Black Bulgar Identification Guide

The first impression

A large oak has come down in a winter gale, or a limb has shed across a woodland ride, and by the following autumn its bark is studded with black. Hundreds of small dark bodies crowd along the trunk in bands, so tightly packed that they press each other into polygons, and the whole surface glistens as if it had been varnished.

Up close each body looks like a black gum drop or a rubber washer. They are firm, springy and cold to the touch, and squeezing one gives the unmistakable resistance of hard jelly that rebounds rather than squashing. That combination, black shining discs in crowded troops on recently fallen oak bark, is the Black Bulgar in a sentence.

Shape and size

Individual fruitbodies are 1 to 4 cm across. They begin as rounded, almost closed knobs, top-shaped or turbinate, and expand into shallow discs or flattened buttons with a slightly incurved rim. There is no true stalk, only a short stubby base narrowing where it grips the bark.

Because they emerge in dense colonies, mature bodies are often distorted, flattened on one or more sides by their neighbours, and a colony seen from above resembles a mosaic of black tiles.

The two surfaces

This is the character to fix in memory, and it is a purely macroscopic one. The upper surface, the disc, is smooth, jet black and distinctly shining, like a drop of polished tar. It is flat to slightly concave in mature material and it is the fertile layer. This is an ascomycete, a cup fungus in origin, so there are no gills, no pores and no teeth anywhere on it.

The outer and lower surface, by contrast, is matt, a dull dark brown to blackish brown, and roughened all over with fine bran-like scurf and granules. Tilt a fruitbody to the light and the contrast between the mirror-black top and the floury brown flank is immediate and unambiguous.

Texture and interior

Cut one through with a sharp blade. The interior is gelatinous but tough, pale to mid brown, and slightly translucent, with a firm rubbery consistency all the way through. It does not run, weep or collapse. Pressed between finger and thumb it deforms and springs back.

That elasticity has a second, seasonal expression. In a dry spell the fruitbodies contract, harden and darken, holding their shape rather than dissolving away, and after the next rain they swell back to full size and resume spore production. Colonies can therefore persist on a trunk for many weeks, right through frost.

The spore deposit

Ripe fruitbodies shed spores freely, and the deposit is sooty blackish brown. Two practical approaches work. Lay several mature discs face down on white paper under a cover for a few hours, or simply handle ripe material and note the black smudges left on your fingers, your lens cloth and the page of your notebook. The species epithet refers precisely to that staining habit, and in the field it is an excellent confirmatory character, because the black jelly fungi it might be confused with leave no such mark.

Under the microscope the arrangement inside each ascus is memorable and definitive: four large, dark brown, asymmetric spores occupy the upper part of the ascus while four much smaller colourless ones sit below them. If you have any microscope at all, this is the character to look for.

Substrate and season

The Black Bulgar is a saprotroph with a strong and useful substrate preference: freshly dead oak, still with its bark on, typically wood that has fallen, been felled or been storm-thrown within the previous one to two years. Ash is a regular secondary host and beech and other broadleaves occur occasionally, but a dense colony on bark-covered oak is the standard encounter.

It fruits from late summer into winter, peaking from September to December in the temperate northern hemisphere, and tolerates frost well. Fruitbodies appear in bands and patches along the upper faces and flanks of trunks and large limbs, often accompanied by other early bark colonisers. As the bark loosens and falls away the fungus disappears from that piece of wood, so old, barkless, weathered timber is the wrong place to search.

Separating it from look-alikes

Exidia glandulosa and Exidia nigricans, the black witches butter. These are the main confusion and they are basidiomycetes, not cup fungi. Their fruitbodies are irregular black lobes, brains or fused sheets with no defined circular disc and no scurfy brown flank; the whole surface is more or less uniform and often warted with tiny pimples. Their texture is softer and more watery, and in a dry spell they collapse into a thin black film or crust plastered on the bark, whereas the Black Bulgar merely shrinks and stays rubbery. Press each with a thumbnail: Exidia squashes, Bulgaria rebounds.

Ascocoryne sarcoides and A. cylichnium. Same gelatinous disc-fungus build, but coloured deep purple, wine-red or pinkish purple, usually on beech. Colour separates them at a glance.

Neobulgaria pura. A near relative with the same rubbery gum-drop shape but pale, translucent pinkish buff to almost flesh-coloured, on beech. It is essentially a pale echo of the same architecture.

Bulgariella pulla. Smaller, on conifer wood, and generally with a less strongly contrasting scurfy exterior.

Daldinia concentrica and Xylaria species. Also black and also on hardwood, but hard and carbonaceous rather than rubbery. Daldinia is a hemispherical cushion that shows concentric silvery rings when cut, and Xylaria forms upright clubs or fingers. Neither has a smooth shining disc.

Old, blackened Peziza or other cup fungi. These have a thin, brittle wall that cracks rather than flexes, and their outer surface is not evenly bran-scurfy.

Field checklist

Crowded troops of black rubbery discs on bark-covered, recently fallen oak; shining jet-black upper surface; matt brown scurfy outer surface; firm elastic gelatinous flesh that rebounds and does not liquefy; sooty black-brown spore deposit that stains fingers and paper; autumn into winter.