Lactarius scrobiculatus

Lactarius scrobiculatus is the type species of a distinctive group of large, bearded, yellow-capped milkcaps found in northern and montane conifer forest. Two features give it its name and its identity: the shaggy, woolly, inrolled cap margin, and the scattered oval pits or 'scrobicules' on the stipe, each a slightly sunken patch of deeper yellow to ochre-brown that looks as though the surface had been dented with a thumbnail.

It is a substantial mushroom, with a firm zoned cap that can reach 20 cm or more, and it fruits in mossy spruce forest often in some quantity. The third key character is the latex, which emerges white and changes to a vivid sulphur-yellow within a few seconds — quickly enough that the change is usually complete before a hand lens is in position.

The species sits alongside relatives such as L. intermedius, L. citriolens and the more southern L. leonis, all of which share yellow tones and a bearded margin. Distinguishing them relies on the intensity of the beard, the presence and colour of stipe scrobicules, odour, and host tree.

Cap

The cap is 6–20 cm or more across, very firm, convex with a deeply inrolled margin at first, then flattening and becoming funnel-shaped. The surface is viscid to sticky when moist, pale straw-yellow to golden or ochre-yellow, and usually shows faint to distinct concentric zoning of darker watery bands. The margin is conspicuously shaggy-woolly with matted pale hairs, especially in young specimens, becoming less bearded with age.

Gills

Gills are crowded, narrow, adnate to decurrent, whitish to pale cream at first and becoming pale yellowish or straw-buff with age. Bruised gills and dried latex stain them sulphur-yellow to dingy ochre.

Stipe

The stipe is 3–8 cm tall and 2–4 cm thick, short, stout, firm and often tapering toward the base. It is pale yellowish and characteristically ornamented with scattered, sunken, oval scrobicules coloured deeper yellow, ochre or brownish-orange. It is viscid in wet weather and becomes hollow with age; there is no ring.

Flesh, latex and spore print

The flesh is white, thick and very firm. Cut tissue exudes copious white latex which turns bright sulphur-yellow within a few seconds of contact with air, and the cut surface itself yellows. The odour is faintly fruity to slightly resinous. The spore print is pale cream to pale yellow; spores are broadly ellipsoid with amyloid warts joined into a partial reticulum.

Lactarius scrobiculatus is ectomycorrhizal with conifers, above all Norway spruce (Picea abies) and other spruces, and occasionally with fir and pine in mixed montane stands. It fruits on mossy, humus-rich forest floor, along shaded forest tracks and on the banks of woodland streams, usually in scattered groups and often reappearing in the same patch annually.

Its distribution is northern and montane across Europe and Asia: common in Fennoscandia and the Baltic, in the Alps, Carpathians and other European mountain ranges, and eastward through Siberian spruce forest. Closely related taxa occur in North American conifer forest. Fruiting is from late summer through autumn, usually a little earlier at higher elevations.

As a spruce symbiont, this species forms ectomycorrhizal root tips on host trees growing in cool, humid, acid forest soils. Spruce forests accumulate thick, slowly decomposing needle and moss layers in which nitrogen is largely bound in organic form, and mycorrhizal fungi like L. scrobiculatus are the principal route by which that nitrogen reaches the tree. In return the fungus receives a share of the tree's photosynthetic carbon.

The robust chemical defence of this species is unusually visible: the pressurised latex system floods any wound with milk that changes colour almost instantly, and grazing marks left by slugs and larvae on the cap are consequently outlined in bright sulphur-yellow. The heavy, firm fruiting bodies persist for a relatively long time in the field, and support a rich fauna of fungus-feeding flies and beetles.

Lactarius torminosus shares a strongly bearded margin but is pink rather than yellow, has a smooth unpitted stipe and white latex that does not yellow, and it grows with birch. Lactarius pubescens is a paler, whitish-pink, weakly zoned bearded species, again with birch. Lactarius intermedius is a paler cream-yellow, less bearded species with fewer or no stipe scrobicules.

Lactarius citriolens is close, with a bearded lemon-yellow cap and yellowing latex, but has a stronger lemon-scented odour and grows with broadleaved trees. Lactarius repraesentaneus is another large bearded northern species, but its cap is more ochre-violaceous and its latex stains the flesh distinctly lilac-violet instead of yellow.