 = Lactarius indigo = 
 
 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 . 
 
 = = Taxonomy and nomenclature = = 
 
 Originally described in 1822 as Agaricus indigo by American mycologist Lewis David de Schweinitz , the species was later transferred to the genus Lactarius in 1838 by the Swede Elias Magnus Fries . German botanist Otto Kuntze called it Lactifluus indigo in his 1891 treatise Revisio Generum Plantarum , 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 subsection Caerulei , 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 : 
 The gradual development of blue to violet pigmentation as one progresses from species to species is an interesting phenomenon deserving further study . The climax is reached in L. indigo which is blue throughout . L. chelidonium and its variety chelidonioides , L. paradoxus , and L. hemicyaneus may be considered as mileposts along the road to L. indigo . 
 The specific epithet indigo is derived from the Latin word meaning " indigo blue " . Its names in the English vernacular include the " indigo milk cap " , the " indigo Lactarius " , the " blue milk mushroom " , and the " blue Lactarius " . In central Mexico , it is known as añil , azul , hongo azul , zuin , and zuine ; it is also called quexque ( meaning " blue " ) in Veracruz and Puebla . 
 
 = = Description = = 
 
 Like many other mushrooms , L. indigo develops from a nodule , or pinhead , 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 5 and 15 cm ( 2 @.@ 0 and 5 @.@ 9 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 noted for not producing as much latex as other Lactarius species , and older specimens in particular may be too dried out to produce any latex . 
 The gills of the mushroom range from adnate ( squarely attached to the stem ) to slightly decurrent ( running down the length of the stem ) , and crowded close together . Their color is an indigo blue , becoming paler with age or staining green with damage . The stem is 2 – 6 cm ( 0 @.@ 8 – 2 @.@ 4 in ) tall by 1 – 2 @.@ 5 cm ( 0 @.@ 4 – 1 @.@ 0 in ) thick , and the same diameter throughout or sometimes narrowed at base . Its color is indigo blue to silvery- or grayish blue . The interior of the stem is solid and firm initially , but develops a hollow with age . Like the cap , it is initially sticky or slimy to the touch when young , but soon dries out . Its attachment to the cap is usually in a central position , although it may also be off @-@ center . Fruit bodies of L. indigo have no distinguishable odor . 
 L. indigo var. diminutivus ( the " smaller indigo milk cap " ) is a smaller variant of the mushroom , with a cap diameter between 3 and 7 cm ( 1 @.@ 2 and 2 @.@ 8 in ) , and a stem 1 @.@ 5 – 4 @.@ 0 cm ( 0 @.@ 6 – 1 @.@ 6 in ) long and 0 @.@ 3 – 1 @.@ 0 cm ( 0 @.@ 1 – 0 @.@ 4 in ) thick . It is often seen in Virginia . Hesler and Smith , who first described the variant based on specimens found in Brazoria County , Texas , described its typical habitat as " along [ the ] sides of a muddy ditch under grasses and weeds , [ with ] loblolly pine nearby " . 
 
 = = = Microscopic features = = = 
 
 When viewed in mass , as in a spore print , the spores appear cream to yellow colored . Viewed with a light microscope , the spores are translucent ( hyaline ) , elliptical to nearly spherical in shape , with amyloid warts , and have dimensions of 7 – 9 by 5 @.@ 5 – 7 @.@ 5 µm . Scanning electron microscopy reveals reticulations on the spore surface . The hymenium is the spore @-@ producing tissue layer of the fruit body , and consists of hyphae that extend into the gills and terminate as end cells . Various cell types can be observed in the hymenium , and the cells have microscopic characteristics that may be used to help identify or distinguish species in cases where the macroscopic characters may be ambiguous . The spore @-@ bearing cells , the basidia , are four @-@ spored and measure 37 – 45 µm long by 8 – 10 µm wide at the thickest point . Cystidia are terminal cells of hyphae in the hymenium which do not produce spores , and function in aiding spore dispersal , and maintaining favorable humidity around developing spores . The pleurocystidia are cystidia that are found on the face of a gill ; they are 40 – 56 by 6 @.@ 4 – 8 µm , roughly spindle @-@ shaped , and have a constricted apex . The cheilocystidia — located on the edge of a gill — are abundant , and are 40 @.@ 0 – 45 @.@ 6 by 5 @.@ 6 – 7 @.@ 2 µm . 
 
