Exidia glandulosa
Black Witches Butter
Names
Exidia glandulosa (Black Witches Butter)
Other names: Exidia truncata
Exidia glandulosa, by Liz Holden
Description
Have you ever been walking through a deciduous forest and wondered what those black, tar-like looking structures are that grow on fallen trunks and branches? Well, Exidia glandulosa also known as Black Witches Butter would be a good guess. This fungus is more often recognised during wetter months as it dehydrates during dry weather and shrinks to a dry, shiny and only millimetre thin membrane on the substrate surface. However, the fungus quickly recovers to a soft gelatinous blob in rainy weather.
These jelly fungi are described by Peter Roberts (a professional mycologist formerly working at Kew Gardens) to have 'turbinate fruit bodies (like an old fashioned spinning top)'. They are often higher than wide, can merge with adjacent fruit bodies and become less stable with age, often starting to hang downwards with age. The upper surface ranges in colour from a shiny black to a dull olive brown and is usually smooth with pit-like depressions and glandular warts (see photo), ranging between 10-60 mm in size and occasionally possessing a stalk.
There are several different ideas concerning the derivation of the name Black Witches Butter. One legend from Eastern Europe is linked to the occurrence of the fruit bodies on wooden house entrances, which was thought to indicate that the house was under a witches spell. To counteract this spell, the fungus had to be punctured to allow the ‘evil juices’ inside the fungus to leak out. Other legends record its use against witchcraft when thrown into fires. Despite the name suggesting edibility (albeit somewhat tenuously), Black Witches Butter lacks a distinctive taste and is considered to be inedible.
Occurrence
The fungus can be found throughout the year but is more recognisable during wet weather as they often dry out during summer months.
Habitat
It grows predominantly on the bark of dead Oak trunks and branches, sometimes hazel and very rarely on other broadleaved trees.
Distribution
A fairly common, widespread species throughout the UK and Ireland (NBN Atlas map). Its distribution worldwide is rather unclear partly due to naming difficulties and the look-a-like species E. nigricans (see below).
Similar species
The following is gleaned from email correspondence with Peter Roberts*, who has investigated the considerable historical confusion over the correct names for the two black Exidia species which exist in Europe - E. glandulosa and E. nigricans (for a full account see the paper – Roberts P., 2009, Mycotaxon 109: 219-220).
In 1789, the French mycologist J. Bulliard described and illustrated Tremella glandulosa. This was accepted by the great Swedish mycologist, Elias Fries, who then transferred the species into the genus Exidia in 1822. However for some reason, Fries also redescribed the species under a new name, Exidia truncata Fr. Fries was very influential and many subsequent mycologists used the name E. truncata instead of E. glandulosa. It was not until 1966 when the Dutch mycologist Donk demonstrated Fries’ mistake that E. glandulosa was accepted as the correct name, thus making E. truncata a redundant name. Thanks to the work of Peter Roberts the other species is now called Exidia nigricans (With.) P. Roberts with E. plana as a synonym and it has been given the English name Warlock's Butter. In contrast to E. glandulosa, E. nigricans rarely occurs on oak, preferring other deciduous trees (NBN Atlas map). The surface of the fruit bodies often become folded like a brain (see photo).
The similar Exidia nigricans, by Liz Holden
Please remember to submit your records to your local recording group or via the Scottish Fungi online recording form.
Text by Peggy Ehrlich and Andy Taylor
Pictures by Liz Holden
*We would like to thank Peter Roberts for his contribution to this article, by helping to clarify the nomenclature between the two species.