Black Bulgar

Black Bulgar forms dense colonies of gelatinous-rubbery buttons on the bark of recently fallen oak and ash. Each fruiting body begins as a small brown top-shaped knob and expands into a disc-topped button 1–4 cm across, the flat upper surface a glossy black while the sides and base remain rough and brown. Handling ripe specimens leaves smudges of dark spores on the fingers, the origin of the epithet inquinans, meaning 'staining' or 'defiling'.

Despite the jelly-like texture it is an ascomycete, not a jelly fungus: the spores are produced in asci on the flat upper disc. Bulgaria sits in the Bulgariaceae within the Helotiales, a small group characterised by tough gelatinous flesh and dark pigmentation.

Its asci are unusual in producing four large, dark brown, kidney-shaped spores and four much smaller colourless ones, a mismatched arrangement that is diagnostic under the microscope and explains the very dark spore deposit.

Fruiting body

Starting as a nearly spherical or top-shaped brown knob, each body expands to a squat button or shallow cup 1–4 cm across and about as tall, broadest at the top and narrowing to a short blunt base. They typically grow in tightly packed troops of dozens, jostling one another into angular shapes along a bark surface.

Upper disc

The fertile upper surface is flat to slightly concave, smooth, glossy and jet black when moist, becoming duller and more matt in dry weather. The rim is usually slightly incurved and paler brown.

Outer surface

The sides and base are dark brown, dry, roughened and finely scurfy or bran-like, contrasting clearly with the polished black disc — this two-tone appearance is the key field character.

Flesh and spore print

The flesh is thick, rubbery-gelatinous and elastic, blackish brown, springing back when squeezed and drying to a hard black lump that revives in rain. A spore deposit is dark blackish brown, and mature specimens shed enough spores to darken the bark and anything that touches them.

Black Bulgar is a specialist of freshly dead hardwood bark, overwhelmingly favouring oak, with ash the next most frequent host and beech, birch and elm occasional. It appears on trunks and large limbs that have fallen or been felled within the previous year or two, and also on standing dead branches, always where the bark is still firmly attached — once the bark sloughs away, the fungus disappears.

Fruiting is chiefly autumn into early winter, and the tough bodies persist through frost, shrivelling in dry spells and rehydrating after rain. It is common and widespread across Europe and temperate Asia, and recorded in parts of North America, being especially abundant in managed oak woodland where recent windfall or felling provides fresh material.

Bulgaria inquinans is saprotrophic, and probably behaves as a latent endophyte: its mycelium is thought to be present in the bark and outer sapwood of healthy living oaks, remaining quiescent until the branch or trunk dies, at which point it fruits rapidly and abundantly before other decay fungi arrive. This explains the characteristic flush of buttons on very recently fallen timber.

Decay is limited to the bark and outermost wood layers rather than the structural heartwood. The gelatinised flesh is an adaptation to a drying, exposed surface, allowing the fruiting body to shut down in dry conditions and resume spore release when rewetted, extending the fruiting period over several months.

Exidia glandulosa, Witches' Butter, is a true jelly fungus on similar oak branches but forms irregular, lobed, brain-like or blistered masses with tiny surface pimples, never a neat two-tone button with a rough brown base and flat glossy disc. Exidia plana is likewise a spreading gelatinous crust rather than a stalked-looking knob.

Ascocoryne sarcoides is a similar rubbery ascomycete on hardwood but is purple to reddish-purple, not black, and is usually more irregular and brain-like. Neobulgaria pura is pale pinkish-buff and translucent with a smooth outer surface. Old, blackened Bulgaria-like discs on conifer wood are more likely to be Ionomidotis or dark Encoelia species, which have thinner, less resilient flesh and paler spore deposits.