Flora of New Zealand Series

Type : Platygrapha coccinea Leight. [=Chroodiscus coccineus (Leight.) Müll.Arg.]

Description : Thallus crustose, effuse, or continuous, corticate or with a thin cortex-like layer. Photobiont green, Phycopeltis or Trentepohlia.  Ascomata apothecia, ±immersed in thallus, but erumpent, their margins formed by the recurved lobes of ruptured tissue originially covering the apothecia. Exciple paraplectenchymatous. Hymenium I−. Hamathecium of paraphyses, simple, apices slightly expanded. Asci cylindrical-clavate, thin-walled, apex typically thickened when young, (1–)4–8-spored. Ascospores transversely septate to submuriform, colourless, halonate or not. I−. Conidiomata unknown.

The taxonomic history of Chroodiscus is outlined by Santesson (1952: 307–308). Hale's view is that taxa in Chroodiscus should be accommodated in Thelotrema (Hale 1980, 1981), a view also held by Eriksson & Hawksworth (1998), but Lumbsch & Vězda (1990) and later workers disagree, preferring to keep it as an independent genus in the Thelotremataceae (Eriksson et al. 2004; Pennycook & Galloway 2004; Eriksson 2005). Thirty, mainly tropical taxa are known (Kirk et al. 2001), the majority being foliicolous (Santesson 1952: Lumbsch & Vězda 1990; Lücking 1992, 1999d; Santesson & Lücking 1999, Frisch 2006), though recently described taxa are corticolous or grow over peaty soil or on bryophytes (Kantvilas & Vězda 2000; Messuti et al. 2003a). A key to the foliicolous taxa is given in Santesson & Lücking (1999: 728). These authors note: "It should also be taken into account that a number of corticolous species currently referred to Ocellularia and Thelotrema in fact belong to Chroodiscus...and even undescribed corticolous species of that genus occur, and a complete survey of these taxa is necessary before conclusions on phylogenetic relationships within the genus can be drawn" (Santesson & Lücking 1999: 728–729). Kantvilas & Vězda (2000) give the most comprehensive recent account of the genus in our area. Chroodiscus is characterised by: a crustose thallus with Phycopeltis or Trentepohlia as photobiont; erumpent apothecia with laciniate and erect or recurved thalline margins formed of ruptured tissue; a non-amyloid hymenium; thick-walled asci with 4–8 ascospores; and colourless, thin-walled, transversely septate, submuriform to muriform ascospores. Three species occur in New Zealand, one on rock, rarely on roots (Malcolm et al. 1995b; Kantvilas & Vězda 2000) and two on bark (Dodge 1971: 457 [as Thelotrema macrocarpum]; Kantvilas & Vězda 2000; Galloway 2001b).