Northern Milkcap

Lactarius trivialis is the archetypal milkcap of the northern conifer and birch forest, so common in Scandinavia, Finland, the Baltic states and northern Russia that it forms a large part of the autumn mushroom crop by weight. Its specific epithet means 'ordinary' or 'commonplace', a fair reflection of how often it turns up.

It belongs to a group of milkcaps with viscid, cool-toned caps in shades of lilac, lead-grey and violet-brown. In the field the combination of a distinctly slimy surface, a large size and pale, crowded gills that discolour to dingy grey-green where damaged is characteristic. Colour is variable, however: young caps can be strongly violaceous, while older or rain-washed specimens fade to a flat greyish ochre.

Like all members of Russulaceae the flesh is brittle rather than fibrous, and the fruiting body has no ring, no volva and no fibrous stipe rind. It is a strictly mycorrhizal species and a useful indicator of moist, acid boreal woodland.

Cap

Large for the genus, 5–20 cm across and occasionally more. Convex with an inrolled margin at first, then flat and finally broadly funnel-shaped with a persistently depressed centre. The surface is markedly viscid to slimy when moist and shines when dry, coloured lead-grey, grey-violet, lilac-brown or dull violaceous, often with faint darker concentric watermarks toward the margin. It fades with weathering to buff or ochre-grey.

Gills

Gills are crowded, narrow, adnate to shortly decurrent, whitish to pale cream at first and darkening to pale ochre or dirty cream with age. Bruised gills and dried latex develop grey to greyish-green stains. Lamellulae are frequent.

Stipe

The stipe is 4–12 cm tall and 1.5–3 cm thick, cylindrical or slightly swollen, hollow at maturity, and greasy to sticky in wet weather. It is coloured like a pale washed-out version of the cap — whitish, pale grey or faintly lilac — and may show faint shallow pits. No ring is present.

Flesh, latex and spore print

The flesh is white, thick in the centre and brittle. Cut surfaces release copious white latex which dries to a grey or olive-grey film, staining the gills and the cut flesh dingy grey-green over several minutes. Odour is faint and slightly fruity. The spore print is pale cream to pale yellow; spores are broadly ellipsoid with an amyloid partial reticulum.

This is a mycorrhizal species of moist, acidic ground, associating principally with birch (Betula) and Norway spruce (Picea abies), and less often with pine and willow. It fruits in mossy conifer forest, damp birch woodland, on the margins of bogs and along ditches and forest tracks, frequently in Sphagnum or feather-moss carpets. Fruiting bodies often come up in large troops.

Its distribution is northern and montane: it is abundant throughout Fennoscandia, the Baltic region, northern Russia and Siberia, occurs more sparsely southward in upland Europe, and has counterparts across northern North America. The main season runs from late summer into autumn, continuing until the first hard frosts.

Lactarius trivialis is ectomycorrhizal, forming root sheaths on birch and spruce and extending a dense mycelial network through the moss and litter layer. In the nutrient-poor, waterlogged and often nitrogen-limited soils of the boreal zone, this partnership is a major route by which trees obtain nitrogen and phosphorus, and the fungus in turn is entirely dependent on carbon fixed by its hosts.

Because it fruits so heavily and so predictably, it is an important seasonal resource for slugs, fungus gnats and small mammals, and its decaying caps return nutrients rapidly to the moss layer. Its strong association with wet, acid, moss-dominated forest also makes it a helpful indicator species when characterising boreal woodland types.

Several grey to violaceous milkcaps overlap with it. Lactarius uvidus is smaller, with white latex that stains the flesh and gills distinctly lilac to violet rather than grey-green. Lactarius flexuosus has a drier, wavy-margined, more lead-grey cap with widely spaced gills and a firm, short stipe. Lactarius vietus is much smaller and thinner-fleshed, greyish, and its white latex dries clearly grey on the gills.

Lactarius blennius is comparable in sliminess but is olive-grey to greenish with darker droplet-like spots arranged in zones, and it grows with beech rather than birch or spruce. Any grey Russula can be eliminated at once, since Russula species release no latex when cut.