Hypoxylon macrocarpum











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.