White Brain Identification Guide

The Setting and the First Glance

This is a cold-weather fungus of damp broadleaved woodland, and you find it by looking at sticks. Walk a beech or hazel wood between late autumn and early spring, a day or two after rain, and turn over the fallen branches lying just clear of the ground. What you are hunting for is a chalk-white to glassy translucent smear on bark: something that looks less like a mushroom than like poured candle wax that set where it fell, or a spill of set egg white on a stick.

Most people walk past it for years. Once you have seen one, they turn up constantly, because the species is common wherever there is dead attached hardwood and enough humidity.

Reading the Fruit Body

There is no cap, no stipe and no gill or pore layer to work through, so the read is different from a gilled mushroom. Instead you assess shape, attachment, surface, internal texture and response to moisture.

Shape and size. Individual fruit bodies begin as small rounded blobs a few millimetres across, then swell and fuse laterally into elongated cushions and irregular ridged masses. A well-developed colony runs 1-10 cm along the branch and is usually only 2-10 mm thick. The surface throws up shallow folds and convolutions, which is where the brain comparison in the common name comes from, although the folding is gentler and more waxy-looking than a true brain fungus.

Attachment. It is broadly sessile, glued flat along the bark or wood over its whole underside. There is no stalk, no point of attachment you could describe as a base, and no free frilly lobes standing away from the substrate.

Surface. Smooth to gently wrinkled, glistening when wet. Crucially, it lacks the small dark pimple-like warts that stud the surface of the black Exidia species. Run a lens over the top: an even gelatinous sheet with folds, not a warty one.

Colour. Pure white to ivory or a translucent hyaline white. Older or weathered material may go faintly greyish, watery or slightly yellowish, but a strong colour of any kind points elsewhere.

The Cut Test

This is the step that separates it from its nearest relative, and it takes five seconds. Slice through a cushion with a sharp knife and look at the cut face against the light.

Exidia thuretiana is homogeneous. The jelly is uniform right through, translucent, with no firm inner core, no distinct skin, no channels and, above all, no hard opaque granules embedded in it. There is no colour change on cutting and no milk or liquid released beyond ordinary water. Texture in the fingers is soft and yielding, wobbling as a unit if you tap the branch, and tearing gives a slight rubbery resistance before it parts cleanly.

Taking and Reading a Spore Print

Jelly fungi will print, but they need patience and humidity. Detach a piece of the fruit body, set it on a glass slide or a strip of dark paper, and put the whole thing under an upturned pot or in a closed box so it does not dry out. Leave it overnight, and expect a faint deposit rather than a heavy one.

The print is white. Under the microscope the spores are allantoid, meaning curved like a small sausage, smooth, colourless, and roughly 13-18 by 5-6 micrometres. If you can get a look at the basidia you have the family character: they are divided longitudinally into four cells by cruciate septa, which places the fungus in the Auriculariales and rules out every superficially similar white jelly formed by an ascomycete.

Substrate and Season

The species is saprotrophic, digesting the wood of dead broadleaved branches. Beech and hazel are the classic substrates, with ash, sycamore, hornbeam, oak and other hardwoods also recorded. It favours branches that are dead but still attached, or recently fallen and still sound, rather than soft, crumbling, long-rotted wood.

The season is the cold, wet part of the year: autumn through to early spring, with the best displays during mild damp spells in winter when almost nothing else is fruiting. Dry weather collapses the fruit body into a thin whitish varnish-like film that is easy to overlook entirely, and a few hours of rain restores it to full size, so the same colony can appear and vanish repeatedly through a season.

Separating It from Look-alikes on Morphology Alone

Crystal brain, Exidia nucleata, also treated as Myxarium nucleatum, is the serious confusion. It is also white, translucent and cushion-forming on hardwood in the same season. The separator is internal: crystal brain contains small hard opaque white granules embedded in the jelly, visible as chalky specks when you hold a piece up to the light or slice it. Exidia thuretiana has none. Get in the habit of cutting every white jelly you find and the two stop being confusable.

Ductifera pululahuana forms white gelatinous cushions too, but they tend to stay as discrete rounded pustules rather than merging into long folded ridges, and it is a warmer-climate fungus.

Tremella species need checking. Tremella encephala grows in company with Stereum sanguinolentum on conifer wood and hides a firm whitish core inside a pinkish-buff jelly, so the cut face settles it. Tremella fuciformis makes thin free frilly lobes standing away from the branch rather than flat cushions. Tremella mesenterica is bright yellow.

The black Exidia species, such as E. glandulosa and E. plana, are the same architecture in dark brown-black, usually with small surface warts, so colour alone divides them.

Neobulgaria pura, an ascomycete, is firmer, rubbery, buff to pinkish, and forms discs with a defined rim rather than shapeless folds, typically in clusters on beech.

Finally, the white slime moulds, including Fuligo and Enteridium species, can look superficially similar as white masses on wood, but they are chalky or powdery rather than gelatinous, they do not wobble, and they collapse into dusty spore mass rather than shrinking to a translucent film.

A Practical Field Sequence

Confirm the substrate is dead hardwood, ideally beech or hazel. Confirm the season is cold and the material is wet. Check the outline: fused folded cushions glued flat to the bark. Check the surface for absence of warts. Cut it and look for absence of internal granules. Print it overnight in a humid box and confirm white spores, allantoid under the lens.