Hypoxylon stygium (Lév.) Sacc. var. annulatum (Rehm) Ju & Rogers.

Stromata pulvinate to hemispherical on bark, erumpent, 2-12 mm diam x 1-2 mm thick, effused on wood, 5-50 mm long x 2-30 mm broad x 0.5 mm thick, with inconspicuous perithecial mounds; surface carbonaceous, bay (6) when young, blackish when mature, long with a brown tinge; dull yellow to pale orange granules beneath surface, with KOH-extractable pigments dull green (70); the tissue below the perithecial layer inconspicuous or up to 0.8 mm, blackish brown.

Perithecia spherical to obovoid, 400-500 µm diam x 500-750 µm high, enclosed in a carbonaceous stromatal layer.

Ostioles papillate, conical, shiny black, encircled with a convex truncatum-type disc 250-380 µm diam.

Asci 85-100 µm total length, the spore bearing-parts 58-75 µm long x 3.5-4 µm broad, the stipes 25-40 µm long, with apical ring discoid, inconspicuous, weakly amyloid or inamyloid, 0.5 µm high x 1 µm broad.

Ascospores light brown, ellipsoid-inequilateral, 6-8.2 x 2.8-3.4 µm (M = 7.3 x 3.1 µm), with straight germ slit spore-length on the flattened side; perispore rarely dehiscent in 10% KOH, smooth.

Anamorph in nature: not observed. Anamorph in culture is Periconiella-like (Ju & Rogers, 1996).

Habitat: on bark and wood of various hosts. Collected on Acer negundo, Betula pendula, Castanea sativa, Quercus robur.

Known distribution: France, Russia.

Specimens examined: FRANCE, Pyrénées Atlantiques (64): Osserain, Bel Air, 07 Nov. 2003, JF-03230, on bark of Castanea sativa; Pau, Bois Bastard, 11 Jun. 1995, JF-95002, on Castanea sativa; Oloron Ste Marie, bois de Larbaig, 26 Oct. 1998, JF-98147, on Quercus robur ; Auterrive, 05 Sept. 1999, JF-99199, on Acer Negundo. La Réunion (974), Saint Philippe, Forêt Mare Longue, 20 Apr. 1999, FC-5285-1, leg. G. Gilles, on unidentified wood (H. stygium).

Notes: Hypoxylon stygium var. annulatum is a distinctive taxon, characterized by the combination of the following features: black, carbonaceous stromata, with papillate ostioles encircled with a disc, dull green KOH-extractable pigments and small-sized ascospores with the germ slit on the flattened side. This last feature is unique among the known European species of Hypoxylon.

Hypoxylon michelianum is another European taxon with ostiolar discs and papillate ostioles, but its stromata are whitish and pruinose when young, KOH-extractable pigments are yellowish, its perithecia are conspicuously exposed, with smaller ostiolar discs, and its ascospores are larger (10-14.5 x 4.5-5.5 µm).

Hypoxylon stygium (Lev.) Sacc., the typical variety, is widespread in the tropics. Ju and Rogers (1996) erected the variety annulatum for collections from Europe having larger perithecia and larger ostiolar discs (250-380 µm diam versus 120-200 µm diam for the former). Comparison with a collection of H. stygium (FC 5285-1) from La Réunion showed that in the variety, not only the perithecia and the ostiolar discs are larger but the ascospores also are slightly larger, apical rings are mostly inamyloid and perispores mostly indehiscent. The results of HPLC analyses (Quang et al., 2005) support the close relationship between H. stygium and its variety annulatum: both contain a BNT derivative named daldinone A as prevailing metabolite, along with BNT, but H. stygium differs in having large amounts of truncatone that is only present in its variety annulatum as traces. Daldinone A is noteworthy in being present as well in Daldinia concentrica (Quang et al., 2002b), in two members of the section Hypoxylon, i. e., H. intermedium and H. hypomiltum. var lavandulocinereum (Hellwig et al., 2004), and in another member of the section Annulata, H. bovei var.microspora (Quang et al., 2005)

Hypoxylon stygium var. annulatum is not uncommon in the French departments Gironde, Landes and Pyrénées Atlantiques, where temperature is usually very mild in winter. It is noteworthy that, despite an active search, it was not collected in Ariège where oceanic influence is less sensible and winters are somewhat colder. Its distribution in Europe is poorly known and should be completed by additional records that can be expected from southern Europe.