Lactarius indigo, commonly known as the indigo milk cap, the indigo (or blue) lactarius, or the blue milk mushroom, is a species of agaric fungus in the family Russulaceae. A widely distributed species, it grows naturally in eastern North America, East Asia, and Central America; it has also been reported in southern France. L. indigo grows on the ground in both deciduous and coniferous forests, where it forms mycorrhizal associations with a broad range of trees. 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 Lactarius genus — is also indigo blue, but slowly turns green upon exposure to air. The cap has a diameter of 5 to 15 cm (2 to 6 in), and the stem is 2 to 8 cm (0.8 to 3 in) tall and 1 to 2.5 cm (0.4 to 1.0 in) thick. It is an edible mushroom, and is sold in rural markets in China, Guatemala, and Mexico.