Yellow Brain Identification Guide

The First Sighting

It is a wet week in late autumn or midwinter, the woods are bare, and a fold of bright golden jelly catches your eye on a dead branch still attached to a shrub. From several metres away it reads as a blob of spilled yolk or a piece of orange plastic caught in the twigs. Up close it is unmistakably alive and unmistakably gelatinous: a lobed, folded, translucent mass two to ten centimetres across, deep golden yellow to orange-yellow, slippery to the touch and quivering when the branch is nudged. The important part of the first impression is the position. This fungus is almost always on dead wood that is still attached to a standing shrub or tree, at chest or head height, rather than on a log on the ground.

Reading the Fruit Body

There is no cap, no stem and no gill or pore layer to work through, which changes the routine. The fertile surface is spread over the entire outside of the mass, so structure means shape and texture.

Begin with the outline. Fresh material is a heap of contorted lobes and convolutions, thin-edged and irregular, often compared to a brain or a crumpled ribbon, arising from a narrow point of attachment where it emerges through a bark crack. Note how it emerges: the jelly pushes out through a split in the bark rather than sitting on the surface, and if you cut the branch across you will often see the fungus continuing inward as a paler gelatinised strand.

Next the texture. Press it. Fresh material is soft, elastic and slippery, and it springs back. It is translucent enough that a lens held against a thin lobe shows light passing through with an amber glow. There is no fibrous flesh, no rind and no distinct interior layer when you cut it: the section is uniformly jelly.

Colour and How Weather Changes It

Colour runs from pale butter yellow through egg yolk to a warm orange, and it usually deepens towards the outer lobes and pales towards the attachment. Weather matters enormously. After days of rain the fungus swells, brightens and becomes almost glassy. In a dry spell it shrinks dramatically, hardening into a small, wrinkled, dark orange to reddish scab pressed against the bark, easily overlooked or mistaken for a resin blob or an old wound. The same body will rehydrate within hours of rain and resume sporulating, and a single fruit body can go through this cycle repeatedly across a winter. Frozen material may look glassy and dull and will recover on thawing. Nothing bruises or changes colour when handled.

Taking a Spore Print

Jelly fungi do print, but you must give them humidity or the surface dries before it drops. Cut a lobe with a short piece of the branch attached, wedge it so a fertile face hangs over a strip of glass or dark card, and enclose everything under an upturned bowl with a damp tissue in the corner. Leave it overnight. The deposit is white to very faintly yellowish and shows best on dark card.

Under a microscope the confirmation is decisive. The basidia are divided lengthwise into four cells by cross walls, giving the characteristic segmented appearance of the genus, and long sterigmata project from them. Spores are broadly ellipsoid to nearly round, colourless and smooth. The jelly fungi that most resemble this species have undivided, two-pronged basidia instead, so a single squash mount settles the question.

Substrate, Season and the Hidden Partner

This is where the ecology becomes an identification tool. Yellow Brain does not digest wood directly. It is a parasite of other fungi, and specifically of crust fungi in the genus Peniophora that are already colonising the branch. That means the wood is only the address, not the food, and it explains why the jelly appears on branches that show no other sign of decay.

Use that. Examine the same branch, above and below the jelly, for a thin smooth crust in pinkish grey, lilac-grey or dull flesh colour, often with a darker cracked look when dry. Finding that crust on the branch strongly supports the identification and helps separate this species from its relatives, which parasitise different hosts.

Typical substrates are dead attached branches of gorse, hazel, beech, oak, ash, birch and many garden shrubs. The season runs from autumn right through winter into early spring, with the best displays after mild wet weather. It is one of the most reliable winter fungi in hedgerows and scrub.

Separating the Look-alikes

The golden ear, Tremella aurantia, is the trickiest. It is a deeper, more uniform orange, tougher and more coarsely lobed, and in section it often shows a paler, denser core of host tissue running through the jelly. Crucially it parasitises the hairy curtain crust, Stereum hirsutum, so check the branch for small tan brackets with hairy, concentrically zoned upper surfaces and smooth orange-buff undersides. Host crust plus a firmer, cored jelly means the golden ear.

Dacrymyces stillatus and its relatives are far smaller, usually only two to five millimetres, forming numerous separate orange cushions or discs rather than one large lobed mass, and they occur mostly on conifer wood, fence rails and worked timber. Their basidia are shaped like a tuning fork with two prongs.

Yellow stagshorn, Calocera viscosa, is bright orange-yellow but grows as an upright branched antler, tough and rubbery rather than soft and floppy, and it roots into buried conifer wood and stumps on the ground.

Witches butter and the other Exidia species share the gelatinous habit but are black, brown or translucent white, and Tremella encephala is pinkish and encloses an obvious hard white core, growing with a different crust host on conifer.

Finally, do not confuse a yellow slime mould such as Fuligo septica with this fungus. Slime moulds are pasty or foamy rather than elastic, dry to a powdery mass of dark spores, and typically creep over mulch, sawdust and mossy stumps rather than emerging from a bark crack on a standing branch.

Field Checklist

• Golden to orange gelatinous lobed mass, 2 to 10 cm, on dead attached branches
• Emerges through cracks in bark, usually above ground level in hedgerow or scrub
• Soft, slippery, translucent and elastic; no cap, stem, gills or pores
• Shrinks to a hard dark orange scab when dry and revives with rain
• Look on the same branch for a smooth pinkish-grey Peniophora crust
• Spore print whitish; basidia divided lengthwise into four cells
• Autumn through winter into early spring