Blue Band Brittlegill | Russula azurea Identification Guide

The Setting and the First Impression

This is a fungus of spruce forest, and it rewards slow walking through mossy, acid ground in mature stands, often in upland or northern country. What catches the eye is a modest, neat mushroom of an unusual colour: a soft lilac-blue, blue-grey or slate-violet cap, frequently paler and creamy-ochre in the middle, sitting on a short, clean white stem. The overall impression is small and tidy rather than imposing, and the cap surface looks dusted, as though someone had brushed it with chalk or flour. That frosted, powdery bloom is visible from standing height once you know to look for it, and it is one of the two or three characters the identification rests on. Fruit bodies come singly or in scattered groups among feather moss, needle litter and bilberry.

Reading the Cap

Cap diameter is typically 3 to 6 cm, occasionally to 8 cm, which makes it one of the smaller members of the genus. Young caps are convex with an inrolled edge; they expand to flat and then to shallowly depressed at the centre, and the margin remains smooth for a long time, only becoming faintly grooved in overmature specimens.

Surface texture matters more than shape. The cuticle is dry and matt, covered by a fine white pruina that looks like frost or bloom on a plum, and rubbing it with a damp finger removes the bloom and darkens the patch. Colour underneath is lilac, violet-blue, greyish-blue or slate, often unevenly distributed, and there is very commonly a paler zone at the centre in cream, ochre, buff or even pale brown, giving the cap a two-tone appearance with a bluish rim and a pale disc. The name refers to that bluish band. In wet weather the cap darkens and looks greasier, and the bloom can be washed off entirely, so judge freshly emerged caps sheltered by moss where possible.

Test the skin by peeling from the margin with a thumbnail. It comes away roughly a third to halfway to the centre: not fully peeling, not stubbornly fixed.

The Gills

Gills are white and stay white, ageing at most to a very pale cream, and they are fairly crowded, narrow, and adnexed to narrowly attached at the stem. Forking occurs only occasionally, mostly near the stem apex, and there are few short intermediate gills. They are brittle, as in every brittlegill, so a finger drawn across them crumbles the edges rather than bending them. There is no bruising to grey, brown, red or black, and no juice of any kind appears when they are cut. Keeping the gills genuinely white through to maturity is one of the strongest characters for this species and it must be checked on a fully expanded cap, not a button.

The Stem, the Base and the Flesh

The stem is 2 to 5 cm long and 0.8 to 1.5 cm thick, cylindrical or very slightly club-shaped, pure white, dry and finely wrinkled lengthwise, sometimes with a faint bloom of its own. It is firm and solid when young and becomes chambered or hollow with age. Look for the absence of things too: there is no ring, no cobwebby veil, no volva at the base, and no coloured flush anywhere on the stem. The base is white and clean, occasionally with faint brownish tints in old material, and it lifts easily from the moss with a small plug of mycelium and litter.

Cut the fruit body from cap to base. The flesh is white, thin in the cap, firm and conspicuously brittle, breaking with a clean snap. Under the cuticle it may be tinged bluish. Exposed flesh remains white or dulls very slightly; there is no reddening, no blackening, no yellowing and no milk. That combination of brittle white unchanging flesh with no exudate places the collection in the brittlegills and excludes the milkcaps immediately.

Taking a Spore Print

Spore colour separates several of the grey-blue brittlegills, so a proper print is essential. Cut the cap free at the top of the stem, lay it gills-down on clean glass or white paper, cover it with a bowl, and leave it four to eight hours in a cool, still place. Scrape a thick heap together with a blade and assess the colour in daylight against white. This species deposits a white to very pale cream print, at the palest end of the genus. Confusing it with a distinctly cream or ochre print is the single commonest error, and thin deposits always look paler than they are, so be patient and use a big cap. Microscopically the spores are small, broadly ellipsoid, and ornamented with isolated warts and short ridges that form only a partial network, staining blue-grey in Melzer's iodine.

Substrate, Season and Ecology

The fungus is ectomycorrhizal with conifers, above all Norway spruce, forming a sheath around the tree's fine roots and exchanging soil nutrients and water for photosynthetic sugars. It grows from soil, never from wood, and it favours acid, mossy, often nutrient-poor ground in mature and old stands, including upland and montane forests. Occasional records come from other conifers, but a spruce association is the expectation. The season is late summer to autumn, with fruitings arriving in flushes after prolonged rain. Because the partnership is with living roots, sites are stable and worth revisiting in later years.

Separating the Look-alikes on Morphology

• Powdery Brittlegill, Russula parazurea, is the closest match and shares the chalky bloom and blue-grey tones, but it is a broadleaf species, typically with oak, beech, lime and birch on richer soils, its colours run more toward grey-green and olive-blue, it usually reaches a larger size, and its spore print is pale cream rather than white.
• Charcoal Burner, Russula cyanoxantha, has a similar violet, blue and green mixed cap but is far larger, and its gills are the giveaway: they are greasy and flexible, bending under a finger like soft plastic instead of crumbling. Its print is white and it grows with broadleaf trees.
• Russula grisea and its relatives are grey-lilac with a broadleaf association and give a distinctly cream spore print, and their gills yellow with age.
• Russula turci is purple to violet, associated with conifers, and has an ochre spore print; the base of its stem carries a strong chemical odour reminiscent of iodine, which is easy to check and immediately separates it.
• Russula amoena and Russula violeipes have velvety caps and stems flushed with yellow or purple, quite unlike the pure white stem here.
• Any grey-blue mushroom that exudes juice from cut gills, bends rather than crumbles, or drops a rust-brown or pink print is not a brittlegill at all.

Field Checklist

Under spruce on acid mossy ground; a small cap, lilac-blue with a paler ochre-cream centre, dry and chalky with a frosted bloom; skin peeling about halfway; white, brittle, crowded gills that stay white; a short, pure white stem with no flush and no veil; white unchanging flesh with no juice; and a white to nearly white spore print. Verify each in turn and the name is secure.