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).
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 natice 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).
Lactarius Indigo is a well-known edible and common fungus with a vast population globally. It has been widely and frequently observed in 137 countries; 735,4391 full records of Lactarius Indigo specimens were recorded on the mycology collection portal.
Lactarius indigo is a mycorrhizal fungus that is closely associated with pine species. In mesoamerica, lactarius indigo co-exist with two widely distributed pine species: P.pseudostrobus and P. oocarpa. The latter represents about 50% of the pine forests in Guatemala and 90% in Nicaragua (Flores 2004). In central america, pine-oak forests occupy an area of 111,400 square kilometers (43,000 sq mi). In north america, the estimated coverage of oak-pine habitats is around 32,323,943 acres according to the Natural Conservancy’s conservation gateway website. With conservative estimation that only 10% of pine-oak forest provides suitable habitat for the growth of lactarius indigo and there are approximately 50 mature individuals per square kilometers, the population of lactarius indigo in America is approximately 70,000. With further extrapolation for its habitats in parts of Europe and Asia, we estimate the global population of Lactarius Indigo to be 100,000 per generation.
Currently, there is no evidence showing any reduction in its population and research suggests that its population is in fact increasing. According to the Global Biodiversity Information Facility, occurrences of the species exponentially have increased since 1975 with the exception of a global pandemic taking place in 2020 drastically reducing human observations. It’s 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 often sold in local farmer markets with other popular edible mushrooms in many countries including Guatemalan, China and Mexico (Montoya and Bandala 1996, Wang 2000, Flores 2005).
Population Trend: Improving
L. indigo is mutualistic, mycorrhizal. The species is found to be associated as ectomycorrhizae with various deciduous and coniferous trees, including floodplain trees, hop-hornbeams, oaks, and pines. The habitats appear to be a broad range of such tree species, however, there are some connections between trees and habitats that aren’t yet fully mapped out: for example, in Arizona, they can be found on ponderosa pine, but not in California ponderosa pine.
The species is not currently undergoing reported decline, but a potential threat would be the bright color attracting curious foragers or destruction of its tree-laden habitat through wildfires.
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.
Educate people on their status as a species, telling them not to pick them. Prevent wildfires that wipe out their habitats.
The mushroom is edible, often sold in China and Central America. Further, 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