Arthonia graphidicola does not form a conspicuous thallus of its own, instead developing within the thallus of
Graphis scripta. The apothecia are tiny and scattered, initially developing beneath the bark surface and then breaking through as small, fleck-like
fruiting bodies. They are rounded to somewhat angular, about 0.06–0.16 mm across, or elongate (often tapering to pointed ends) and up to about 0.42 mm long; they are reddish- to dark-brown and lack . Under the microscope, the apothecia are about 60–85
micrometres (μm) tall, with a reddish-brown
hymenium that turns dull olivaceous in
potassium hydroxide solution (
K), and gives a red reaction with iodine. The is hyaline and iodine-positive (I+ blue). The
asci are and 8-spored (about 35–40 × 15–17 μm), and in K/I typically show only a minute
amyloid ring at the apex. The ascospores are oblong to ovoid, macrocephalic, and 2–3-septate, measuring (13–)14–17 × 4.5–5.5 μm; they are initially hyaline and smooth, but later become covered in dark brown granular warts.
Pycnidia are uncommon. When present they are immersed (about 40 μm across) and produce hyaline, rod-shaped
conidia around 4.5–5.5 × 0.8 μm.
Arthonia graphidicola is one of the few lichenicolous
Arthonia species reported from hosts in the Graphidaceae, and it can be confused with
A. diorygmae because both have hyaline ascospores of similar size (about 13–17 × 4–5.5 μm). It differs in its host and in fruiting-body characters:
A. graphidicola grows on the thallus of
Graphis scripta and forms very small, often cleft, fleck-like apothecia (typically 0.06–0.16 mm across, sometimes elongate), with a pale reddish-brown asciogenous layer; its ascospores are 2–3-septate and become brown and warty at maturity. A second comparable species is
A. thelotrematis (on
Thelotrema lepadinum), which has slightly smaller ascospores (about 11–14 × 4.5–5.0 μm) and a reddish-brown hypothecium. ==Habitat and distribution==