Mycobank MB 517715
Thallus thin, grey-green powdery, effuse, turning lemon-yellow in herbarium or almost absent except as thin veil at margins of excipulum. Pigments in thallus dissolving K+ yellow in section (best observed adjacent to or below lirellae). Photobiont trentepohlioid. Lirellae erumpent-sessile, mostly simple, few- or stellate-branched, scattered evenly over the thallus, with thin powdery lateral thalline cover, 0.3–0.6(–1) mm long, 0.1–0.3(–0.4) mm broad. Exciple margins completely carbonized and raised above disc, generally slit-like at first and mostly gaping to broadly exposed later, brownish-yellow at inside margins, K– in section. N+ red. Disc mostly exposed, dark brown and often with green (yellow in herbarium) pruina, dissolving in K to give bright yellow. Hymenium 40–60 µm, I+ red. Ascospores 8/ascus, (3–)4–5 septate, (14)–15–19(–22) x (3–)4–5(–6) µm, hyaline, excluding perispore (1 µm), with medial cells somewhat enlarged, and infrequent slight constriction below these. Perispore of old ascospores turning brown and roughly wrinkled. Pycnidia frequent, pale-green pruinose, sessile 0.07–0.12 mm diam., with short bacilliform conidia 2.5–3 x 1–1.3 µm.
Opegrapha is a large genus of ca. 300 species, with both lichenized and lichenicolous species (Kirk et al., 2008). The genus is generally characterized by having crustose, non-corticate thalli and carbonized, elongate to rounded lirellae, transversely-septate and I– ascospores, but the large size of the genus results in few reliable morphological features common across the genus (Ertz, 2009). The new species is most likely to be confused with O. varia Pers., but is distinguished by smaller and fewer septate ascospores (mostly 5–6-septate in O. varia), generally smaller lirellae and pycnidia, and smaller conidia. In O. varia the pycnidia are mostly (0.1–)1.2–2(–2.5) mm diam., with conidia that are either ellipsoid to pyriform, 3.5–4.5 x 1.5–2 µm, or bacilliform, 3.8–5.5(–6.5) x 1–1.5 µm. Opegrapha varia generally has a whiter thallus, lacking the yellowish coloration present in many collections of the new species, even when the apothecia or pycnidia are greenish pruinose. The same is true for O. herbarum Mont, and is very evident in collections on lignum where it grows side-by-side with O. viridipruinosa. Opegrapha herbarum differs further in having 3-septate ascospores, and apparently never producing pycnidia. Opegrapha xerica Torrente & Egea differs in having a K+ green epithecium and exciple, and longer conidia. The new species is most frequently found in the eastern, drier parts of Britain and Northern Ireland, and is found in both pycnidiate and sexual forms. It is so far known from SE England, E Scotland, W Scotland (rare), and Northern Ireland. The earliest collection dates only from 1991, and it is tempting to suggest that it has only recently entered the British Isles or, at least, expanded greatly in E Scotland and E England following the amelioration of high SO2 levels since the 1980s. It grows on a range of hardwood trees, mainly in woodland or woodland edge situations, and associated species include Anisomeridium biforme, A. polypori, Bacidia delicata, Chaenotheca brachypoda, Chrysothrix candelaris, Cliostomum griffithii and, on lignum, Opegrapha herbarum and O. ochrocheila. It occurs on dry bark and does not seem to enter the Xanthorion communities, as can the similar O. herbarum and O. varia. It is frequent in secondary woodland and is not strongly linked to old-growth woodland or veteran trees, as is O. xerica.
From the yellowish-green powdery pruina, conspicuous on the discs and pycnidia.
British Isles; on bark and lignum of hardwood trees (Acer platanoides, A. pseudoplatanus, Betula, Fraxinus, Quercus, Sambucus and Ulmus).
Type:—ENGLAND: East Suffolk, Needham Market; 52°9’N, 1°4’E; on Sambucus bark in sheltered, disused chalk pit; Jun 2004, Earland-Bennett & Hitch s.n. (holotype E).
Additional specimens examined (paratypes):—ENGLAND: East Suffolk: Sudbourne; Jan 2002, Earland-Bennett & Hitch s.n. (E); Pettistree, Byng Hill Lane; Mar 1997, Earland-Bennett (E). NORTHERN IRELAND: Londonderry, SSW of Dungiven, Banagher Woods; Coppins et al. 14438 (E). SCOTLAND: East Lothian: Oak Wood; Coppins 22638 (E); Binning Wood (NW); Coppins 22505 (E); Peebles: West Linton, Coaly Burn; Coppins 22819 (E).
Sicut Opegrapha varia sed thallo tenui, griseoviridi, pulverulento, saepe sulphureo suffuso, ascosporis brevioribus, 4-5 septatis, pycnidiis et conidiis parvioribis differt.