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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 µm thick,
blackish.
Perithecia spherical, 600-700 µm diam.
Ostioles higher than the stromatal surface, conical, papillate,
black, encircled with a concave truncatum-type disc 190-220 µm
diam.
Asci 180-220 µm total length x 5-6 µm broad, the spore-bearing
parts 70-95 µm long, the stipes 110-140 µm long, with apical ring
inamyloid to weakly amyloid, discoid, 1.5 µm high x 2-2.7 µm broad.
Ascospores brown to dark brown, ellipsoid-inequilateral with
narrowly rounded ends, 9.5-13 x 4-4.8 µm (M = 10.8 x 4.5 µm), 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 µm vs.11-15 x 4.5-5.5 µm, 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 µm.
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