Birch Brittlegill

The Birch Brittlegill, Russula betularum, is a slight, pale mushroom of wet birch woodland. Where most red-capped brittlegills are vividly coloured, this one is washed-out — a soft rose or shell-pink fading almost to white, often with the faintest yellow or lilac tint near the centre.

It belongs to the Russula emetica group, a cluster of fragile, brightly to palely coloured, white-spored species whose caps peel easily and whose margins become grooved early. Within that group, R. betularum is defined by its small size, its pale colours and its strict association with birch.

Because birch is a pioneer tree of heaths, moorland fringes and boggy hollows, this brittlegill is a characteristic member of a distinctive fungal community that also includes the birch bolete group, milkcaps and the Fly Agaric. It is a useful indicator that the surrounding trees are birch even when the canopy is hard to see.

Cap

Small for the genus, usually 3-6 cm across, convex then flat and finally slightly depressed. The surface is smooth, greasy when wet and quickly drying matt. Colour is a delicate pale pink or rose, strongest at the centre and fading towards the margin, and in many collections the cap bleaches almost to white or cream with age or after rain. The cuticle peels easily and almost completely, and the thin margin becomes distinctly grooved or furrowed at maturity.

Gills

White, staying white, moderately spaced rather than crowded, adnexed to almost free, and extremely brittle. Short intermediate gills are present. The gill edges are entire and smooth under a lens.

Stipe

Slender and fragile, 4-8 cm long but only 0.6-1.5 cm thick, white, smooth and often slightly club-shaped at the base. It becomes hollow and easily crushed with age, and may develop faint grey tones in wet weather. No ring or volva is present.

Flesh & spore print

The flesh is white, thin and very fragile, unchanging when cut. The spore print is white, which together with the birch host and pale cap separates it from most similar small brittlegills. Spores are broadly ellipsoid with tall isolated warts.

Russula betularum is mycorrhizal exclusively with birch (Betula pendula and B. pubescens). It favours damp, acid ground, and is typically found in Sphagnum moss, among heather and bilberry, in birch carr, on boggy heathland margins and in wet hollows within birch scrub.

Fruiting extends from July to late October, with peaks after prolonged rain. The species is common throughout northern and central Europe wherever birch grows on acid soils, and extends into upland and boreal birch woodland; comparable pale birch-associated brittlegills occur across the northern hemisphere.

This is an ectomycorrhizal partner of birch. The fungus forms a mantle around the birch's fine roots and a network of hyphae between the outer root cells, transferring water, nitrogen and phosphorus into the tree in exchange for photosynthetic sugars. Birch is a pioneer species colonising poor acid ground, and mycorrhizal partners like this one are central to its ability to establish on nutrient-starved soils.

The underground mycelium spreads through the moss and peat layer and can persist for many years, fruiting repeatedly in the same hollows. The fragile fruit bodies decay within days, and their soft tissues are quickly used by fly larvae, springtails and slugs, recycling nutrients into the shallow acid litter.

The Sickener (Russula emetica) is larger, with a far brighter scarlet cap and a stouter stipe, and grows with conifers in wet moss rather than with birch. The Beechwood Sickener (Russula nobilis, also called R. mairei) is firmer, more vividly red and tied to beech, with more crowded gills.

The Fragile Brittlegill (Russula fragilis) is a similar size and equally frail but its cap mixes purple, wine and olive tones, and under a hand lens its gill edges are finely serrated rather than smooth. Russula gracillima, another birch species, is more slender still and has a cap combining pink with olive-grey patches. Pale colour, entire gill edges, white spore print and a birch host together point to the Birch Brittlegill.