Black Witches' Butter

Black Witches' Butter (Exidia glandulosa) is a jelly fungus of the Auriculariales, the group that also contains the wood ears. Its fruit bodies are made largely of water held in a gelatinous matrix, which allows them to dry to almost nothing in a dry spell and rehydrate to full size within hours of rain, a trick that lets them fruit through the cold, wet months when most agarics are finished.

Fresh fruit bodies begin as separate blackish, top-shaped or button-like blobs attached to the wood at a narrow point. As they grow they coalesce along the branch into undulating, lobed, brain-like masses several centimetres long. The upper surface carries scattered tiny pimple-like warts, the glands that give the species its epithet.

The genus Exidia has been repeatedly revised and the black species are notoriously hard to separate, with E. glandulosa and the spreading E. nigricans long confused under one name.

Fruiting body

Initially discrete blackish to dark olive-brown cushions or tops 1-3 cm across, narrowed to a small point of attachment, later fusing into irregular, lobed, wrinkled masses up to 10 cm or more long. When fully hydrated the texture is soft, rubbery and elastic; when dry it becomes a thin, hard, shiny black film or scab pressed against the bark.

Surface

The fertile upper surface is smooth to undulating, matte to slightly shiny black, and dotted with numerous minute wart-like glands visible with a hand lens. The underside and attachment point are usually slightly paler and finely downy.

Structure and flesh

There are no gills, pores or teeth; the entire outer surface bears the spore-producing tissue. The interior is uniformly gelatinous, translucent dark brown to black when wet, with no distinct layers and no colour change when cut. It quivers when the branch is tapped.

Spore print

White. Sausage-shaped, colourless spores are produced across the upper surface, and a heavy fruiting can leave a whitish bloom on twigs and leaves below.

This fungus fruits on dead attached branches and recently fallen wood of deciduous trees, with oak by far the most frequent host; beech, hazel, ash, birch and other hardwoods are also used. It typically appears on wood that still has its bark, along the upper sides of dead limbs where rain collects.

It is common and widespread throughout temperate Europe, North America and Asia. Fruiting peaks from autumn through winter and into early spring, and fruit bodies can persist for months, alternately shrivelling and re-expanding with the weather. Frost has little effect on them.

Exidia glandulosa is a saprophyte and an early coloniser of recently dead hardwood, often taking hold in attached branches that have died on the standing tree. It causes a white rot, breaking down lignin as well as cellulose in the outer sapwood.

The gelatinous construction is an adaptation to intermittent moisture: the fungus can suspend spore production during dry periods and resume within hours of rain, spreading its release over a long season instead of a short flush. Spores are actively discharged from the outer surface and dispersed by air currents to newly dead wood.

Exidia nigricans (long known as E. plana) is the closest match and forms broader, thinner, more continuously spreading black sheets over the bark with a more deeply folded, convoluted surface, whereas E. glandulosa starts as separate cushions with narrow attachment points that only later merge. Exidia recisa is brownish amber rather than black and forms tongue- or top-shaped bodies mainly on willow.

Bulgaria inquinans, the black bulgar, looks superficially similar on fallen oak but forms firm, rubbery, top-shaped discs with a smooth shiny black upper face and a rough brown outside, and it drops a blackish spore print rather than a white one. The bright orange-yellow Tremella mesenterica shares the gelatinous lobed form but never the black colour.