Hypoxylon crocopeplum

              

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Hypoxylon crocopeplum Berk. & Curtis.

Stroma effused to more rarely pulvinate, 4-50 mm long x 3-20 mm broad x 0.9-1 mm thick, irregularly elongate, with inconspicuous perithecial mounds and a yellow effused margin while immature; surface at first pale fulvous (43), apricot (42), sienna (8), eventually dull bay (6) to sepia (63) with a greyish tone due to a whitish film; rust (39) to scarlet (5) granules beneath surface and between perithecia with KOH-extractable pigments rust (39) to bay (6); the tissue below the perithecial layer blackish, up to 0.2 mm thick.

Perithecia obovoid to slightly tubular, more rounded at the periphery of the stroma, 0.25-0.35 mm diam x 0.45-0.6 mm high. An underlying layer of old empty perithecia can be observed in some stromata.

Ostioles umbilicate, surrounded by greyish discs 100-120 m diam when immature, appearing as black dots contrasting with the stromatal surface in mature stromata.

Asci cylindrical, originating from ascogenous hyphae, 170-190 m total length, the spore-bearing parts 75-95 m long x 9-12 m broad, the stipes 80-110 m long, with apical ring discoid, amyloid, 1.2-1.8 m high x 2.4-3 m broad.

Ascospores brown, ellipsoid-inequilateral with narrowly rounded ends, 11.4-14.4 x 6-7.2 m, M = 12.6 x 6.5 m (n=30), with a conspicuously sigmoid germ slit spore-length, uniseriate in the ascus; perispore dehiscent in 10 % KOH, smooth.

Anamorph in nature: a Virgariella-like conidiogenous structure was observed in culture on YMG (Stadler, unpublished data) and on JF-03233.

Habitat: so far recorded in France on Fraxinus and Ulmus, on bark or on rotten decorticated wood of trunks lying on river banks.

Known distribution: pantropical and eastern North America (Ju & Rogers, 1996), Europe (present study).

Specimens examined: FRANCE. Haute Garonne (31): Martres-Tolosane, Le Moulin, 13 Dec. 2003, JF-03233, on a rotten trunk of Ulmus sp. (immature). Loiret (41): Saint Georges du Cher, Camping, 16 Nov. 2003, on Fraxinus excelsior, leg Paul Leroy (immature). Pyrnes Atlantiques (64): Auterrive, le du Gave d'Oloron, 06 Nov. 2003, JF-03223, on a rotten trunk of Ulmus sp.(mostly immature); same location and same date, JF-03224, on bark of Fraxinus excelsior (mostly immature); same location, 30 Jun. 2004, JF-04101, on bark of Fraxinus excelsior, leg. JF and M. Stadler,

Notes: Hypoxylon crocopeplum primarily is a tropical species which was so far unknown from Europe. The above findings might give the illusion it is a recent invader, but it more likely has been overlooked as, in the field, it is much like H. rubiginosum and its numerous allies. However its immature stromata are of a peculiar tone of orange which is different from other related species in Europe and should have been noticed by field mycologists. Aside this striking colour, immature stromata are distinctive in that their ostioles are as a rule at the centre of a grayish disc, a feature not reported by Ju & Rogers (1996) for the specimens they examined. Mature stromata become dull brown but remain noticeable on account of their black ostioles. The whitish film present on the surface of some mature stromata was also reported by Ju & Rogers (1996). The salient microscopic features of this species are the rather broad ascospores with narrowly rounded ends and their conspicuously sigmoid germ slit. Such a sigmoid germ slit is known only from H. fuscum among European species of Hypoxylon.

Hypoxylon crocopeplum is said to have a typical form in eastern North America with thin stromata and obovoid perithecia, while the tropical form displays thicker stromata with a thick basal layer and tubular perithecia (Ju & Rogers, 1996). Our collections are morphologically referrable to the North American form. During his investigations on chemotaxonomy of Xylariaceae by the means of HPLC, Dr. Marc Stadler obtained the secondary metabolite profile of H. crocopeplum from the type collection at Kew. The HPLC profile of the collection JF-03223 proved to be identical to that of the type material (unpublished data).

Regarding its secondary metabolites, H. crocopeplum is allied to the rubiginosum group in containig mitorubrin, rubiginosin A and orsellinic acid, but contains mitorubrinol which is lacking in H. rubiginosum (Hellwig et al., 2004; Quang et al., 2004).