Perithecia spherical to obovoid, 500-700 µm diam x 600-700 µm high, discretely surrounded by a carbonaceous stromatal layer.

Ostioles papillate, conical, black, sometimes at the centre of a discoid depression 150-230 µm diam.

Asci 150-195 µm total length, the spore bearing-parts 64-86 µm long x 5.5-6 µm broad, the stipes 80-120 µm long, with apical ring discoid, amyloid, 0.8-1.2 µm high x 2-2.8 µm broad.

Ascospores light brown to brown, ellipsoid-inequilateral, frequently crescent-shaped, 8.8-10.3 x 3.7-4.9 µm (M = 9.9 x 4.2 µm), with faint straight germ slit less than spore-length; perispore usually indehiscent in 10% KOH, smooth, with a thickening on the more convex side.

Anamorph in nature not observed

Habitat: on bark or decorticated wood of branches, trunks or stumps. Mainly found during this study on Alnus, Betula and Corylus, more rarely on Rosaceae (Prunus avium, Prunus spinosa, Sorbus aucuparia) and once on Populus tremula.

Known distribution: Asia, Europe and North America

Specimens examined: FRANCE, Aričge (09): Montseron, Roquebrune, 01 Aug. 1999, JF-99171, on Prunus spinosa; Rimont, Las Muros, ruisseau de Peyrau, 10 May 1996, JF-96052, on Corylus avellana; Rimont, Las Muros, 25 Mar. 1997, JF-97022, on Alnus glutinosa; Rimont, Grand Bois, 22 Feb. 1999, JF- 99030, on Betula pendula; Saurat, Bois de Lagrié, 16 Aug. 1999, JF-99201, on Sorbus aucuparia. Pyrénées Atlantiques(64): Sauveterre de Béarn, ile de Sauveterre, 26 Jun. 1999, JF-99129, on Alnus glutinosa.

Notes: Hypoxylon multiforme is characterized by pulvinate to widely effused-pulvinate coalescent stromata, with conspicuous perithecial mounds and conical papillate ostioles, growing on various hosts. In the field it can be confused with H. cohaerens and its variety microsporum, although they both are constricted at base and restricted to Fagus for the former and to Quercus and Castanea for the latter. Microscopically, H. multiforme differs from H. cohaerens in its faint, less than spore-length germ slit, while it is conspicuous and spore-length in H. cohaerens. It will be more easily distinguished from H. cohaerens var. microsporum which has smaller ascospores and vinaceous KOH-extractable pigments.

As pointed out by Professor Rogers (pers. communication) and Ju et al. (2004), and as observed in most of our specimens, KOH-extractable pigments of H. multiforme are frequently with orange shades, even from mature stromata. The taxonomic value of these variations is unknown but could correspond to chemical races (Rogers, pers. communication).

As to secondary metabolites (Quang et al., 2005), BNT is prevailing in H. multiforme, along with unknown compounds identical to metabolites found in H. cohaerens var. microsporum and H. michelianum, which might be azaphilones related to cohaerin A and B isolated from H. cohaerens. The four aforementioned species differ from other members of the Annulata section in having KOH extractable-pigments that are not dull green (70) and in lacking BNT derivatives (Quang et al., 2005).

Unlike Ju and Rogers (1996) but like Petrini and Müller (1986) we observed in all collections that perispores were mostly indehiscent in 10% KOH. Moreover, in all collections examined, and in some collections of H. cohaerens and H. cohaerens var. microsporum as well, some perithecia display ostioles at the centre of a discoid depression, a feature not previously described and supporting the close relationship of H. multiforme with other members of the section Annulata erected by Ju & Rogers (1996).