Indigo Milk Cap

Lactarius indigo is one of the very few mushrooms in the world that is genuinely, saturatedly blue. Every part of a fresh fruit body, the zoned cap, the crowded decurrent gills, the short stout stem and the latex that bleeds from a cut, is indigo to dark blue, often with a silvery-grey bloom over the cap that gives it a metallic sheen.

It belongs to the same section of Lactarius as the Saffron Milkcap, the group whose members have coloured rather than white latex and whose tissues oxidise to green. In L. indigo the green staining is superimposed on blue, so ageing or damaged fruit bodies develop verdigris to olive patches and old caps can look mottled blue-green.

The species is centred on the Americas and eastern Asia rather than Europe, and it is a well-known and much-photographed find in the oak and pine woods of the southeastern United States, Mexico and Central America. Its appearance is so distinctive that it is effectively unmistakable once the blue latex has been confirmed, and it is one of very few larger fungi that a beginner can name with confidence on colour alone.

Cap

5-15 cm across, convex with an inrolled margin at first, then flat and finally deeply funnel-shaped or vase-like with a depressed centre. The surface is smooth to slightly sticky, indigo to dark blue with a distinct silvery or greyish bloom, and marked with darker concentric zones like a target. Older caps fade toward pale grey-blue and develop green blotches.

Gills

Decurrent and running down the stem, crowded, narrow, and sometimes forking near the attachment. They are the same indigo blue as the cap, often a shade brighter, and stain green where scratched or bruised. Latex beads readily along a cut gill edge.

Stipe

2-8 cm tall, 1-2.5 cm thick, short and stout, cylindrical or tapering downward, and hollow at maturity. It is indigo to pale blue, often with a silvery bloom, and frequently marked with shallow darker rounded pits or scrobicules like those of the Saffron Milkcap. There is no ring.

Flesh, latex and spore print

The flesh is pale blue to whitish, marbled with blue, and thick in the cap. Cutting releases indigo-blue latex, sparse in dry weather and copious in damp conditions, which slowly turns dull green on exposure to air and stains the cut surface green. The spore print is cream to pale yellow, and the spores are ornamented with warts and ridges forming a partial reticulum, as is typical for the genus.

Lactarius indigo grows in both broadleaved and coniferous woodland, most often under oaks and pines and also with other hardwoods in mixed stands. It favours warm temperate to subtropical conditions and is found in open woodland, along forest trails and in leaf litter on well-drained ground, sometimes in company with other members of the orange and blue latex group.

Its range covers eastern and southern North America, particularly the Gulf states and the southern Appalachians, extending through Mexico and Central America into northern South America, and it also occurs in East and Southeast Asia including China, Japan and India. It is absent as a native from Europe. Fruiting is chiefly from summer to autumn, following the wet season, and fruit bodies appear singly, scattered or in loose groups.

The Indigo Milk Cap is ectomycorrhizal. It wraps the fine feeder roots of oaks and pines in a fungal mantle and forms a Hartig net between the root cortical cells, through which it delivers water, phosphorus and nitrogen drawn from a wide soil volume and receives sugars from the tree in exchange. The relationship is essential to the fungus, which cannot fruit away from a compatible host.

The blue pigment responsible for the colour is an azulene compound held in the latex, and the shift to green reflects its oxidation once exposed to air. The latex system itself is a defence, sealing damaged tissue and discouraging grazing by slugs and insect larvae. As with other milkcaps, the fruit bodies are grazed by small mammals and invertebrates that help disperse spores through the forest floor.

No other large mushroom in its range combines a blue cap with blue latex, so a cut gill settles the identification immediately. Lactarius paradoxus is the nearest relative, but its cap is greyish blue to brownish with only a bluish flush and its latex is reddish brown to purplish rather than indigo. Lactarius chelidonium has a dull yellowish to blue-grey cap and yellow-brown to bluish latex.

Faded or old Lactarius indigo that has greened extensively might be confused with the green-staining orange milkcaps such as Lactarius deliciosus, but those have orange rather than blue latex and orange rather than blue gills where undamaged. Blue-tinted Russula species such as members of the cyanoxantha group have no latex whatsoever, white gills and white spore prints, and blue Entoloma species have pink spore prints, attached rather than decurrent gills and fibrous, non-brittle flesh.