Hypoxylon michelianum







Hypoxylon michelianum Ces. & de Not.

Stromata effused, 7-25 mm long x 4-8 mm broad x 1 mm thick, surface black, carbonaceous, with conspicuous perithecial mounds; dark brown to blackish granules beneath surface with KOH-extractable pigments honey (64) to olivaceous buff (89); the tissue below the perithecial layer 250 Ám thick, blackish.

Perithecia spherical, 600-700 Ám diam.

Ostioles higher than the stromatal surface, conical, papillate, black, encircled with a concave truncatum-type disc 190-220 Ám diam.

Asci 180-220 Ám total length x 5-6 Ám broad, the spore-bearing parts 70-95 Ám long, the stipes 110-140 Ám long, with apical ring inamyloid to weakly amyloid, discoid, 1.5 Ám high x 2-2.7 Ám broad.

Ascospores brown to dark brown, ellipsoid-inequilateral with narrowly rounded ends, 9.5-13 x 4-4.8 Ám (M = 10.8 x 4.5 Ám), with straight germ slit spore-length; perispore indehiscent in 10% KOH.

Anamorph: not seen.

Habitat: on bark or wood, possibly specific for Laurus sp.

Known distribution: southern Europe: Italy, Portugal, Spain.

Specimen examined: SPAIN, Vigo, Island of Cortegada, Villagarcia, 21 Nov. 1985, FC-5285-4, on Laurus nobilis.

Notes: our description of H. michelianum is based on only one specimen, identified by Prof. J.D. Rogers, and is incomplete in one respect: the material we studied is fully mature and lacks the typical white pruinose covering encountered in young stromata. In addition, ascospores are slightly smaller than reported by Ju & Rogers (1996), 9.5-13 x 4-4.8 Ám vs.11-15 x 4.5-5.5 Ám, and ascal apical rings are weakly amyloid to frequently inamyloid.

Hypoxylon michelianum is a rare species, known from only a few collections, and its intraspecific variations are probably still poorly known. It has been included in a recent investigation of some Hypoxylon species of the Annulata group (Quang et al., 2005) and, as shown by HPLC profiling, contains traces of BNT, with BNT derivatives and cohaerin-like azaphilones as major metabolites. It belongs to the group of few species with KOH-extractable pigments that are not dull green.

Its field identification relies on the combination of two features: perithecia are conspicuously prominent and ostioles are papillate and encircled with a small concave disc. Hypoxylon stygium var. annulatum is another European taxon with black, effused, carbonaceous stromata and papillate ostioles at the centre of a concave disc. In this species, discs are broader and perithecial mounds are inconspicuous. Moreover, it differs from H. michelianum in having dull green KOH-extractable pigments and much smaller ascospores 6-8.2 x 2.8-3.4 Ám.