Leptogium is a genus of lichen-forming fungi in the family Collemataceae. It has about 110 species. Species formerly classified under Leptogium have since been divided among the genera Leptogium, Pseudoleptogium, and Scytinium. Leptogium lichens are predominantly found on tree bark or soil, often among mosses, and sometimes on rocks in moist environments.
In 2013, a proposal supported by molecular phylogenetics data was made to conserve the genus Leptogium with a conserved type, aiming to maintain the current broader classification including both small- and larger foliose species within Leptogium, while segregating the smaller squamulose species into Scytinium. This conservation was recommended by a vote of 14-0-1 to prevent the necessity of reclassifying about 100 species into new genera such as 'Mallotium. The proposal was widely supported by the Nomenclature Committee for Fungi as it simplifies the taxonomy and maintains historical continuity for the genus Leptogium.
Leptogium is characterized by its foliose, gelatinous thallus, which varies in colour from blue-grey to olive-brown or blackish. The upper and lower of these lichens consists of a single layer of roughly equal-sized cells. The medulla, on the other hand, is made up of loosely intertwined hyphae intermingled with chains of cells. The upper surface of the thallus can range from smooth to wrinkled or ridged, often exhibiting a glossy appearance, while the lower surface may be smooth, web-like, or hairy, sometimes featuring small clusters of white rhizines.
Leptogium species often possess isidia and have Nostoc as their photobiont. The ascomata are, which can be sessile or shortly stalked and are predominantly . The may persist or become excluded, occasionally displaying a form. The is raised, cup-shaped, and mainly composed of arranged hyphae, with a colour that varies from colourless to reddish-brown. The of the lichen is either concave or flat.
The is colourless to reddish-brown and does not react with solutions of either potassium hydroxide or ammonia. The hymenium is colourless and turns blue when exposed to iodine. The is shallow and either colourless or pale yellowish. The consists of numerous, with slightly swollen apices. The asci are club-shaped (clavate) and contain four to eight spores, with the wall and turning blue in response to potassium hydroxide and iodine. are ellipsoidal, ovoid, or fusiform, sometimes featuring an apiculate end, and are septate and colourless. Lichen substances have not been detected by thin-layer chromatography in Leptogium species.
, Species Fungorum (in the Catalogue of Life) accepts 84 species of Leptogium.