Jelly Rot

Phlebia tremellosa, long known as Merulius tremellosus, is one of the most eye-catching wood crusts of the late season. It begins as a flat sheet spreading over the underside or cut face of dead wood, then lifts its outer margin into a narrow, wavy bracket, so a single colony can be simultaneously resupinate and shelf-forming.

The genus Phlebia is defined by its merulioid fertile surface: instead of clean tubes or spines, the spore-bearing layer is thrown into a network of low wrinkles, folds and shallow pits that only imitate pores. In this species that surface is soft, translucent and gelatinous, and coloured a warm salmon, apricot or pinkish orange that contrasts sharply with the white, felty upper side.

The fungus is common and widespread across the temperate Northern Hemisphere, and is one of the more thoroughly studied white rot species because of its efficient breakdown of lignin. In the field it is most often noticed on the shaded underside of large fallen trunks, where the gelatinous fruit bodies stay plump long after other autumn fungi have collapsed.

Fruit body

The colony spreads as an effused sheet from a few centimetres to more than 30 cm across, often coalescing from several patches. Along the upper edge it becomes effused-reflexed, forming shelves 5–25 mm wide with a wavy, sometimes lobed outline. The upper surface of these shelves is white to cream, densely covered with fine soft hairs, and may pick up green algal staining with age.

Fertile surface

The underside is the diagnostic feature: a soft, waxy-gelatinous layer thrown into radiating wrinkles, ridges and irregular shallow pits, giving a merulioid or half-formed poroid look. Colour ranges from pale salmon and apricot to deep orange-pink, fading to dull ochre or brownish as the fruit body dries.

Flesh and spore print

The flesh is thin, translucent and rubbery-gelatinous, drying to a hard, shrunken, horn-like film that can rehydrate after rain. It shows no colour change when cut and exudes no liquid. The spore deposit is white; spores are small, cylindrical and slightly curved.

Jelly Rot fruits on dead broadleaf wood — birch, beech, alder, willow, poplar and oak are all frequent hosts — and appears less often on conifer logs. It favours large fallen trunks, stumps and thick branches lying in damp shade, and typically colonises wood that has already lost its bark.

Fruiting is concentrated in autumn and can continue into early winter, with the gelatinous fruit bodies tolerating light frosts and reviving after rain. The species is widespread throughout Europe, temperate Asia and North America.

The fungus is a saprotroph causing a white rot, degrading lignin along with cellulose so that the residual wood becomes pale, soft and fibrous. It is a strong secondary coloniser, moving into logs after bark loss and often occupying wood already partly broken down by other decay fungi.

Colonies are annual, forming new gelatinous sheets each autumn from mycelium that persists inside the log. The dried remains of an old season's fruiting frequently stay attached as a thin varnished crust and can be found alongside the new growth.

Phlebia radiata shares the wrinkled merulioid surface and orange colouring but stays fully resupinate, never forming a hairy white reflexed shelf, and its ridges radiate outward from a central point like the spokes of a wheel. Byssomerulius corium has a similarly white, effused-reflexed form but its fertile surface is pale cream to buff, only faintly wrinkled, and its texture is membranous and leathery rather than jelly-like.

Serpula himantioides and its relatives produce a comparable folded fertile surface but in rusty brown tones and yield a brown, not white, spore deposit. Trametes and other true bracket polypores are firm and dry with distinct, regularly bounded tubes rather than a soft mesh of folds, and their upper surface is tough rather than gelatinous.