Hypoxylon rutilum

              

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Hypoxylon rutilum Tul. & C. Tul.

Stromata pulvinate to nearly hemispherical when growing on bark, erumpent through the lenticels, 3-12 mm long x 3-5 mm broad x 1-1.5 (-2) mm thick, effused to effused-pulvinate when growing on decorticated wood, 4-23 mm long x 2-5 mm broad x 0.4-0.5 (-0.9) mm thick, with inconspicuous to conspicuous perithecial mounds; surface sienna (8), rust (39) to dark brick (60); orange red granules beneath the stromatal surface and between the perithecia, with KOH-extractable pigments orange (7) to scarlet (5); the tissue below the perithecial layer blackish, inconspicuous or up to 100 m thick; underlying wood with a sweetish smell reminiscent of coconut.

Perithecia spherical to obovoid, 150-200 m diam x 200-300 m high, the fertile perithecial layer may be seated on up to four layers of old and empty perithecia.

Ostioles higher than the stromatal surface, black, minutely papillate.

Asci 145-160 m total length x 6-7.5 m broad, the spore bearing-parts 55-75 m long, the stipes 85-95 m long, with apical ring discoid, amyloid, 0.8-1 m high x 2-2.8 m broad.

Ascospores brown to dark brown, ellipsoid-inequilateral, 7.5-10 x 4-4.8 m (M = 8.8 x 4.5 m), with straight germ slit spore-length; perispore dehiscent in 10% KOH, smooth.

Anamorph in nature: on young or old stromata, fawn (87) velvety. The conidiogenous structure is Virgariella-like, pale brown, roughened. Conidiogenous cells 21-27 x 2.5-3 m; conidia ellipsoid, hyaline, 3.5-5(-6) x 2.8-3.5 m.

Habitat: on bark and wood of twigs and branches lying on the ground in damp and shadowy places. Recorded on Alnus glutinosa, Corylus avellana, Fagus sylvatica, Fraxinus excelsior, Populus tremula, Prunus avium, Prunus padus, Tilia platyphyllos.

Known distribution: Europe (France, Italy, Switzerland, UK) and Asia (Taiwan).

Specimens examined: FRANCE: Arige (09): Rimont, Las Muros, ruisseau de Peyrau, 23 Apr. 1997, JF-97044, on Fraxinus excelsior; Rimont, Las Muros, ruisseau de Peyrau, 21 Mar. 1998, 98036, on Corylus avellana; Rimont, Las Muros, ruisseau de Peyrau, 25 Nov. 1998, JF-98179, on Corylus avellana; Rimont, Grand Bois, 26 Feb. 1999,JF-99027, on Fagus sylvatica; Rimont, Las Muros, 31 May 1999, JF-99106, on Prunus avium; Rimont, Las Muros, 11 Sept. 2000, JF-00201, on Prunus avium; Rimont, Las Muros, ruisseau de Peyrau, 13 Jun. 2002, JF-02105, on Prunus avium. Aude (11): Belcaire, Bois du Pinet, 1000m, 28 Sept. 2003, JF-03178, on Prunus padus. Pyrnes Atlantiques (64): Oloron Ste Marie, Bois de Larbaig, 23 Aug. 1998, FC-603, on Fagus sylvatica.

Notes: Hypoxylon rutilum is a rarely recorded species, owing to its discrete orange brown stromata easily confused with those of H. perforatum and H. rubiginosum. It is distinctive in having minutely papillate ostioles and red granules beneath the stromatal surface. Moreover, its field identification is possible thanks to the characteristic sweetish smell of the underlying wood of fresh specimens; unfortunately this smell vanishes in herbarium material. Ju and Rogers (1996) already noticed this smell when culturing H. rutilum on Oatmeal agar and described it as reminiscent of coconut oil.

It is also noteworthy that pulvinate stromata are usually composed of several stromatal layers, each one growing year after year on top of the other. This feature is also reported by Mathiassen (1989) for H. macrosporum on Salix bark, and occasionally encountered in some other taxa of Hypoxylon (H. macrocarpum, H. rubiginosum, H. ticinense).

In the field, H. rutilum may also be easily confused with the rarely recorded H. julianii which has similar stromata with ostioles opening sometimes higher than the stromatal surface, similar small perithecia and similar red granules under the stromatal surface. Hypoxylon julianii differs in having much larger ascospores (15-18 x 6-7.5 m) and in lacking specific smell. A possible confusion also exists with H. laschii, which differs in having larger perithecia, thicker basal tissue under the perithecia and lacks specific smell.

As inferred from HPLC profiling (Hellwig et al., 2002; Stadler et al., 2004b), H. rutilum belongs to the rubiginosum chemotype with mitorubrin as prevailing compound, with rubiginosin A and orsellinic acid. Apart from the lack of rubiginosin C, HPLC profile of H. rutilum appears almost identical to that of Hypoxylon julianii. Moreover, these two taxa share a common unknown metabolite identified as Peak HR1 (Stadler et al., 2004b).

Our records on several different hosts show that H. rutilum is not restricted to Fagus, and has a wider host range than previously assumed (Miller 1961, Petrini & Mller 1986, Ju & Rogers 1996).