Hypoxylon cohaerens var microsporum

              

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Hypoxylon cohaerens (Pers.: Fr.) Fr. var. microsporum Rogers & Candoussau.

Stromata pulvinate, constricted at base, often coalescent, with inconspicuous perithecial mounds, 2-15 mm diam x 1.5-5 mm thick; surface dark brick (60) to sepia (63), becoming blackish ; dark orange brown granules beneath surface, with KOH-extractable pigments varying from livid red (56) to bay (6) or purplish grey (128); the tissue below the perithecia brown to blackish brown, up to 4 mm thick.

Perithecia obovoid, 400-500 m diam x 500-750 m high, discretely surrounded by a carbonaceous stromatal layer.

Ostioles papillate, black, at times at the centre of a discoid depression 120-150 m diam.

Asci 130-170 m total length x 4-5.5 m broad, the spore bearing-parts 48-73 m long, the stipes 75-115 m long, with apical ring inamyloid, 1.5 m high x 2 m broad.

Ascospores light brown to brown, ellipsoid-inequilateral, 6.8-8.8 x 3.4-4 m ( M = 7.3 x 3.8 m), with faint germ slit less than spore-length; perispore dehiscent in 10% KOH, smooth, with a thickening on the more convex side.

Anamorph in nature: dark rosy vinaceous (58) on young stromata or at margins, turning livid purple (81) in 10% KOH. Conidiogenous cells 15-30 x 3 m; conidia ellipsoid, 3.5-4.8 x 2.5-2.8 m. The conidiogenous structure is Virgariella-like.

Habitat : primary saprophyte on bark of fallen or cut off branches or trunks of Quercus and Castanea in Europe; also reported on Erica arborea and Laurus azorica in Portugal; a wider host range is encountered in tropics.

Known distribution: Europe (southern England, France, Portugal, Spain) and tropics (Colombia, Honduras, India, Mexico, Taiwan).

Specimens examined: FRANCE, Arige (09): Ganac, ruisseau de Ganac, 02 Jul. 2000, JF- 00111, on Castanea sativa ; Montseron, Ouzeich, 20 Dec. 1996, JF-96181, on Castanea sativa; Rimont, Las Muros, 19 Jan. 2001, JF-01006, on Quercus robur; Charente Maritime (17): Ile de R, St. Martin en R, Les Sallires, 28 Apr. 2004, JF-04065, on bark of Quercus ilex. Haute Garonne (31): Saleich, Artihagure, 10 Feb. 2000, JF- 00009, on Quercus robur; Landes (40): Capbreton, 06 Jun. 1982, FC-5252-82, on Quercus suber. PORTUGAL: Ilha de Madeira, Levada da Sawa de Faial (790 m), 23 Apr. 2004, leg. W. Jger, det. M. Stadler, STMA 04W19, on bark of Laurus azorica.

Notes: In all collections examined, ascal apical rings were inamyloid, while Ju & Rogers (1996) describe the ascal apical rings as being occasionally amyloid. Hypoxylon cohaerens var. microsporum mainly differs from the type in smaller ascospores having a faint shorter germ slit and in vinaceous KOH-extractable pigments. Although it is reported to have a wide host range in tropics (Ju & Rogers, 1996), Hypoxylon cohaerens var. microsporum seems restricted to Quercus and Castanea in our collecting area, while H. cohaerens is restricted to Fagus and to northern hemisphere.

It may be easily confused in the field with H. multiforme, which differs in having more conspicuous perithecial mounds, sienna (8) to olivaceous (48) KOH-extractable pigments, amyloid ascal apical rings and larger ascospores. Moreover, H. multiforme is found mainly on Alnus,Betula and Corylus, and hiterto has never been reported from Quercus or Castanea.

When host is unknown, identification of H. cohaerens var. microsporum relies on the combination of the following characters: stromata pulvinate, restricted at base, with papillate ostioles, vinaceous KOH-extractable pigments and ascospores averaging less than 8 m long with a short faint germ slit.

In a recent account on the chemotaxonomy of some members of Hypoxylon section Annulata, Quang et al. (2005) reported that, besides BNT, H. cohaerens contains specific azaphilones cohaerins A and B which lack in the variety microsporum, HPLC profile of which appears closer to that of H. multiforme. Awaiting further investigations on the secondary metabolites of these latter taxa which probably are azaphilones close to cohaerins A and B, Quang et al. (2005) demonstrated that collections of H. cohaerens var. microsporum from Colombia, France, Portugal and Taiwan all had identical metabolite profiles, confirming that it was a single species with a wide distribution.