Lactarius indigo is widely distributed and commonly found in North and Central America into montane Colombia and temperate Asia. It forms ectomycorrhizal associations with a number of different tree species, primarily species of Quercus and Pinus. It is assessed as Least Concern.

Lactarius indigo, originally described in 1822 by American mycologist Lewis David de Schweinitz (de Schweinitz LD. 1822), is a species of agaric fungus in the family Russulaceae. The latin-derived epithet indigo means “indigo blue” (Roody WC. 2003), so lactarius indigo is commonly known in English as the indigo milk cap (Arora D. 1986) , the indigo lactarius (Russell B. 2006), or the blue milk mushroom (Fergus CL. 2003). It is known as añil, azul, hongo azul, zuin, and zuine in Spanish; it is also called quexque meaning “blue” in central Mexico—Veracruz and Puebla (Montoya L, Bandala VM 1996).

Lactarius indigo is fairly widespread in its distribution, from Asia (China, Japan and India) to most of North America and Central America along the gulf coast, Mexico, Costa Rica and Guatemala (Wang 2000; Sharma JR, Das K 2002; Upadhyay RC, Kaur A 2004; Mueller et al. 2006) with its southernmost distribution found in the Humboldt oak cloud forests of Colombia (Winkler 2013). In Europe, it has only been sighted in the UK and southern France (Marcel 1988) but its identity needs to be confirmed and it is excluded from this assessment..

The species commonly grows scattered or in groups living symbiotically in mycorrhizal associations with oak and pine woods in North America (Hesler and Smith 1979) but in Mexico, it is associated with Mexican alder, American Hornbeam, American Hophornbeam, and Liquidambar macrophylla (Montoya and Bandala 1996). In Costa Rica and Colombia, it has been found to be associated with several native oaks of the genus Quercus (Halling 2009). It has also demonstrated considerable variability in its appearance in different environments. As a mycorrhizal fungus, this change of host plant and appearance in different ecosystems could be the result of migration and its adaptations to new environments (Wu and Mueller 1997) or that this is a species complex in need of revision.

Lactarius Indigo is a well-known edible and common fungus with a vast population in North and Central America into montane Colombia.  It is also reported from temperate Asia.  Given its distribution and abundance, the population is stable. It is described to be “occasional to locally common” in terms of its frequency of appearance (Roody 2003) in the Appalachian Mountains of the United States. Lactarius Indigo has a seasonal harvest time; fruit bodies are often widely collected during the rainy season between June and September and

Lactarius indigo is mutualistic, forming ectomycorrhizal associations with a number of different tree species, primarily species of Quercus and Pinus. I

The species is not currently undergoing reported decline.

Old forests containing the trees in which L. indigo grows should be protected, and wildfires should be prevented against. Ensure that hikers or foragers are aware of the impact they can bring upon mycological and ecological systems.

A rigorous taxonomic study of specimens from throughout its range and tree associations should be undertaken to determine if this is a species complex.

Often sold in local farmer markets in many countries including Guatemalan, China and Mexico (Montoya and Bandala 1996, Wang 2000, Flores 2005). Its bright blue pigment has been used for a variety of uses, including as a basis for fluorescent pigments.

Wang X-H. (2000). “A taxonomic study on some commercial species in the genus Lactarius (Agaricales) from Yunnan Province, China”. Acta Botanica Yunnanica. 22 (4): 419–27. ISSN 0253-2700.

Sharma JR, Das K (2002). “New records of Russulaceae from India”. Phytotaxonomy. 2: 11–15. ISSN 0972-4206.

Upadhyay RC, Kaur A (2004). “Taxonomic studies on light-spored agarics new to India”. Mushroom Research. 13 (1): 1–6.

Mueller GM, Halling RE, Carranza J, Mata M, Schmit JP (2006). “Saphrotrophic and ectomycorrhizal macrofungi of Costa Rican oak forests”. In Kappelle M (ed.). Ecology and
Conservation of Neotropical Montane Oak Forests. Berlin; New York: Springer. p. 62. ISBN 978-3-540-28908-1.

Winkler, D (2013). “Die erstaunliche Funga eines tropischen Bergnebel-Eichenwaldes in Kolumbien” (PDF). Der Tintling (in German). 85 (6): 27–35.

Marcel B. (1988). Pareys Buch der Pilze (in German). Hamburg & Berlin, Germany: Paul Parey. p. 80. ISBN 3-490-19818-2.

Montoya L, Bandala VM (1996). “Additional new records on Lactarius from Mexico”. Mycotaxon. 57: 425–50.

Roody WC. (2003). Mushrooms of West Virginia and the Central Appalachians. Lexington, Kentucky: University Press of Kentucky. p. 93. ISBN 0-8131-9039-8.

Russell B. (2006). Field Guide to Wild Mushrooms of Pennsylvania and the Mid-Atlantic. University Park, Pennsylvania: Pennsylvania State University Press. p. 78. ISBN 978-0-271-02891-0.

Fergus CL. (2003). Common Edible and Poisonous Mushrooms of the Northeast. Mechanicsburg, Pennsylvania: Stackpole Books. p. 32. ISBN 978-0-8117-2641-2.

Näsholm T, Ekblad A, Nordin A, Giesler R, Högberg M, Högberg P (1998). “Boreal forest plants take up organic nitrogen”. Nature. 392 (6679): 914–6. Bibcode:1998Natur.392..914N. doi:10.1038/31921. S2CID 205001566.
Arora D. (1986). Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi. Berkeley, California: Ten Speed Press. ISBN 0-89815-169-4.
Healy RA, Huffman DR, Tiffany LH, Knaphaus G (2008). Mushrooms and Other Fungi of the Midcontinental United States (Bur Oak Guide). Iowa City, Iowa: University of Iowa Press. p. 103. ISBN 978-1-58729-627-7.

Hesler LR, Smith AH (1979) North American species of Lactarius. University of Michigan Press, Ann Harbor.

Halling RE. “Lactarius indigo (Schw.) Fries”. Macrofungi of Costa Rica. The New York Botanical Garden. Retrieved 2009-09-24.

Wu QX, Mueller G (1997) Biogeographic relationships between the macrofungi of temperate eastern Asia and eastern North America. Can J Bot 75:2108–2115

de Schweinitz LD. (1822). “Synopsis fungorum Carolinae superioris”. Schriften der Naturforschenden Gesellschaft in Leipzig (in Latin). 1: 87.

Flores R, Díaz G, Honrubia M (2005). “Mycorrhizal synthesis of Lactarius indigo (Schw.) Fr. with five Neotropical pine species”. Mycorrhiza. 15 (8): 563–70. doi:10.1007/s00572-005-0004-y. PMID 16133250. S2CID 195072384