Sebacina incrustans

Sebacina incrustans grows as a spreading sheet that does not stay on one substrate. Starting on bare soil or litter, it advances outward and simply engulfs whatever it meets — grass and herb stems, moss, fallen twigs, dead leaves — encasing them in a smooth waxy layer so that a patch of ground looks as if it has been dipped in candle wax.

The genus Sebacina belongs to the Sebacinales, an order of heterobasidiomycetes distinguished by longitudinally septate basidia. Unlike the gelatinous Tremella species in the same broad grouping, Sebacina fruit bodies are firm, waxy and closely appressed, and their texture is closer to a rubbery paint layer than to a jelly.

Colonies can be large, sometimes running for half a metre or more along a bank or path edge, and are unmistakable once seen. Both the habit of enveloping living plant bases and the pale greyish-white, slightly translucent colour are hallmarks of the species, which occurs widely across Europe, Asia and North America.

Fruit body

An effused sheet, 1–3 mm thick, spreading indefinitely over soil, litter and low vegetation and coalescing into large patches. The advancing margin is usually paler, sometimes fringed or fibrillose, and can lift away from the substrate into short, irregular, blunt lobes or ridges up to a few millimetres high.

Surface

The fertile surface is smooth to very slightly bumpy or warted, without pores, teeth, wrinkles or gills of any kind. It is dull waxy rather than shiny, and colours run from white and greyish white through pale cream to dingy ochre-buff or faintly lilac-grey with age. In dry weather it becomes chalky and cracks into small plates.

Flesh and spore print

The flesh is thin, waxy and slightly rubbery when fresh, becoming brittle when dry; it does not change colour when cut and yields no liquid. Spore deposits are white. Under the microscope the basidia are longitudinally septate into four cells, the defining feature that separates Sebacina from ordinary corticioid crusts.

The species fruits on soil, moss and litter, most often at woodland margins, along shaded path and track edges, on banks and in grassy clearings among trees. It is strongly associated with ectomycorrhizal broadleaf trees, and colonies are usually within the root zone of oak, beech, hazel or lime.

Fruiting runs from late summer through autumn, occasionally persisting into early winter in mild years. It is widespread and locally frequent across temperate Europe and has been recorded across Asia and North America, though its low profile means it is often overlooked.

Sebacinales are among the most ecologically flexible fungal orders known, with members forming ectomycorrhizas, orchid mycorrhizas and root endophytic associations. Sebacina incrustans is generally treated as ectomycorrhizal with broadleaf trees, which explains its consistent occurrence on soil beneath oak and beech rather than on isolated dead wood.

The enveloping habit of its fruit bodies is a surface phenomenon: the crust simply grows over anything in its path, living or dead, without decaying it, using the enveloped stems and twigs as extra scaffolding for spore-bearing surface area. Colonies are annual, breaking down over winter, but frequently reappear on the same bank in successive seasons from a persistent soil mycelium.

Sebacina epigaea is closely related but forms thinner, more distinctly greyish or brownish patches directly on bare soil and banks, without the same wholesale enveloping of standing stems, and it often shows a more sharply defined, slightly reflexed edge. Tremellodendron pallidum, another sebacinoid fungus, rises into tall fused branched clumps rather than remaining a crust.

Ordinary corticioid crusts in genera such as Athelia, Hypochnicium and Byssocorticium can look similar from a distance, but they are cottony, membranous or floccose rather than waxy, are almost always on wood rather than soil, and lack the smooth continuous coating that unites soil, stems and litter into one sheet. Peniophora species are pinkish, thin and confined to attached twigs and branches. White slime moulds occasionally spread over litter in a comparable way but are ephemeral, collapsing to powdery masses within a day or two.