Yellow Swamp Brittlegill Identification Guide

First impression and the ground under your boots

The habitat comes first with this species. You are in damp birch woodland or on the edge of a bog, walking on sphagnum, sedge and wet peaty soil, with water not far below the surface. Among the moss sits a clean, bright chrome-yellow cap, glossy from the rain, five to ten centimetres across, on a short white stem. Russula claroflava is one of the more brightly coloured brittlegills of northern woodland, and the combination of pure yellow cap, white stem and saturated wet ground is close to a signature in itself. The confirmation, though, comes from watching how the fungus discolours over the next quarter of an hour.

The genus check

Before naming a species, confirm you have a Russula at all. Three checks take fifteen seconds. First, the stem should snap cleanly across like a stick of chalk, with a short brittle break and no fibrous stringiness. Second, the flesh should be crumbly and granular rather than fibrous, because the tissue is built of round sphaerocyst cells. Third, break a gill with a fingertip: brittlegill gills shatter rather than bending, and no milk or latex is released. If a white or coloured drop appears on the broken gill, you are in Lactarius or Lactifluus instead.

Reading the cap

Size and shape. Four to ten, sometimes twelve centimetres across. Convex when young, then flattening and finally becoming shallowly depressed at the centre in mature specimens.

Colour. Bright, clear chrome yellow to lemon or egg yellow, generally even across the whole surface, occasionally a little paler toward the margin and slightly deeper at the disc. It is a clean yellow, not muddied with olive, ochre or brown.

Surface. Smooth, slightly greasy or viscid when wet, matt and finely felted when dry. The cuticle peels from the margin roughly halfway to the centre, a proportion worth checking because peelability is a standard brittlegill character.

Margin. Even and smooth in young caps, becoming faintly grooved or tuberculate-striate for a few millimetres in old ones.

Gills, stem and the greying reaction

Gills. Adnexed to almost free, fairly crowded, moderately broad, brittle, pale cream at first and maturing to a light ochre or pale yellowish tone that reflects the spore colour. They are usually unequal, with some forking near the stem.

Stem. Four to ten centimetres tall, cylindrical or slightly clavate, white, dry, and finely wrinkled or veined longitudinally. It becomes soft and cavernous with age.

The reaction to watch. This is the key character. Handle the fungus and cut it, then leave it on the moss for ten to twenty minutes. The white flesh, the stem surface and the bruised parts of the gills slowly turn grey, then a darker ash grey, and eventually approach blackish grey in old or heavily damaged material. The change is progressive, never passes through red or pink, and never involves the sudden coral-red flush of the truly blackening brittlegills. Old fruit bodies in the field are often recognisable from a distance by having a bright yellow cap sitting on a distinctly grey, grubby-looking stem.

Flesh. White, firm when young and quickly softening, thick at the disc, and mild in odour with at most a faintly fruity note.

Spore print

The print matters here because it separates the yellow-capped brittlegills more cleanly than cap colour does. Cut the stem flush, place a fully opened cap gills-down on white paper or a glass slide, cover, and leave for three or four hours. Russula claroflava gives a pale ochre to light yellowish deposit, sitting toward the middle of the standard brittlegill spore-colour chart rather than at either extreme. Print on white, not coloured, paper, and scrape the deposit into a thick ridge with a blade before judging the tone, because thin deposits always read paler than they are.

Under the microscope the spores are broadly ellipsoid, around 7 to 10 by 6 to 8 micrometres, ornamented with warts joined by fine connecting lines into a partial network, and the ornament stains blue-black in Melzer's iodine, as in all brittlegills.

Substrate, partners and season

Russula claroflava is ectomycorrhizal and strongly associated with birch. Look for it wherever birch grows on wet ground: sphagnum bog margins, wet birch carr, alder-birch mires, damp heathland with scattered birch and the boggy hollows of pine-birch woodland. Willow and aspen also feature as partners in some areas. It is a northern and montane species in general distribution, common in Scandinavia, northern Britain, northern Europe and boreal North America, and it fruits from midsummer through autumn, with the best flushes after prolonged wet weather. It grows singly or in small scattered groups directly from wet moss and peat rather than from wood.

Separating the look-alikes on morphology alone

• Russula ochroleuca is the common yellow brittlegill of drier mixed and coniferous woodland. Its cap is a duller, greener ochre-yellow, the stem greys only faintly and slowly, the flesh does not develop the strong ash tone, and the spore print is much paler, whitish to pale cream. Habitat helps too: it is a fungus of ordinary woodland floor, not sphagnum.
• Russula risigallina (lutea) has a smaller, more delicate cap in apricot to golden yellow tones, distinctly yellow to deep egg-yellow gills, no greying at all, and a rich deep ochre spore print, the darkest end of the brittlegill scale.
• Russula fellea is uniformly straw to pale honey-coloured throughout, cap, gills and stem alike, with no white stem to contrast against the cap and no greying, and it is a beech-wood species on drier soil.
• Russula solaris and Russula raoultii are pale, fragile, whitish-yellow species of beech woodland, again without the greying reaction and with much paler prints.
• Russula nigricans and the blackening group do turn grey and black, but the flesh first flushes distinctly red or salmon on cutting before darkening, their gills are notably thick and widely spaced, and their caps are dirty white to brown, not yellow.
• Amanita species with yellow caps are separated instantly by free gills, a ring, a bulbous or sheathed base and fibrous rather than brittle flesh.

The compact decision path

Brittle chalk-snap stem and crumbling flesh, so a brittlegill. Bright clean chrome-yellow cap. White stem that greys steadily over minutes with no red intermediate. Pale ochre spore print. Birch overhead and wet peaty ground below. That sequence resolves Russula claroflava with confidence, and the greying test plus the birch association are the two characters most worth writing into your notes.