Black Witches' Butter

Exidia glandulosa forms dark sepia to blackish, rubbery-gelatinous basidiocarps that are turbinate (top-shaped, like an inverted cone) and up to 5 cm (2.0 in) across. They are firm when fresh, but become lax and distorted with age or in wet weather. The fruit bodies occur singly or in small groups. The upper, spore-bearing surface is shiny and dotted with small pimples or pegs. The undersurface is hirsute, developing a dense covering of small, gelatinous spines. The spore print is white. When the fruit bodies are dried they can shrink to form a flattened black crust.

Microscopic characters. The microscopic characters are typical of the genus Exidia. The basidia are ellipsoid, septate, 15–25 by 9–13 μm. The spores are allantoid (sausage-shaped), 11–18 by 4–6.5 μm.

Similar species. Exidia truncata is very similar, but has a rough, punctate or pimply undersurface and (microscopically) slightly longer spores. Exidia pithya is also similar, but produces button-shaped fruit bodies that quickly coalesce, forming an effused, lobed mass that can be up to 30 cm (12 in) across. The ascomycete Bulgaria inquinans forms similar, rubbery-gelatinous, blackish fruit bodies on oak. Their upper surfaces are entirely smooth, however, and they produce copious black (not white) spore prints, often leaving a black stain if wiped with the hand.

Exidia glandulosa is a wood-rotting species, typically found on dead attached branches of broadleaf trees, especially oak, occasionally hazel, beech, alder, and other trees. It is a pioneer species capable of colonizing living or recently dead wood. A study of the wood decay process in attached oak branches showed that E. glandulosa is a member of a community of eight basidiomycetous fungi consistently associated with the decay of dying branches on living trees. Specifically, its role is to disintegrate the tissue of the vascular cambium, which loosens the attached bark. It persists for some while on fallen branches and logs. The fruit bodies are normally produced in the autumn. Its global distribution outside of Europe is uncertain.

While nonpoisonous, one American field guide says E. glandulosa is inedible, though it has also been reported to be edible.