Jelly Rot Identification Guide

First impression and where you are standing

Autumn, damp woodland, and a fallen birch or beech trunk lying in shade. What draws the eye is a pale, waxy-looking crust spreading across the underside or flank of the log, with a row of narrow white shelves projecting from it. Get level with the wood and the shelf undersides show as a soft salmon-pink to orange sheet.

Touch it. That settles a great deal at once: the whole structure is cool, soft and gelatinous, yielding under a fingertip like set jelly rather than resisting like a bracket. You can slide a knife under an edge and peel off a whole sheet of it, which no woody polypore will allow.

Overall growth form

Phlebia tremellosa grows effused-reflexed. In practice that means it starts as a resupinate patch pressed flat against the wood and then turns outwards along the upper edge to form a narrow shelf, usually up to about 2 cm deep, occasionally more.

Individual patches fuse into strips that can run 20 cm or more along a log, so the outline is irregular and rambling rather than the neat fan of a true bracket. On the underside of a horizontal trunk it may stay almost entirely flat with only a fringe of turned-up edge.

The upper, sterile surface

Look at the top of a reflexed shelf. It is white to cream and densely hairy — under a lens, a soft felted tomentum, sometimes matted into low ridges. Old or long-exposed material often picks up a green tinge where algae have colonised the hairs.

The margin is thin, whitish, translucent and often finely fringed, and it fades out onto the bare wood where the fungus is actively spreading. That combination of a white woolly top and a translucent fringed edge is a useful confirmation.

The spore-bearing surface

Turn the log or bend down and look at the underside. This is the diagnostic surface, and it needs careful reading because it sits halfway between smooth and porous.

The surface is merulioid: a network of low, radiating, wrinkle-like folds that repeatedly branch and rejoin, enclosing shallow, angular, irregular pits. These pits look like pores at a glance, but they are not true tubes — there is no separate tube layer, and the "pores" are simply the gaps between folds. Sectioned, they are shallow depressions, typically well under a millimetre deep, with roughly one to two folds per millimetre.

Colour runs from pale pinkish-buff through salmon to orange-ochre, and it is strongest in fresh, well-developed material. On drying, the whole surface darkens and dulls to a brownish-orange film.

Section and flesh

Cut a piece vertically with a razor. In section, the flesh is thin — a millimetre or three — soft, translucent and whitish, with the coloured hymenial layer as a distinct band on one face and the woolly white layer on the other.

There is no colour change on cutting and no latex. The flesh does not have the corky or leathery consistency of a polypore, nor the brittle texture of an agaric: it is a gel with fibres in it.

A useful behavioural note: on drying, the whole fruit body shrinks dramatically to a thin, hard, dull film that may be hard to recognise. Wetting it restores much of the shape and colour, and a rehydrated specimen is far easier to identify than a dried one.

Getting spores

A print is possible if you find a well-developed reflexed shelf. Cut a piece and lay it hymenium-down over a glass slide or dark paper in a covered container overnight. The deposit is white and usually very sparse — this is a fungus with a modest hymenial area and it is easy to end up with nothing visible.

Microscopy is more dependable. Scrape or section the fertile surface and mount in water or dilute potassium hydroxide. Expect:

• Spores that are colourless, smooth, small and allantoid — sausage-shaped, curved along one side — roughly 4–4.5 by 1–1.5 micrometres, and not turning blue in iodine.
• A monomitic hyphal system — that is, generative hyphae only, with no skeletal or binding hyphae — and clamp connections present at the septa.
• No cystidia of any kind on the hymenium, which distinguishes it from a number of otherwise similar crusts.

Substrate, ecology and season

The substrate is dead hardwood. Birch is the classic host, with beech, alder, willow, hazel and poplar all frequent; conifer records exist but are much less common.

It occupies fallen trunks, large branches and stumps, and it strongly favours wood that is already damp, shaded and slightly decayed, which is why it is a fungus of ditch sides, stream valleys and the shadier interiors of woodland rather than open, sun-baked timber. It is a white-rot fungus, degrading lignin as well as cellulose and leaving the wood pale, soft and fibrous.

Fruiting runs from autumn into early winter, and the fruit bodies persist as dried films long after frost, rehydrating in mild wet spells. The species is widespread across the Northern Hemisphere.

Separating the closest look-alikes on morphology alone

Phlebia radiata is entirely resupinate, with no turned-out hairy cap, and its texture is waxy rather than truly gelatinous. Its surface forms radiating wrinkles arranged around a central point, in vivid orange, pink and violet tones, and it lacks the pit-like network.

Phlebia rufa is also resupinate, is tougher, and has a merulioid ochre to rust-toned surface without a white hairy shelf.

Byssomerulius corium is the trickiest, because it is also white and effused-reflexed on hardwood. Separate it on texture and colour: it is leathery, papery and pliant when dry rather than gelatinous, and its hymenium is white to cream, nearly smooth or only faintly wrinkled, never salmon-orange.

Serpula lacrymans and S. himantioides have a genuinely merulioid folded surface, but the folds are deeper and coarser, the mature surface is rusty-brown from the spore mass rather than salmon, the margin bears thick white or lilac-tinted cords, and the spores are yellow-brown and much larger.

Leucogyrophana species are similarly brown-yellow with brownish spores.

Gloeoporus dichrous has a white hairy cap like a small shelf, but its fertile layer is a sharply demarcated, dark purple-brown, gelatinous sheet with true, small, round pores that peels away cleanly from the flesh.

True polypores such as Trametes have a distinct tube layer visible in section and tough, corky flesh.

Jelly fungi such as Tremella are gelatinous but form lobed, brain-like masses with no organised folded hymenium at all.