Arthonia radiata (Pers.) Ach.

The extensive thallus is white to light grey with often a brownish or olivaceous hue. It is smooth and immersed in the tree bark to superficial. The margin is either not determinate or bordered by a thin brown line. The photobiont is a species of the family Trentepohliaceae.

The apothecia are brownish black, without pruina, and highly variable in shape. They are irregularly maculate to bluntly star-shaped, but more often clearly elongated and branched. The apothecia are immersed in the thallus to partially raised, 0.2–1.5 × 0.1–0.8 mm in size or in stellate clusters up to 2.2 mm in diameter. They are 100–130 μm tall.

The epithecium is 8–15 µm tall and olive-brown.

The hymenium is colorless or tinged pale olive-brown, and 35–60 μm tall.

The hypothecium is unpigmented or pale olive-brown, and 5–30 μm tall.

The paraphysoids are 1–1.5 μm wide. Their tips are slightly widened to 2.5 µm but without distinct pigment caps or plaques.

The asci are clavate, without stipe, 28–50 × 13–18 µm in size, and 8-spored.

The spores are colorless, narrowly obovoid, 11–18 × 3–6 μm in size, and divided by 3 transverse septa. The apical cell is not enlarged.

The pycnidia are brownish black, immersed in the thallus or slightly raised, and 60–100 µm in size. The wall is olive-brown. The rod-shaped conidia are 4–7 × ca 1 μm in size and straight.

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). Lichen secondary compounds have not been detected by TLC. 

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

The olive-brown pigment in the exciple, epithecium, and the wall of the pycnidia changes to green in K solution.