Hypoxylon intermedium











Hypoxylon intermedium (Schwein.: Fr.) Ju & Rogers.

Stromata hemispherical, narrowly connected to the substrate at the centre, rarely coalescent, with inconspicuous perithecial mounds, 2-7 mm diam x 1-3 mm thick; surface greyish sepia (106) to dark brick (60); pale orange granules beneath surface, with KOH-extractable pigments greenish yellow (16); the tissue below the perithecial layer light grey to grey brown, 1-2.5 mm thick, extending upwards between the perithecia.

Perithecia spherical to obovoid, 250-320 Ám diam x 400-600 Ám high.

Ostioles lower than the stromatal surface, on mature stromata conspicuously encircled with a prominent ring of white substance 150-200 Ám diam.

Asci 180-230 Ám total length x 12-18 Ám broad, the spore bearing-parts 100-128 Ám long, the stipes 85-110 Ám long, with apical ring lacking or highly reduced, inamyloid.

Ascospores dark brown to blackish brown, ellipsoid-inequilateral, 16-23 x 8.2-10.8 Ám ( M = 19.1 x 9.4 Ám), with straight germ slit spore-length; perispore dehiscent in 10% KOH, smooth.

Anamorph in nature: not observed.

Habitat: primary saprophyte on bark of dead branches of Fraxinus excelsior before their fall on to the ground.

Known distribution: reported on Fraxinus sp. from Europe and North America.

Specimens examined: FRANCE, AriŔge (09): Orlu, RÚserve nationale d'Orlu, Jasse de Justiniac, 29 Jul. 2001, JF- 01155, on Fraxinus excelsior; Rimont, Las Muros, 09 Sept. 2000, JF-00198, on Fraxinus excelsior; Rimont, Las Muros, 04 Feb. 2001, JF-01011, on Fraxinus excelsior.

Notes: Hypoxylon intermedium, previously known as H. fraxinophilum Pouzar, is highly distinctive in its stromatal, microscopic and ecological features. It will be identified in the field thanks to its hemispherical greyish sepia stromata with conspicuous white substance surrounding the ostioles, growing on fallen dead branches of Fraxinus found after a gust of wind or a tempest. It is noteworthy that H. intermedium stops growing if branches are in contact with the soil but is able to keep on growing during the following season if branches remain hanging above the soil level. Contact with the soil involves more humidity in the substrate and apparently favours more competitive ascomycetes or basidiomycetes.

Field identification will be readily confirmed by the observation of the following characters: KOH-extractable pigments greenish yellow, large blackish brown ascospores and apical rings lacking.

Secondary metabolites of H. intermedium are hypomiltin and derivatives, along with orsellinic acid (Hellwig et al., 2004). Hypomiltin is a new azaphilone which is also the prevailing compound in H. perforatum and other tropical species such as H. hypomiltum and H. trugodes which, however, are morphologically much different. This is an interesting and rather unusual case of noncorrelation between morphology and metabolites profiles.