 = = = Similar species = = = 
 
 The characteristic blue color of the fruiting body and the latex make this species easily recognizable . Other Lactarius species with some blue color include the " silver @-@ blue milky " ( L. paradoxus ) , found in eastern North America , which has a grayish @-@ blue cap when young , but it has reddish @-@ brown to purple @-@ brown latex and gills . L. chelidonium has a yellowish to dingy yellow @-@ brown to bluish @-@ gray cap and yellowish to brown latex . L. quieticolor has blue @-@ colored flesh in the cap and orange to red @-@ orange flesh in the base of the stem . Although the blue discoloration of L. indigo is thought to be rare in the genus Lactarius , in 2007 five new species were reported from Peninsular Malaysia with bluing latex or flesh , including L. cyanescens , L. lazulinus , L. mirabilis , and two species still unnamed . 
 
 = = Edibility = = 
 
 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 . 
 
 = = = Chemical composition = = = 
 
 A chemical analysis of Mexican specimens has shown L. indigo to contain moisture at 951 mg / g of mushroom , fat at 4 @.@ 3 mg / g , protein at 13 @.@ 4 mg / g , and dietary fiber at 18 @.@ 7 mg / g , much higher in comparison to the common button mushroom , which contains 6 @.@ 6 mg / g . Compared to three other wild edible mushroom species also tested in the study ( Amanita rubescens , Boletus frostii , and Ramaria flava ) , L. indigo contained the highest saturated fatty acids content , including stearic acid with 32 @.@ 1 mg / g — slightly over half of the total free fatty acid content . 
 The blue color of L. indigo is due to ( 7 @-@ isopropenyl @-@ 4 @-@ methylazulen @-@ 1 @-@ yl ) methyl stearate , an organic derivative of azulene . It is unique to this species , but similar to a compound found in L. deliciosus . 
 
 = = Distribution , habitat , and ecology = = 
 
 L. 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 United States , the species is found with ponderosa pine in Arizona , but is absent in California 's ponderosa pine forests . It has also been collected from China , India , Guatemala , and Costa Rica ( in forests dominated by oak ) . In Europe , it has so far only been found in southern France . A study on the seasonal appearance of fruiting bodies in the subtropical forests of Xalapa , Mexico , confirmed that maximal production coincided with the rainy season between June and September . 
 L. indigo 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 . The subterranean hyphae of the fungus grow a sheath of tissue around the rootlets of a broad range of tree species , forming so @-@ called ectomycorrhizae — an intimate association that is especially beneficial to the host , as the fungus produces enzymes that mineralize organic compounds and facilitate the transfer of nutrients to the tree . 
 Reflecting their close relationships with trees , the fruit bodies of L. indigo are typically found growing on the ground , scattered or in groups , in both deciduous and coniferous forests . They are also commonly found in floodplain areas that have been recently submerged . In Mexico , associations have been noted with Mexican alder , American Hornbeam , American Hophornbeam , and Liquidambar macrophylla , while in Guatemala the mushroom associates with smooth @-@ bark Mexican pine and other pine and oak species . In Costa Rica , the species forms associations with several native oaks of the Quercus genus . Under controlled laboratory conditions , L. indigo was shown to be able to form ectomycorrhizal associations with the neotropical pine species Mexican white pine , Hartweg 's pine , Mexican yellow pine , smooth @-@ bark Mexican pine , and the Eurasian pines Aleppo pine , European black pine , maritime pine , and Scots pine . 
 
 = = = Cited literature = = = 
 
 Arora D. ( 1986 ) . Mushrooms Demystified : A Comprehensive Guide to the Fleshy Fungi . Berkeley , California : Ten Speed Press . ISBN 0 @-@ 89815 @-@ 169 @-@ 4 . 
 Hesler LR , Smith AH ( 1979 ) . North American Species of Lactarius . Ann Arbor , Michigan : The University of Michigan Press . ISBN 0 @-@ 472 @-@ 08440 @-@ 2 . 
 
 