indigo milk cap

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 distributed throughout southern and eastern North America but is most common along the Gulf Coast, Mexico, and Guatemala. Its frequency of appearance in the Appalachian Mountains of the United States has been described as "occasional to locally common". Mycologist David Arora notes that in the U.S., the species is found with ponderosa pine in Arizona, but is absent in California's ponderosa pine forests. It is also found in Mexico, Guatemala, Costa Rica (in forests dominated by oak), and as far south as the Humboldt oak cloud forests of Colombia. In Europe, it has so far only been found in southern France. It has also been collected from China and India.

A study on the seasonal appearance of fruiting bodies in the subtropical forests of Xalapa, Mexico, found that maximal production of L. indigo coincided with the rainy season between June and September. It is a mycorrhizal fungus, and as such, establishes a mutualistic relationship with the roots of certain trees ("hosts"), in which the fungi exchange minerals and amino acids extracted from the soil for fixed carbon from the host. …

Although L. indigo is a well-known edible species, opinions vary on its desirability. For example, American mycologist David Arora considers it a "superior edible", while a field guide on Kansas fungi rates it as "mediocre in quality". It may have a slightly bitter, or peppery taste, and has a coarse, grainy texture. The firm flesh is best prepared by cutting the mushroom in thin slices. The blue color disappears with cooking, and the mushroom becomes grayish. Because of the granular texture of the flesh, it does not lend itself well to drying. Specimens producing copious quantities of milk may be used to add color to marinades. In Mexico, individuals harvest the wild mushrooms for sale at farmers' markets, typically from June to November; they are considered a "second class" species for consumption. L. indigo is also sold in Guatemalan markets from May to October. It is one of 13 Lactarius species sold at rural markets in Yunnan in southwestern China.