Exidia nigricans

Exidia nigricans (common name Witches' butter) is a jelly fungus in the family Auriculariaceae. It is a common, wood-rotting species throughout the Northern Hemisphere, typically growing on dead attached branches of broadleaf trees. It has been much confused with Exidia glandulosa.

Taxonomy
The species was originally described from England as Tremella nigricans by Withering in 1776, based on a phrase name published by Dillenius in 1741. It was subsequently considered a synonym of Exidia glandulosa, until Donk revised species concepts in 1966 and placed it in synonymy with E. plana. Changes in the starting point for fungal nomenclature delegitimized E. plana, making E. nigricans the species' earliest name.

Molecular research has shown that E. nigricans has distinct DNA from E. glandulosa, despite similarities, even on a microscopic level.

Common name
Both Dillenius (1741) and Withering (1776) gave the English name for this species as "Witches' butter", though this name has subsequently also been applied to other gelatinous fungi, including E. glandulosa and the yellow Tremella mesenterica. It is said to be based on a folk belief that witches milk cows at night and scatter the butter they create in the process around.

Description
Exidia nigricans forms dark sepia to blackish, rubbery-gelatinous fruit bodies that are button-shaped and 1-2 cm across. The fruitbodies occur in clusters and quickly coalesce to form effused, irregular masses up to 20 cm across. The upper, spore-bearing surface is shiny and dotted with small pimples or pegs. The individual fruitbodies are each attached to the wood at the base. The spore print is white.

Microscopic characters
The microscopic characters are typical of the genus Exidia. The basidia are ellipsoid, septate, 15–25 × 8–13 μm. The spores are allantoid (sausage-shaped), 14–19 × 4.5–5.5 μm.

Similar species
Exidia nigricans and E. glandulosa are frequently confused. The two are similar (even microscopically), but E. glandulosa produces discrete, top-shaped fruitbodies that rarely if ever coalesce.

Fruitbodies of Phaeotremella foliacea and P. frondosa are typically a warmer, lighter brown, but can sometimes be dark sepia to black. The fruitbodies are gelatinous, but usually foliaceous (with flattish lobes or fronds) and never have warts or pegs on the surface. Non-gelatinous black specimens can be found in Annulohypoxylon, Biscogniauxia, Camarops, and Hypoxylon.

Habitat and distribution
Exidia nigricans is a wood-rotting species, typically found on dead attached branches of a wide range of broadleaf trees. It persists for some while on fallen branches and logs. The species typically fruits in autumn and winter. It is widely distributed in the Northern Hemisphere, including Europe and North America. In the latter, it can be found from November to March on the west coast, and around the year elsewhere.