Lactarius torminosus, commonly known as the woolly milkcap or the bearded milkcap, is a large basidiomycete fungus in the genus Lactarius. It is found in the United Kingdom, Northern Europe, and is common in North America, where it grows in mixed forests in a mycorrhizal association with various trees, most commonly birch. Although it is valued for its peppery flavour and eaten after pickling in Russia and Finland, it is highly irritating to the digestive system when eaten raw. The toxins responsible for the acrid taste are destroyed by cooking.
The fruit bodies produced by the fungus have caps which can reach up to 10 cm (3.9 in) in diameter. They are pink-flesh coloured, with whitish concentric zones. The edge of the cap is rolled inwards, and shaggy when young. On the underside of the caps are flesh-colored gills crowded closely together. It is on these fertile gill surfaces that the spore-bearing cells, the basidia are found. The cylindrical stem is a pale flesh colour with a delicately downy surface, reaching lengths of up to 8 cm (3.1 in). When cut or injured, the fruit bodies ooze a white latex that does not change colour upon exposure to air. The variety normandensis, in contrast, has latex that changes colour from white to yellow.
The cap is initially convex, but as it matures, the center of the cap forms a depression, and the outer edges become raised until eventually it assumes the shape of a shallow funnel; its width is typically between 2 and 12 cm (0.8 and 4.7 in). The margin (edge of the cap) is curled inwards, and when young is covered with a dense thicket of "hair" that withers in age to whitish fibers. Similarly, the cap surface is initially tomentose (covered with a thick matting of hairs) but eventually the hairs wear off, leaving the surface smooth. The surface is sticky to viscid and smooth on the central portion in early stages, often with whitish concentric zones. The overall cap color is pinkish-orange to pale dull pink, becoming orange to whitish on the margin, with the pink gradually fading. The white to flesh-colored flesh is firm, but becomes flaccid in age. The latex that is produced when the mushroom tissue in cut or injured is white to cream, and does not change color with prolonged exposure to air, nor does it stain the gills. It has an acrid taste, with a slight to pungent odor.
The gills are short-decurrent (running a little ways down the length of the stem), close to crowded together, narrow, and are sometimes forked near the stem. Their colour is whitish, becoming pale vinaceous (the colour of red wine) to pale orange or cream tinged vinaceous, turning pale tan with age. The stem is 1.5–8 cm (0.6–3.1 in) long, 0.6–2 cm (0.2–0.8 in) thick, fragile, more or less equal in width throughout, cylindrical or narrowed at the base. The surface of the stem is dry, and either smooth to pruinose (with a very fine whitish powder on the surface). Its colour is pale light pinkish to yellowish tinged or slightly pinkish orange to orange white, sometimes spotted. The flesh of the stem is firm, beige white, and stuffed (as if filled with cotton), but it eventually becomes hollow.
The spore print of L. torminosus is cream to pale yellow. The spores are 8–10.2 by 5.8–6.6 μm, roughly spherical to broadly elliptic in side view, and hyaline (translucent). The ornamentation on the surface of the spores is amyloid, partially reticulate, with interrupted ridges and a few isolated warts. The prominences are roughly 0.5–0.7 μm high, with a prominent apiculus. The basidia (spore-bearing cells) are 30–47.7 by 7.3–8.2 μm, four-spored, club-shaped to cylindrical, and hyaline. The pleurocystidia are only present in the form of macrocystidia (very long cystidia) embedded and originating in the hymenium and just below it, measuring 40.3–80.0 by 5.1–9.5 μm. The macrocystidia are abundant, spindle-shaped to ventricose (swollen on one side), gradually tapering in width, with granular hyaline contents. The cheilocystidia are 30–52 by 4.5–8.0 μm, also in the form of macrocystidia.
