Hypoxylon cohaerens











Hypoxylon cohaerens (Pers.: Fr.) Fr.

Stromata pulvinate, constricted at base, often containing few perithecia and then shortly stipitate, becoming coalescent, with inconspicuous perithecial mounds, 1.5-5 mm diam x 1.5-4 mm thick; surface dark brick (60) to chestnut (40), eventually black, orange brown granules just beneath surface with KOH-extractable pigmentsdark vinaceous (82) when young, greyish olivaceous (90) to olivaceous (48) when mature; the tissue below the perithecia brown to blackish brown, up to 3.5 mm thick.

Perithecia spherical to ovoid, 450-520 m diam x 500-680 m high.

Ostioles papillate, black, frequently, on mature stromata, at the centre of a discoid depression 120-150 m diam.

Asci 118-180 m total length, the spore bearing-parts 56-88 x 6-6.8 m, the stipes 60-95 m long, with apical ring amyloid or inamyloid, 1-1.2 m high x 2.2-2.5 m broad.

Ascospores brown to dark brown, ellipsoid-inequilateral, 8.8-12.2 x 3.4-5.5 m ( M = 9.9 x 4.3 m), with straight germ slit spore-length; perispore dehiscent in 10% KOH, smooth, with a thickening on the more convex side.

Anamorph in nature: fawn (87) to greyish sepia (106), velvety, covering the young stromata.Conidiogenous cells 15-30 x 2-2.8 m; conidia ellipsoid, 5-6 x 3-4 m. The conidiogenous structure is Virgariella-like.

Habitat: primary saprophyte on bark or wood of Fagus sp.

Known distribution: Europe, North America.

Specimens examined: FRANCE, Arige (09): Rimont, Las Muros, 11 Sept. 1996, JF-96091, on Fagus sylvatica; Rimont, Grand Bois, 26 Apr. 1999, JF-99080, on Fagus sylvatica; Rimont, Grand Bois, 23 Sept. 1999, JF-99236, on Fagus sylvatica; Rimont, Las Muros, 03 Aug. 2000, JF-00141, on Fagus sylvatica; Rimont, Las Muros, 17 Jan. 2001, JF-01005, on Fagus sylvatica; Ustou, Cirque de Cagateille, 26 Aug. 2001, JF- 01175, on Fagus sylvatica.

Notes: Hypoxylon cohaerens is a common saprophyte of beech forests, characterized by black pulvinate stromata constricted at base with papillate ostioles. Two other taxa, H. cohaerens var. microsporum and H. multiforme, share similar stromatal features, and field identification relies mainly on host identification: H. cohaerens is restricted to Fagus while H. cohaerens var. microsporum is restricted to Quercus and Castanea and H. multiforme grows mostly on Betula, Alnus and Corylus. However, one collection made on Ligustrum vulgare, a member of Oleaceae, [Hautes Pyrnes (65), Bagnres de Bigorre, L'Arbizon, 04 Sept. 2002, JF-02154, leg. FC] proves that host specificity is rarely absolute.

Stromatal KOH-extractable pigments vary as stromata become mature, they are dark vinaceous when recorded from anamorphic state, while black mature stromata yield typical olivaceous pigments. Along with BNT, unknown compounds were already reported from H. cohaerens by Mhlbauer et al. (2002), which were identified by Quang et al. (2005) as two new azaphilones named cohaerins A and B, which lack in related species H. cohaerens var. microsporum and H. multiforme and other members of section Annulata submitted to HPLC analyses. These cohaerins are only present in mature stromata, while yet unknown metabolites occur in younger stromata (Quang et al., 2005), which corroborates coloured reactions in KOH we observed. This peculiarity makes this last feature somewhat difficult to use in distinguishing H. cohaerens from its allies, while HPLC profiling allows for an unambiguous identification, even from very old material like the Persoon's type collection dating back prior to 1797 (Quang et al., 2005).

When HPLC is not available, the separation of H. cohaerens from its allies relies on microscopic observation of ascospores germ slit: it is spore-length in H. cohaerens but shorter in H. cohaerens var. microsporum and H. multiforme. The both latter differ in ascospore size.

It is noteworthy that, beside its variability in KOH-extractable pigments, H. cohaerens shows variability in the reaction of ascal apical ring to Melzer's reagent and a tendency to have stromata containing very few perithecia, some of them being even uniperitheciate.