 Originally described in 1822 as Agaricus indigo by American mycologist Lewis David de <unk> , the species was later transferred to the genus Lactarius in 1838 by the <unk> Elias Magnus Fries . German botanist Otto <unk> called it <unk> indigo in his 1891 treatise <unk> <unk> <unk> , 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 <unk> <unk> , 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 : 
