, often in some quantity, and it is one of the brittlegills most likely to be met by a casual walker in an English oak wood in September.

This species is ectomycorrhizal, forming sheathed root tips with oak, beech and birch and exchanging soil-derived water, phosphorus and nitrogen for the tree's photosynthetic sugars. Its tolerance for a range of soil types and host trees accounts for its abundance, and it is a regular member of the fungal community sampled from oak root systems.

The fruiting bodies are relatively short-lived and are quickly exploited by fungus gnat larvae, beetles and slugs, whose feeding both destroys the mushroom and helps distribute spores. Rain progressively leaches the purple pigments from the cap, which is why late-season specimens look so faded and blotchy; the pigments are water-soluble and can stain damp litter beneath a decaying cap.

Russula xerampelina is similarly dark wine-purple but its white stipe browns ochre where handled, its gills mature deep ochre, its spore print is ochre and it smells strongly of shellfish — R. atropurpurea browns not at all and prints pale cream. Russula queletii grows with spruce, has a stipe washed with violet and a distinctly gooseberry-like odour.

Russula fragilis is much smaller and more fragile, with a purple cap, a strongly toothed or serrated gill edge visible with a lens, and a pure white spore print. Russula nobilis and other red brittlegills are scarlet rather than purple-black and lack the near-black cap centre. Checking the stipe for greying versus browning, and taking a spore print, resolves this group quickly.