Lactarius indigo, commonly known as the indigo milk cap, indigo milky, indigo lactarius, blue lactarius, or blue milk mushroom, is a species of agaric fungus in the family Russulaceae. The fruit body color ranges from dark blue in fresh specimens to pale blue-gray in older ones. The "milk", or latex, that oozes when the mushroom tissue is cut or broken (a feature common to all members of the genus Lactarius) is also indigo blue, but slowly turns green upon exposure to air. The cap has a diameter of 4–15 cm (2–6 in), and the stem is 2–8 cm (3⁄4–3+1⁄8 in) tall and 1–2.5 cm (3⁄8–1 in) thick. It is a widely distributed species, growing in the Americas and Eurasia. L. indigo grows on the ground in both deciduous and coniferous forests, where it forms mycorrhizal associations with a broad range of trees. It is an edible mushroom and is sold in rural markets in Mexico, Guatemala, and China.
Taxonomy Originally described in 1822 as Agaricus indigo by American mycologist Lewis David de Schweinitz, the species was later transferred to the genus Lactarius in 1838 by the Swede Elias Magnus Fries. German botanist Otto Kuntze called it Lactifluus indigo in his 1891 treatise Revisio Generum Plantarum, but the suggested name change was not adopted by others. Hesler and Smith, in their 1960 study of North American species of Lactarius, defined L. indigo as the type species of subsection Caerulei, a group characterized by blue latex and a sticky, blue cap. In 1979, they revised their opinions on the organization of subdivisions in the genus Lactarius, and instead placed L. indigo in subgenus Lactarius based on the color of latex and the subsequent color changes observed after exposure to air. As they explained:The gradual development of blue to violet pigmentation as one progresses from species to species is an interesting phenomenon deserving further study. The climax is reached in L. indigo which is blue throughout. L. chelidonium and its variety chelidonioides, L. paradoxus, and L. hemicyaneus may be considered as mileposts along the road to L. indigo. The specific epithet indigo is derived from the Latin word meaning 'indigo blue'. Its names in the English vernacular include the "indigo milk cap", the "indigo Lactarius", the "blue milk mushroom", and the "blue Lactarius". In central Mexico, it is known as añil, azul, hongo azul, zuin, and zuine; it is also called quexque (meaning 'blue') in Veracruz and Puebla.
Description Like many other mushrooms, L. indigo develops from a nodule that forms within the underground mycelium, a mass of threadlike fungal cells called hyphae that make up the bulk of the organism. Under appropriate environmental conditions of temperature, humidity, and nutrient availability, the visible reproductive structures (fruit bodies) are formed. The cap of the fruit body, measuring between 4–15 cm (1+1⁄2–6 in) in diameter, is initially convex and later develops a central depression; in age it becomes even more deeply depressed, becoming somewhat funnel-shaped as the edge of the cap lifts upward. The margin of the cap is rolled inwards when young, but unrolls and elevates as it matures. The cap surface is indigo blue when fresh, but fades to a paler grayish- or silvery-blue, sometimes with greenish splotches. It is often zonate: marked with concentric lines that form alternating pale and darker zones, and the cap may have dark blue spots, especially towards the edge. Young caps are sticky to the touch.
The flesh is pallid to bluish in color, slowly turning greenish after being exposed to air; its taste is mild to slightly acrid. The flesh of the entire mushroom is brittle, and the stem, if bent sufficiently, will snap open cleanly. The latex exuded from injured tissue is indigo blue, and stains the wounded tissue greenish; like the flesh, the latex has a mild taste. Lactarius indigo is noted for not producing as much latex as other Lactarius species, and older specimens in particular may be too dried out to produce any latex. The gills of the mushroom range from adnate (squarely attached to the stem) to slightly decurrent (running down the length of the stem), and crowded close together. Their color is an indigo blue, becoming paler with age or staining green with damage. The stem is 2–8 cm (3⁄4–3+1⁄8 in) tall by 1–2.5 cm (3⁄8–1 in) thick, and the same diameter throughout or sometimes narrowed at base. Its color is indigo blue to silvery- or grayish blue. The interior of the stem is solid and firm initially, but develops a hollow with age. Like the cap, it is initially sticky or slimy to the touch when young, but soon dries out. Its attachment to the cap is usually in a central position, although it may also be off-center. Fruit bodies of L. indigo have no distinguishable odor. L. indigo var. diminutivus (the "smaller indigo milk cap") is a smaller variant of the mushroom, with a cap diameter between 3–7 cm (1+1⁄8–2+3⁄4 in), and a stem 1.5–4 cm (5⁄8–1+5⁄8 in) long and 0.3–1 cm (1⁄8–3⁄8 in) thick. It is often seen in Virginia. Hesler and Smith, who first described the variant based on specimens found in Brazoria County, Texas, described its typical habitat as "along [the] sides of a muddy ditch under grasses and weeds, [with] loblolly pine nearby".
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