Leucodecton forms a thin, crust-like thallus that lies flat on the bark or rock surface. Its upper layer is usually cream to pale fawn; the outer "skin" (cortex) is only weakly developed, giving the surface a smooth to slightly knobbly look. Large, irregular crystals may be embedded in the interior, but the border (prothallus) that some lichens display is missing. Vegetative propagules used for asexual spread—powdery soredia or tiny finger-like isidia—occasionally occur. The photosynthetic partner is a filamentous green alga from the genus Trentepohlia, which lends an orange tint when exposed in damaged areas.[2]
The reproductive bodies are flask- to urn-shaped apothecia that start out immersed in the thallus and often push up in small wart-like clusters. Their black discs are usually concave, sometimes dusted with a grey bloom (pruina), and are framed by a rim of thallus tissue (thalline exciple) that may split irregularly as the fruiting body matures. Beneath this rim lies a true exciple—an internal ring of densely intertwined fungal hyphae—seen in a view from above. Within the apothecium, the colourless hymenium houses slender asci, each with a single functional wall layer that thickens abruptly at the tip. The asci typically hold one to eight ascospores.[2]
Spores are elongated to spindle-shaped, with several internal walls; they begin colourless and thin-walled but often darken and develop thick, laminated walls as they age. No conidiomata (structures that produce asexual spores) have been observed. Chemically, most species produce stictic or norstictic acid, secondary metabolites that are useful for species identification through thin-layer chromatography.[2]