Lactarius normandensis was described by Alexander H. Smith in 1960 to account for the North American species closely resembling L. torminosus, but having instead latex which changed colour from white to yellow upon exposure, and which stained tissues and paper yellow. Hesler and Smith reduced this taxon to the status of variety in 1979. Lactarius torminosus var. normandensis has been collected from Idaho, Michigan and Wisconsin in the United States, from Quebec in Canada, and from Switzerland. The variety resembles Lactarius pubescens var. betulae, but differs in its longer pleurocystidia, larger spores with slightly different spore ornamentation, and its strongly burning acrid taste. The holotype specimen, collected by Smith near Nordman, Idaho in 1956, is kept in the fungus collection at the University of Michigan Herbarium.
The intensely peppery taste of the raw mushroom can blister the tongue if sampled in sufficient quantity. Some authors have reported the species as poisonous, or causing "mild to fatal gastroenteritis". In a 1930 publication, Hans Steidle reported that although the mushroom was not toxic to "unicellular and cold-blooded organisms", the liquid extract and the pressed juice of the fruit bodies, when injected under the skin of a frog, resulted in disturbed breathing, paralysis, and eventually death. Despite these reports, in Finland, Russia and other North European countries it is eaten, after boiling or five-day soaking, pickled and valued for its hot, burning taste. In Norway it is roasted, and added to coffee. The nutritive composition of the fruit bodies of Finnish specimens has been analysed and found to contain the following components (as a percentage of dry weight): protein, 17.2%; phosphorus, 0.46%; calcium, 0.12%; magnesium, 0.088%; potassium, 2.97%; sodium 0.011%.
Lactarius torminosulus is a dwarf version of L. torminosus, an arctic species associated with Betula nana or B. glandulosa. Immature fruit bodies of L. scrobiculatus resemble L. torminosus, but they have a white latex that soon changes yellow upon exposure to air, and their stems have depressed shiny spots. The caps of the poorly known species L. cilicioides are zoned, and its spores are smaller. L. pubescens is physically quite similar, but can be distinguished by its paler colour and smaller spores (6–8.5 by 5–6.5  μm). L. controversus has a cap margin that is not as hairy, whitish to cream-coloured gills, and larger spores measuring 7.5–10 by 6–7.5 μm.
Lactarius torminosus is a mycorrhizal species, and it plays an important role in nutrient and water uptake by trees. It grows in association with birch (Betula) and hemlock (Tsuga) in mixed forests. It is also known to grow in urban settings where birch trees are nearby. Fruit bodies grow on the ground, scattered to grouped together. It is found throughout the United Kingdom, Northern Europe, and is common in North America, where it sometimes grows with aspen (Populus species). The North American distribution extends north into the Yukon and Alaska, and south to Mexico.
The fruit bodies are a component of the diet of the red squirrel Sciurus vulgaris. Lactarius torminosus mushrooms may be parasitized by the mold Hypomyces lithuanicus.
Lactarius torminosus contains the lactarane-type sesquiterpene lactone 15-hydroxyblennin A. This sesquiterpenoid has a lactarane skeleton, similar to compounds found in other Lactarius species, such as Lactarius deliciosus, Lactarius blennius (blennin A), and Lactarius rufus (lactarorufin N, deoxydihydroketolactarorufin N). Fruit bodies contain a number of sterols, of which ergosterol is the predominant component at 60.5% (of total sterols), followed by the ergosterol derivatives ergost-7-en-3-ol (13.7%), ergosta-7-22-dien-3-ol (8.3%), and ergosta-5/7-dien-3-ol (17.0%). The mushrooms also contain the compound velleral, a breakdown product of stearyl-velutinal, which is contained within the lactifers—specialized hyphal cells that produced the mushroom's latex. It has been suggested that broken lactifers leak precursors whose breakdown products act as defensive agents toxic to humans and effectively deter certain vertebrates that might consume the mushroom. Velleral, which has an extremely pungent taste and is thought to be responsible for the toxicity of the raw mushroom, is present at a concentration of 0.16 mg/g mushroom.