Vegetative morph: bulbils (sclerotia) 70-100 µm diam., singly or in small clusters, pale orange, not corticate (thus appearing lich lichen soredia), developing on the surface of the host thallus which is substantially degraded.
Teleomorph: basidiomata to ca 10 mm diam., pale orange, thin and effuse, granular to floccose, the margin indeterminate, closely adnate to the host thallus surface. Hymenium comprising one or several layers of basidia on vertically branching, thin-walled hyphae. Basidia 25–40 x 12–15 mm, distinctly wider than the supporting hyphae, initially narrowly cylindrical, sometimes with a basal or lateral, ellipsoidal probasidial bladder; when mature, generally becoming clavate to suburniform, occasionally with one thin, transverse septum in the upper third. Basal clamps absent, the basal septum with a dolipore visible by light microscopy. Sterigmata four per basidium, to 6.5 µm long and 3 µm wide at the base, curved. Basidiospores 13–17.5 (–18.5) x 8–11.5(–13.5) µm, pyriform or lacrymiform, one side frequently flattened or slightly concave, hyaline, smooth, usually aseptate, not staining in iodine, with a prominent truncate apiculus 1.5–3 µm diam, occasionally with one thin, transverse septum, the wall then distinctly constricted. Hyphidia, cystidia and other sterile hymenial elements absent.
Assessed by Woods & Coppins (2012) as of Least Concern. The species is common and widespread, and probably significantly under-recorded.
Distinguished most easily from Marchandiomyces species and other similar basidiomycetous lichen parasites by the pale orange rather than pink bulbils.
Marchandiobasidium aurantiacum is an aggressive parasite of corticolous Physcia species, sometimes also spreading onto neighbouring lichens, especially Xanthoria parietina.
Scattered throughout the British Isles and Ireland, probably more or less coextensive with its host lichens. BLS map here.