Hypoxylon rubiginosum













Hypoxylon rubiginosum (Pers.: Fr.) Fr.

Stromata effused, effused-pulvinate to pulvinate, with inconspicuous to very conspicuous perithecial mounds, 2-80 mm long x 3-30 mm broad x 1-1.3(-2) mm thick; surface rust (39), sienna (8), bay (6), umber (9) to dark brick (60), usually wrinkled between the perithecial mounds, with margins yellow to orange and frequently widely effused when immature; orange brown granules beneath surface and between perithecia, with KOH-extractable pigments orange (7) to rust (39) ; the tissue below the perithecial layer usually conspicuous, up to 1(-1.5) mm thick, dark brown to blackish.

Perithecia spherical to obovoid, 300-650 m diam x 450-800 m high.

Ostioles lower than the stromatal surface, on mature stromata infrequently surrounded by a ring of white substance 50-70 m diam.

Asci 130-180 m total length x 6-9(-10) m broad, the spore bearing-parts 56-95 m long, the stipes 60-98 m long, with apical ring amyloid to weakly amyloid, discoid, 0.5-1.5 m high x 2-2.7 m broad.

Ascospores brown to dark brown, ellipsoid-inequilateral, 8.8-11(-12) x 4-5.5 m ( M = 10.1 x 4.4 m), with straight germ slit spore-length; perispore dehiscent in 10% KOH, smooth.

Anamorph in nature at margins of young stromata or on old stromata, buff (45) to honey (64), velvety. Conidiogenous structure Nodulisporium-like, slightly roughened, yellowish to light brown. Conidiogenous cells 10-30 x 2-3 m. Conidia ellipsoid, 5-6 x 3-4 m, yellowish.

Habitat: Very common on bark or decorticated wood of various hosts, mostly Fraxinus, Salix and Ulmus. Also occasionally found on Acer campestre, Betula pendula, Corylus avellana, Crataegus oxyacanthae, Cydonia oblonga, Fagus sylvatica, Juglans regia, Lonicera sp., Malus sylvestris, Populus tremula, Prunus avium, Prunus spinosa, Quercus robur, Rhamnus catharticus, Tilia platyphyllos.

Known distribution: Northern temperate areas. Europe, North America.

Specimens examined: FRANCE. Arige(09): Allires, la Grangette, 10 Sept. 1999, JF-99211, on Tilia platyphyllos; Aulus, Agnesserre, 14 Sept 1997, JF-97162, on Fagus sylvatica; Rimont, Las Muros, 03 Nov. 1996, JF-96161, on Fraxinus excelsior; Rimont, Las Muros, 29 Jul. 1998, JF-98082, on Quercus robur; Rimont, Las Muros, 15 Sept. 1998, JF-98124, on Salix caprea; Rimont, Las Muros, 02 Sept 2000, JF- 00180, on Fraxinus excelsior; Rimont, Las Muros, 06 Sept. 2000, JF-00185, on Cydonia oblonga; Rimont, Las Muros, ruisseau de Peyrau, 25 Sept. 2001, JF-01206, on Juglans regia; Aude (11): Belcaire, chemin de Trassoulas, 25 May 2001, JF- 01105, on Salix caprea. Pyrnes Atlantiques (64): Maulon, Hpital Saint Blaise, les Barthes, 26 Jun. 1999, JF-99130, on Fraxinus excelsior.

Notes. Hypoxylon rubiginosum has long been a broadly circumscribed complex of species (Miller, 1961). Petrini & Mller (1986) were the first to recognize H. rubiginosum in the narrow sense adopted herein, separating it from two taxa to which they gave the varietal rank, H. rubiginosum (Pers.:Fr.) Fr.var. cercidicola (Berk.& Curtis ex Peck) L.E. Petrini and H. rubiginosum (Pers.: Fr.) Fr.var. perforatum ( Schw.) L. E. Petrini. In their revision of the genus Hypoxylon, Ju & Rogers (1996) assessed that all tropical species formerly included under the epithet rubiginosum could be separated from the North temperate taxon based on pigments yielded in KOH and morphological and cultural data. They likewise considered H. perforatum (Schw.:Fr.)Fr. a distinct species owing to its amber pigments in KOH and its Virgariella-like anamorph. More recently, chemical data provided by HPLC techniques confirmed the presence of a peculiar secondary metabolite (hypomiltin) in stromata of H. perforatum (Stadler et al., 2005) and confirmed that the morphological differences between H. rubiginosum and its variety cercidicola were supported by differences in secondary metabolites (Stadler et al., 2004b). The new name H. petriniae Stadler & Fournier was given to this former variety, for the epithet cercidicola remains ambiguous. (See notes under H. petriniae).

As to HPLC profile, H. rubiginosum is characterized by the association of mitorubrin, rubiginosin A and orsellinic acid as main secondary metabolites (Stadler et al., 2004b). Rubiginosins B and C, rubiginosic acid, entonaemin A and daldinin C were additional compounds recently isolated (Quang et al., 2004)

Among the other Hypoxylon species sharing with H. rubiginosum its stromatal colour and its orange KOH-extractable pigments, H. rutilum differs in having papillate ostioles and ascospores averaging less than 10 m long, H. cercidicolum differs in having a swollen stellate stromatal margin and an inamyloid apical ring, H. ferrugineum and H. julianii differ in having ascospores averaging more than 15 m long, and H. laschii differs in having erumpent pulvinate stromata and ascospores averaging less than 10 m long. Hypoxylon crocopeplum, a taxon recently recorded from Europe for the first time, is much like H. rubiginosum in gross morphology. Its immature stromata are distinctive in having a brighter orange colour and greyish discs around ostioles; moreover, its ascospores are larger and have a sigmoid germ slit.

H. ticinense and H. subticinense differ mainly from H. rubiginosum in their peculiar discoid undulate stromata, lying on a thick black basal tissue, lined with a yellow fimbriate margin when young. Moreover, the former differs in having ascospores averaging less than 7 m long, while the latter differs in having ellipsoid-equilateral ascospores.

Two recently described species closely related to H. rubiginosum are H. liviae (Granmo, 2001) and H. salicicola (Granmo, 1999). Both are host specific, the former is specific for Sorbus aucuparia, the latter for Salix sp., and are likely to have a boreal distribution. These two species are still unknown from France. Hypoxylon liviae differs from H. rubiginosum in having whitish pruinose brown stromata, yellow stromatal granules yielding luteous (12) to sienna (8) pigments in 10% KOH, and dark brown nearly equilateral ascospores. Hypoxylon salicicola is much like H. rubiginosum and differs primarily in having smaller perithecia 0.1-0.35 mm diam and smaller ascospores 7-10 x 3-4.5 m. Ju and Rogers (1996) cultured it and observed a Nodulisporium-like anamorph, but growing more slowly than that of H. rubiginosum.