Lactarius indigo, also known as the Indigo Milkcap, is a medium to large mushroom with an indigo-blue colour. The cap surface is sticky and exhibits a silvery sheen. When the cap is injured, it releases indigo-blue milk that turns dark green upon exposure. This mushroom typically grows either solitarily or in groups on forest soil during the summer and fall.

Cap firm, convex with a depressed disc and inrolled margin at first, becoming shallowly funnel-shaped. The surface is smooth, pale to light- or purplish-blue when fresh, later greyish or silvery. Gills attached to the stem or beginning to run down it and crowded close together. Their colour is an indigo blue, becoming paler with age or staining green with damage. Stem cylindric or tapering to base, and hard. The surface is sticky and sometimes spotted. Flesh whitish, turning indigo blue when cut; staining slowly greenish. Spore print pale yellowish.

Microscopic Features: The spores are broadly ellipsoid to subglobose, measuring 7-10 x 5.5-7.5 µ. They have an ornamentation of amyloid warts and connecting lines, reaching a height of approximately 0.5 µ. Occasionally, these warts and lines form partial reticula.

Lactarius indigo on the MushroomExpert.Com web site.

The second photo is by Bernard Dupont and licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.