Charcoal Burner

Russula cyanoxantha belongs to the brittlegills, a very large genus of mycorrhizal agarics recognised by their brittle, chalky flesh, absence of any ring or volva, and stems that snap cleanly like a stick of blackboard chalk rather than shredding into fibres. The Charcoal Burner is among the most abundant brittlegills of European broadleaf woodland.

What makes it exceptional is a character that contradicts the genus name. In almost every other Russula the gills are rigid and shatter under a sideways stroke of the thumb; in R. cyanoxantha the gills are soft, pliable, slightly greasy to the touch and bend without breaking, feeling almost soapy. This single tactile test, done by drawing a finger across the gill faces, identifies the species faster than any colour character.

The cap colour is famously unreliable, running through wine-purple, slate blue, olive-green, grey, lilac and mixtures of all of these, sometimes on a single cap and often changing between young and old fruit bodies. A distinctly green-capped form is common enough to have been named separately in the past. The English name refers to the sooty, charcoal-like tones many caps develop.

Cap

5-15 cm across, convex at first, then flattening and finally becoming shallowly depressed in the centre. The surface is smooth, often slightly greasy or sticky when wet and matt when dry, with the cuticle peeling only a short way in from the margin. The margin is smooth and only becomes faintly grooved in old fruit bodies. Colour is highly variable: mixtures of violet, wine-purple, slate grey, blue-grey, olive and green, frequently radially streaked and paler toward the centre.

Gills

Adnexed to almost free, moderately crowded, and white, remaining white or very pale cream throughout life. They are the key feature: soft, flexible, faintly greasy and forking near the stem, and they can be stroked without shattering. In most other brittlegills the same stroke crumbles them.

Stipe

5-10 cm tall, 1.5-3 cm thick, cylindrical or slightly clubbed, firm then softening, and white, sometimes flushed with lilac or grey where the cap colour has run down. It is ringless and has no volva, and it snaps cleanly rather than splitting into strands.

Flesh and spore print

The flesh is white, thick and brittle in the cap, and unchanging or ageing very slightly greyish. The spore print is white. A drop of iron salts on the stem surface gives a greenish rather than pink reaction, a classic laboratory confirmation for this species and a useful separator within the genus.

The Charcoal Burner is a woodland fungus of broadleaved and mixed stands, most consistently associated with beech, and also frequent under oak, birch, hornbeam and sweet chestnut. It occurs on a wide range of soils, from acid sands to calcareous clays, which contributes to its abundance, and it also appears in parkland and along wooded roadsides wherever suitable host trees grow.

Fruit bodies appear from early summer through to late autumn, with the main flush from August to October. They emerge singly or scattered across the woodland floor, pushing up through leaf litter, and are often among the first larger mushrooms of the summer after prolonged rain. The species is common throughout Europe, extends across temperate Asia, and closely similar taxa are reported from North America.

Russula cyanoxantha is ectomycorrhizal. Its mycelium sheathes the fine root tips of its host trees and grows between the outer root cells, forming the structure known as a Hartig net. Through this interface the fungus supplies the tree with water, phosphorus and nitrogen scavenged from a soil volume far larger than the roots can reach, and receives sugars produced by photosynthesis in return.

Because the relationship depends on a living host, the fungus cannot be grown apart from its tree and fruit bodies appear only within the root zone of mature beech, oak and their associates. Brittlegills as a group are among the dominant ectomycorrhizal partners of temperate forests, and the wide soil tolerance of this species makes it one of the most frequently encountered root symbionts in European woodland.

Other purple and green brittlegills are the main source of confusion. Russula parazurea has a dull bluish-grey, distinctly bloomed and powdery cap and gills that shatter normally. Russula grisea and the Russula ionochlora group have similarly mixed grey-violet-green caps but cream rather than pure white gills and brittle, non-flexible gill plates. Russula virescens, the Greencracked Brittlegill, is unmistakable once seen: its green cap surface breaks into a mosaic of small angular patches like crazed paintwork.

The flexible, greasy gills of the Charcoal Burner remain the fastest field separator from all of them, backed up by the greenish reaction to iron salts on the stem where other members of the group turn salmon-pink. Away from the genus, Lepista nuda can show a comparable violet cap, but it has pink spores, sinuate lilac gills, a fibrous non-brittle stem and a much later fruiting season.