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Hypoxylon macrocarpum Pouzar.
Stromata pulvinate on bark, 3-11 mm long x 4-6 mm broad x 1-2 mm
thick, widely effused and elongated on decorticated wood, 25-70 (-450) mm
long x 4-40 mm broad x 1-1.2 mm thick, with conspicuous to inconspicuous
perithecial mounds; surface livid vinaceous (83) to livid red (56), later
dark vinaceous (82), with a
metallic shine when mature; margins effused, hazel (88) to livid red (56); yellowish
orange granules beneath surface, with
KOH-extractable pigments greyish sepia (106) to umber (9); the tissue
below the perithecial layer inconspicuous or up to 0.5 mm thick, greyish
sepia (106); underlying wood with a strong aromatic smell.
Perithecia obovoid to frequently tubular, 250-550 µm diam x
600-1000 µm high, rarely in two layers.
Ostioles lower than the stromatal surface.
Asci 120-165 µm total length x 6.5-7.5 µm broad, the spore
bearing-parts 56-78 µm long, the stipes 60-95 µm long, with apical ring
amyloid, discoid, 1 µm high x 2-2.5 µm broad.
Ascospores brown, ellipsoid-inequilateral, frequently slightly
crescent-shaped, 9.5-11.5 x 4-4.8 µm (M = 10.5 x 4.4 µm), with straight
germ slit spore-length; perispore dehiscent in 10% KOH, smooth.
Anamorph in nature: vinaceous buff (86), velvety, on young or old stromata.
Conidiogenous cells hyaline, smooth, 17-35 x 1.5-2.5 µm; conidia hyaline,
ellipsoid, 4-7 x 3-3.5 µm; conidiogenous structure
Virgariella-like.
Habitat: on bark or decorticated wood of various hosts.
Recorded during this study mainly on Acer campestre, Fraxinus
excelsior, Salix caprea and Ulmus minor. Occasionally found on
Alnus glutinosa, Corylus avellana, Crataegus oxyacanthae, Fagus
sylvatica, Prunus spinosa, Quercus robur and Tilia platyphyllos.
Known distribution: Europe (Czech Republic, Denmark, France, Germany,
Switzerland) and North America (Canada, U.S.A.).
Specimens examined: Ariège (09): Aulus les bains,
Agnesserre, 14 Sept. 1997, JF-97159, on Fagus sylvatica;
Rimont, Las Muros, ruisseau de Peyrau, 19 Jul. 1997, JF-97095, on Ulmus
minor; Rimont, Grillou, 14 Jul. 1999, JF-99147, on Corylus
avellana; Rimont, Grillou, 14 Jul. 1999, JF-99148, on Tilia
platyphyllos; Rimont, Las Muros, 27 Aug. 1999, JF-99194, on
Crataegus oxyacanthae; Rimont, Las Muros, 23 Sept. 2000, JF-00213,
on Salix caprea.
Notes: Prior to Pouzar's (1978) description of H. macrocarpum as a
distinct species, it has been long included within the
Miller's (1961)
wide concept of H.
rubiginosum.
It is readily identified in the field and distinguished from
H.
rubiginosum by its large to very large stromata, with a typical
metallic shine and by the strong aromatic smell of the underlying
substrate. Moreover it yields olivaceous brown to grey brown pigments in
10% KOH, while those of H.
rubiginosum are orange.
When growing on bark, H. macrocarpum usually has small pulvinate
stromata which could be confused with those of
H. fuscum
and H.
perforatum which also have stromata with purplish tones and
KOH-extractable pigments with olivaceous tones. Hypoxylon
macrocarpum differs mainly in having much larger, frequently tubular
perithecia, and darker KOH-extractable pigments which are rather brown
while those of
H. fuscum and
H. perforatum are more
yellow. Moreover H. fuscum
and H. perforatum
both lack the metallic shine of mature stromata and the
characteristic aromatic smell of the substrate.
This aromatic smell is absent from the stromata, or very faint, but is
striking when underlying wood of fresh specimens is cut off.
Unfortunately, it vanishes from dried herbarium material, even if it is rehydrated.
This smell is variously interpreted as reminiscent of vanilla,
Filipendula ulmaria, flowers of Crataegus oxyacanthae,
methyl salicylate or bitter almonds, at the same time sweet and aromatic,
and is a very distinctive feature among Pyrenomycetes. It should be noted
that a similar sweetish smell, although less strong, is encountered in
another member of Hypoxylon, H. rutilum.
Ju and Rogers (1996) report
that cultures of H. rutilum
yield this smell, described as coco-like, while
no smell is reported to occur when culturing H. macrocarpum.
Secondary metabolites of H. macrocarpum are distinctive in being almost
specific among those of Hypoxylon species whose HPLC profile is known.
Along with BNT and orsellinic acid,
Mühlbauer et al. (2002) isolated
three new compounds named macrocarpones A, B and C, of which the former is also known
from the xylariaceous fungus Pulveria porrecta Malloch & CT Rogerson
(Stadler et al., 2004a). According to
Mühlbauer et al. (2002), the distinctive
smell of H. macrocarpum is due to volatil compounds that were not revealed
by HPLC.
Another striking characteristic of H. macrocarpum
is that mature stromata left for drying at room temperature
are rapidly covered with a thick layer of tendrils of massively
discharged ascospores and turn completely black. To a lesser
extent, this can also be observed in
H. howeianum and
H. rubiginosum.
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