Green Elfcup

Green Elfcup (Chlorociboria aeruginascens) is best known not for its fruit bodies but for what it does to wood. The mycelium deposits a pigment called xylindein through the timber it colonises, turning fallen branches an unmistakable blue-green that persists long after the fungus has finished with them. This green-stained wood was gathered for centuries for the decorative inlay work known as Tunbridge ware.

The fungus is an ascomycete in the Helotiales, a group of small disc and cup fungi that discharge spores from asci lining the upper surface. In Chlorociboria the apothecia are shallow, saucer- or goblet-shaped, and carried on a short off-centre stalk, and they share the same blue-green pigment as the mycelium.

Finding the stain is easy; finding the cups is a matter of timing, since they need sustained damp weather and often appear only on the underside of well-rotted branches lying in contact with the ground.

Fruiting body

Apothecia are small, 2-8 mm across, at first cup- or goblet-shaped with an inrolled margin, expanding to shallow saucers or irregular discs, often wavy or split at the edge with age. They are frequently gregarious, dozens scattered over one stained branch.

Surfaces

The upper, spore-bearing surface is smooth and blue-green to verdigris or turquoise, sometimes with a yellowish tinge in old specimens. The outer surface is a similar colour but duller and finely downy or minutely roughened.

Stalk

Each cup has a short, slender stalk, 1-3 mm long, attached off-centre or laterally, the same blue-green as the rest of the fruit body and often curved so the disc faces outward from the wood.

Flesh, wood stain and spores

The flesh is thin, waxy-gelatinous and blue-green throughout. The colonised wood beneath is stained a strong blue-green well into its interior, which is the most reliable field character. Spores are colourless, narrowly spindle-shaped, and give a whitish print, though prints are rarely taken from such small cups.

Green Elfcup colonises well-rotted, barkless deciduous wood lying on damp ground, with oak and ash particularly favoured; hazel, beech, birch, willow and other hardwoods are also used. It thrives in shaded, humid woodland where fallen branches stay wet, along stream banks and in old coppice.

It is widespread across Europe, North America, and temperate Asia, with related species in the Southern Hemisphere. The stained wood can be found in any month, while the cups themselves are most frequent from late summer through autumn after prolonged wet weather, sometimes lingering into mild winters.

The fungus is saprophytic, a white-rot decomposer that breaks down lignin and cellulose in downed hardwood, generally arriving after the bark has fallen away and the wood has already been softened by earlier colonisers. Its bright pigment is thought to help it defend the territory it has claimed inside the log against competing fungi.

Spores are shot from asci on the upper surface of the cups and carried on air currents to new wood. Because the mycelium can occupy a branch for years before fruiting, the density of green-stained wood in a wood is a better measure of the fungus's presence than the number of cups visible in any one season.

Chlorociboria aeruginosa is essentially identical in the field, producing the same green stain and similar cups; it is separated on spore size under a microscope, its spores being noticeably longer. Any reliable distinction between the two requires measurement rather than field characters.

Other small cup fungi on wood differ in colour: Bisporella citrina forms bright lemon-yellow discs, and Ascocoryne sarcoides makes purplish, gelatinous, brain-like blobs. Neobulgaria pura is pinkish and rubbery. Green algae or lichen films on wood can also colour a branch, but they sit only on the surface, whereas Chlorociboria stains the wood right through, visible when the branch is snapped open.