Arthonia Ach.
Type : Arthonia radiata (Pers.) Ach. [=Opegrapha radiata Pers.]
Description : Thallus crustose, effuse or delimited by brown lines, ecorticate or immersed, corticolous or foliicolous or lichenicolous. Photobiont Trentepohlia, some taxa saprophytic, weakly lichenised or lichenicolous. Ascomata apothecia, adnate to sessile or immersed, immarginate presenting a somewhat "rubbed down" appearance, rounded to irregular, stellate or ±lirellate. Disc red-brown to black. Thalline exciple absent. Epithecium colourless to red-brown or dark-brown. Hymenium I+ blue or red, IKI+ blue. Hypothecium ±absent to conspicuous, often poorly delimited from hymenium. Hamathecium of few to numerous, sparingly to richly branched and anastomosing paraphysoids in a gel matrix, often more richly branched above and there pigmented or with crystals; apices often with thin, well-defined, dark caps 2–4 μm wide. Asci 8-spored, clavate, ellipsoidal or subglobose, semi-fissitunicate, with large apical dome and usually distinct ocular chamber; apical dome mostly IKI− or IKI+ bluish in lower part near apex of ocular chamber. Ascospores mostly ovoid to oblong-ovoid 1–7-septate, sometimes with an enlarged apical cell, often constricted at the mid-septum, colourless and smooth; mature spores often brownish and ±warted. Conidiomata pycnidia, usually immersed, very small and inconspicuous. Conidia colourless, simple, bacillar to thread-like.
Key
Arthonia is a large, cosmopolitan genus of c. 500 species (Sundin & Tehler 1998; Kirk et al. 2001), included in the family Arthoniaceae (Eriksson et al. 2004; Pennycook & Galloway 2004; Eriksson 2005), of which c. 45 are lichenicolous fungi (Alstrup & Hawksworth 1990; Etayo 2002; Follmann & Werner 2003; Calatayud et al. 2004a; Ihlen et al. 2004b), the remainder being lichenised. Although the genus is much in need of revision, much useful information on it is to be found in the following accounts (Santesson 1952; Coppins 1989b, 1992a; Alstrup & Hawksworth 1990; Kantvilas & Vězda 1992; Wedin 1993b; Egea & Torrente 1995; Grube et al. 1995, 2004c; Lücking 1995b; Matzer 1996; Wedin & Kondratyuk 1997; Sundin & Tehler 1998; Wedin & Hafellner 1998; Sundin 2000; Etayo 2002; Follmann & Werner 2003; Calatayud et al. 2004a). As presently circumscribed, Arthonia is paraphyletic (Sundin & Tehler 1998) and includes taxa with two different ascus types, one is broadly clavate to saccate, the other pyriform with a broad apex. New Zealand taxa are still very imperfectly known and the genus strongly deserves collection and monographing here. The present account discusses 29 species, 11 of them being lichenicolous.
