Arthonia didyma Körber

The thin thallus is pale fawn to olive-grey with often a pink hue when fresh. It is smooth and immersed in the substrate. The margin is not determinate. The thalli are extensive or occur in tiny patches that are often found around small elevations on the host bark like lenticels, leaf scars, or bases of twigs and branches. The photobiont is a species of the family Trentepohliaceae.

The apothecia are maculate, dark orange-brown to brown-black, and without pruina. They are flat to slightly convex, irregularly rounded, elongated or indistinctly branched, 0.06–0.6 × 0.06–0.3 mm in size, and 45–70(–100) μm tall.

The epithecium is reddish brown or indistinct.

The hymenium is colorless or pale orange-brown, and 30–50 μm tall.

The hypothecium is reddish brown and up to 30 μm tall.

The paraphysoids are ca 1 μm wide. Their tips are often enlarged to 2.5 µm and brown-walled.

The asci are narrowly clavate, 42–55 × 12–18 µm, and 8-spored.

The spores are colorless first, with a brown pigmentation and granular ornamentation when old. They are obovoid, 12–18 × 5–8 μm in size, and divided by 1 transverse septum.

The pycnidia are reddish brown, immersed in the thallus, and 40–60 µm in size. The wall is red-brown. The rod-shaped conidia are 3.5–4 × ca 0.7 μm in size and straight.

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). Unidentified anthraquinones have been detected in some individuals by TLC.

The hymenium reacts I+ blue and KI+ blue. A KI+ blue ring structure has been observed in the asci.

The reddish-brown pigment in the apothecia and the wall of the pycnidia changes to grey to olive in K solution. If present, the additional orange-red pigment in the apothecia changes to purple-violet in K solution.