Lactarius

Description

Macromorphology

The eponymous "milk" and the brittle consistency of the flesh are the most prominent field characters of milk-cap fruitbodies. The milk or latex emerging from bruised flesh is often white or cream, but more vividly coloured in some species; it can change upon exposition or remain unchanged. Fruitbodies are small to very large, gilled, rather fleshy, without veil, often depressed or even funnel-shaped with decurrent gills. Cap surface can be glabrous, velvety or pilose, dry, sticky or viscose and is often zonate. Several species have pits (scrobicules) on the cap or pileus surface. Dull colors prevail, but some more colorful species exist, e.g. the blue Lactarius indigo or the orange species of section Deliciosi. Spore print color is white to ocher or, in some cases, pinkish. Some species have angiocarpous, i.e., closed fruitbodies.

Micromorphology

Microscopically, Lactarius species have elliptical, rarely globoid spores with amyloid ornamentation in the form of more or less prominent warts or spines, connected by ridges, like other members of the family Russulaceae. The trama (flesh) contains spherical cells that cause the brittle structure. Unlike Russula, Lactarius also have lactiferous, i.e. latex-carrying hyphae in their trama.

Species identification

Distinguishing Lactarius from Lactifluus based on morphology alone is difficult; there are no synapomorphic characters known so far that define both genera unequivocally but tendencies exist: zonate and viscose to glutinose caps are only found in Lactarius, as well as closed (angiocarpous) and sequestrate fruitbodies. All known annulate and pleurotoid (i.e., laterally stiped) milk-caps, on the contrary, belong to Lactifluus.

Characters important for identification of milk-caps (Lactarius and Lactifluus) are: initial colour of the latex and color change, texture of cap surface, taste (mild, peppery, or bitter) of latex and flesh, odor, and microscopical features of the spores and the cap cuticle (pileipellis). The habitat and especially the type of host tree can also be critical. While there are some easily recognizable species, other species can be quite hard to determine without microscopical examination.

Distribution and habitat

Lactarius is one of the most prominent genera of mushroom-forming fungi in the Northern hemisphere. It also occurs natively in Northern Africa, tropical Africa, tropical Asia, Central America, and Australia. Its possible native distribution in South America and different parts of Australasia is unclear, as many species in those regions, poorly known, might in fact belong to Lactifluus, which has a more tropical distribution than Lactarius. Several species have also been introduced with their host trees outside their native range, e.g. in South America, Southern Africa, Australia, and New Zealand.

Ecology

Lactarius belongs to a lineage of ectomycorrhiza obligate symbionts. As such, they are dependent on the occurrence of possible host plants. Confirmed habitats apart from temperate forests include arctic tundra and boreal forest, mediterranean maquis, tropical African shrubland, tropical Asian rainforest, mesoamerican tropical oak forests, and Australian Eucalyptus forests.

While most species display a preference towards either broadleaf or coniferous hosts, some are more strictly associated with certain genera or species of plant hosts. A well-studied example is that of alders, which have several specialized Lactarius symbionts (e.g. L. alpinus, L. brunneohepaticus, L. lilacinus), some of which even evolved specificity to one of the Alnus subgenera. Other examples of specialized associations of Lactarius are with Cistus shrubs (L. cistophilus and L. tesquorum), beech (e.g. L. blennius), birches (e.g. L. pubescens), hazel (e.g. L. pyrogalus), oak (e.g. L. quietus), pines (e.g. L. deliciosus), or fir (e.g. L. deterrimus). For most tropical species, host plant range is poorly known, but species in tropical Africa seem to be rather generalist.

Lactarius species are considered late-stage colonizers, that means, they are generally not present in early-colonizing vegetation, but establish in later phases of succession. However, species symbiotic with early colonizing trees, such as L. pubescens with birch, will rather occur in early stages. Several species have preferences regarding soil pH and humidity, which will determine the habitats in which they occur.

Fungi of this genus have been recorded as a main food of the pleasing fungus beetle Tritoma angulata.

Edibility

Several Lactarius species are edible. L. deliciosus notably ranks among the most highly valued mushrooms in the Northern hemisphere, while opinions vary on the taste of other species, such as L. indigo or L. deterrimus. Several species are reported to be regularly collected for food in Russia, Tanzania and Hunan, China. Some Lactarius are considered toxic, for example L. turpis, which contains the mutagenic compound necatorin, or L. helvus. There are, however, no deadly poisonous mushrooms in the genus. Bitter or peppery species, for example L. torminosus, are generally not considered edible, at least raw, but are nevertheless consumed in some regions, e.g. in Finland. Some small, fragrant species, such as the "candy caps", are sometimes used as flavoring.

L. deliciosus is one of the few ectomycorrhizal mushrooms that has been successfully cultivated.

Chemistry

Different bioactive compounds have been isolated from Lactarius species, such as sesquiterpenoids, aromatic volatiles, and mutagenic substances. Pigments have been isolated from colored Lactarius species, such as L. deliciosus or L. indigo.

Pharmacology

An extract of Lactarius badiosanguineus exhibits inhibitory activity on thrombin.