Arthonia graphidicola

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 pruina. 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 hypothecium is hyaline and iodine-positive (I+ blue). The asci are clavate 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. …

In Britain, Arthonia graphidicola has been recorded only as a commensal on Graphis scripta growing on hazel (Corylus) bark. The type collection is from Benderloch (Argyll), and additional specimens were reported from other western Scottish localities, including Loch Sunart (West Inverness-shire), Seil (Argyll), and several sites on Skye (Tokovaig Wood, Kilmore hazelwood, and Coille Gaireallach). These records suggest a distribution centred on mild, oceanic districts in western Scotland, in habitats supporting well-developed populations of the host lichen on hazel.