Black Witch's Butter

Black Witch's Butter (Exidia glandulosa) is a dark, rubbery jelly fungus that appears as glistening black cushions and lobes on dead hardwood, most typically oak. When fully hydrated it looks like a smear of tar or a spill of black jelly along a branch; in dry weather it contracts to a thin, hard, shiny black crust that can be mistaken for old paint or a burn mark on the bark.

The genus Exidia belongs to the Auriculariales and is characterised by longitudinally septate basidia and by fruiting bodies whose fertile surface is peppered with tiny sterile projections. In E. glandulosa these projections are unusually well developed, forming the conspicuous dark warts or pimples that give the species its epithet and its main field character.

Unlike the Tremella jellies, Exidia species feed directly on wood. Black Witch's Butter is therefore a genuine decomposer and can be found on branches with no other visible fungus present, which is a helpful distinction when working through a group of similar-looking jellies.

Fruiting body

Cushion-shaped to top-shaped gelatinous bodies 1–3 cm across, usually in rows or dense groups along a branch, often fusing into irregular masses 10 cm or more long. Individual fruiting bodies are attached at a narrowed base and expand upward and outward into a rounded, lobed cushion.

Surface

The key feature: the upper, fertile surface is covered in numerous minute blackish warts or pimples, giving a granular, studded appearance under a hand lens and a slightly matt, roughened look to the naked eye. The lower and lateral surfaces are smoother and duller.

Colour

Olive-black to black when fresh and wet, sometimes with dark brown or bronze tones in the thinner parts. Drying material becomes shiny jet black and paper-thin, adhering closely to the bark.

Flesh and spore print

Flesh gelatinous but firmer and more rubbery than the pale jellies, springing back when pressed; no fibrous core, ring, volva or stipe, and no colour change on cutting. Spore print white, in striking contrast to the black tissue.

Black Witch's Butter grows on dead attached branches and recently fallen wood of broadleaved trees, with oak by far the most frequent host. Beech, hazel, ash and other hardwoods are also used. It is characteristic of branch wood still held in the canopy or resting in brash piles, where alternating wetting and drying is the norm.

Fruiting spans autumn through winter and into spring, and it is one of the fungi most reliably seen on a wet winter woodland walk. The species is widespread across temperate Europe and reported from North America and Asia, though some North American material has been reassigned to related species.

Exidia glandulosa is a saprotrophic white-rot fungus of dead hardwood. Its mycelium permeates the outer wood of dying and dead branches, degrading lignin and cellulose, and fruits out through the bark in response to sustained wet weather. It is typically an early coloniser of oak branch wood, appearing while the bark is still intact.

The rubbery, water-storing tissue lets a single fruiting body survive repeated freeze, thaw and drought cycles through a temperate winter, revive within hours of rain, and shed spores across many separate episodes. The dark pigmentation is thought to help protect the exposed tissue from ultraviolet light and desiccation during the dry intervals.

Exidia nigricans (often called Warlock's Butter) is extremely similar but forms broad confluent sheets that merge into a single large brain-like mass with a folded surface, rather than the discrete pimpled cushions of E. glandulosa, and its warts are less prominent. Exidia recisa (Amber Jelly Roll) shares the narrow attachment but is translucent amber-brown and smooth-surfaced.

Bulgaria inquinans (Black Bulgar) can look similar on oak but forms firm, rubbery, top-shaped discs with a flat shiny black fertile face and a brown scurfy outer surface, and its structure is much tougher and more regular. Dark slime moulds and tarry exudates on bark lack any defined gelatinous fruiting body outline.