Black Witches' Butter Identification Guide

Choose Your Weather First

Most identification guides start with the fungus. For this one, start with the forecast. A gelatinous fungus is only readable when it is fully swollen with water, and in a dry week Black Witches' Butter is a black stain on bark that almost nobody notices. Plan a session for the day after heavy rain, in autumn, winter or very early spring, and head for broadleaved woodland with plenty of standing and hanging deadwood. Under those conditions the species is one of the easiest fungi in the wood to see: glossy black lumps strung along the upper side of dead branches, catching the light like beads of oil.

Read the Colony, Not the Blob

The overall pattern along the branch is the first real character. Individual bodies start as small, top-shaped or button-like cushions, roughly 1-3 cm across, each one attached by a narrow constricted point. They expand, meet their neighbours and coalesce into a lumpy, ridged, convoluted band, but even in a well-developed colony the constituent cushions usually remain traceable as bumps and lobes rather than dissolving into one smooth expanse. Run a fingertip along the band and you should feel that undulating, knuckled outline.

The Squeeze Test and the Section

Pick a piece up and press it gently between finger and thumb. It should feel firmly gelatinous and rubbery-elastic, deforming under pressure and then rebounding. A specimen that squashes to slime is over-mature or frost-damaged, and one that feels dry, leathery and unyielding when wet is almost certainly a different genus. Cut across it with a sharp blade: the interior is uniformly gelatinous, translucent brown-black held up to the light, with no rind, no separate layers and no contrasting core. Nothing changes colour on the cut face.

Surface Detail Under Magnification

With a 10x lens on the exposed upper surface of a wet specimen you should find the diagnostic detail: numerous minute wart-like or pimple-like projections scattered across the shiny jelly, giving it a finely studded texture. Wipe the surface clean of grit first, because dirt trapped in the jelly can mimic them. Colour is olive-black, blackish-brown or near-pure black; at the thin margin the jelly is often slightly translucent, and against the sky it can look dark amber rather than black.

Where the Spores Come From

There is no cap, no gills, no pores and no toothed layer to examine. The whole outer surface of the jelly is fertile, and this is the reason a piece of Exidia lying on card will drop a proper spore print in a way that many people do not expect. Set a well-hydrated fruit body on black glass or dark card, invert a glass over it to trap humidity, and leave it overnight somewhere cool. The result is a white deposit, sometimes surprisingly abundant. If you have a microscope, the spores are narrow and strongly curved like tiny sausages, and the basidia are pear-shaped and divided lengthwise into four compartments. Those two microscopic features together place the specimen firmly among the true jelly fungi and rule out the black-disc ascomycetes at a stroke.

The Collapse-and-Revive Cycle

Watch the same branch across a season and the behaviour itself becomes a character. In dry conditions the fruit bodies shrivel to a thin brittle blackish crust, barely thicker than paint, and appear dead. Rain restores them to full volume within hours. A colony can repeat this many times, which is why the species has such a long apparent season and why old, unremarkable-looking black films on oak branches are worth wetting and re-checking. Collected material behaves the same way and can be revived from a dried packet with a spoonful of water.

Substrate, Host and Season

The species is a wood decayer of dead broadleaved branches, with oak dominating the records and beech, hazel, birch and other hardwoods appearing regularly. It strongly favours wood that is still firm: attached dead branches in the crown, recently snapped limbs and brash piles. It is far less common on soft, crumbling, ground-rotted logs. The season runs from autumn through to spring, peaking in mild wet winter spells, and colonies are gregarious, so if one branch is occupied, the others on the same fallen limb usually are too.

Sorting Out the Other Black Jellies

Exidia nigricans, sometimes published as E. plana, is the species you must actively exclude. It grows on the same hosts at the same time of year but forms broad, flat, spreading, brain-like sheets that fuse early and sit on a wide attachment, and its surface is comparatively smooth, lacking well-developed pimples. Cushion-shaped units with pinched bases and a studded surface point to Black Witches' Butter; a wide, flattish, smooth convoluted crust points to E. nigricans.

Exidia recisa grows mainly on willow and is translucent amber to reddish-brown, so colour alone divides them.

Exidia thuretiana is whitish or greyish-translucent on beech, again separated by colour.

Bulgaria inquinans on fallen oak forms discrete goblet or button shapes with a glossy black upper disc but a dull, brown, scurfy outer wall, and the texture is tough and rubbery rather than jelly-like. Its spore deposit is dark brown to blackish, which is the cleanest single separation from a white-printing Exidia.

Tremella foliacea builds upright, frilly, leaf-like lobes in warm brown tones, and Auricularia auricula-judae forms ear-shaped, tan-brown, internally veined discs, neither of which resembles a row of small black cushions.

Finally, do not forget the non-fungal impostors: tar-spot residues, black slime moulds in their plasmodial stage and actual bitumen splashes on roadside timber all fool people at a distance. The squeeze test settles all three in a second.

Summary Sequence

Wet day, dead firm hardwood, black cushions merging into a ridged band, narrow attachments, pimpled surface at 10x, elastic gelatinous flesh with no internal layering, white spore print. Every one of those can be checked in the field without a laboratory, and together they leave very little room for doubt.