Hypoxylon fuscum











Hypoxylon fuscum (Pers.: Fr.) Fr.

Stromata pulvinate on bark, rounded, at times coalescent, 2-5 mm diam x 0.5-2 mm thick, or effused-pulvinate on decorticated wood, 15-60 mm long x 3-10 mm broad x 0.4-2 mm thick, with inconspicuous perithecial mounds; surface vinaceous purple (101) to dark vinaceous (82) or vinaceous brown (84), at times wrinkled between perithecial mounds; yellowish granules beneath surface with KOH-extractable pigments amber (47), luteous (12) to citrine (13), olivaceous (48) or dilute umber (9); the tissue below the perithecial layer brown to dark brown, 0.1-1.5 mm thick.

Perithecia spherical to obovoid, 200-380 m diam x 250-400(-600) m high.

Ostioles lower than the stromatal surface, at times, on mature stromata, surrounded by a ring of white substance 40-60 m diam.

Asci 105-170 m total length x 7-10(-13) m broad, the spore bearing-parts 65-108 m long, the stipes 35-90 m long, with apical ring discoid, amyloid, 0.8-1.5 m high x 2.8-3.5 m broad.

Ascospores brown to dark brown, ellipsoid-inequilateral with narrowly rounded ends, 11-16 x 5-8 m ( M = 13.2 x 5.8 m), (on Carpinus betulus 9.5-12.2 x 4-6.2 m, M = 11.4 x 5.2 m), with slightly sigmoid to sigmoid spore-length germ slit; perispore dehiscent in 10% KOH, smooth; uniseriate to infrequently biseriate in the ascus.

Anamorph in nature buff (45) to honey (64), velvety, occurring at first stages of stromatal development, more rarely on old stromata. Conidiogenous cells hyaline, smooth, 20-25 x 2-2.5 m. Conidia ellipsoid, hyaline, 3.5-5.5 x 2-3.5 m. The conidiogenous structure is Virgariella-like.

Habitat: on bark or decorticated wood of various trees, usually members of Betulaceae ( Alnus, Betula, Carpinus and Corylus), very common on dead standing stems of Corylus avellana. Also recorded during this study on Acer campestre, Frangula alnus, Prunus spinosa, Rhamnus catharticus and Salix caprea.

Known distribution: reported from North temperate areas (Europe, North America) and from tropics as well (Guyana, Philippines, Taiwan) (Ju & Rogers, 1996).

Specimens examined: France : Arige (09): Montseron, Roquebrune, 20 Sept. 1999, JF-99221, on Rhamnus catharticus; Prat Communal, Loumet, 950 m, 25 Aug. 2004, JF-04187, on Alnus glutinosa; Rimont, Las Muros, ruisseau de Peyrau, 13 Apr. 1996, JF-96008, on Alnus glutinosa; Rimont, Las Muros, 13 Mar. 1997, JF-97042, on Acer campestre; Rimont, Las Muros, ruisseau de Peyrau, 17 Jul. 1999, JF-99154, on Betula pendula; Rimont, Las Muros, ruisseau de Peyrau, 17 Jul. 1999, JF-99155, on Corylus avellana; Rimont, Las Muros, ruisseau de Peyrau, 29 Jul. 2000, JF- 00137, on Salix caprea; Rimont, Las Muros, ruisseau de Peyrau, 11 Aug. 2000, JF-00156, on Corylus avellana; Rimont, Las Muros, 08 Apr. 2001, JF-01062, on Corylus avellana; Rimont, Saurine, 29 Jun. 2001, JF-01124, on Frangula alnus. Cte d'Or (21): Etevaux, Fort de Saint Lger, 04 Oct. 1990, P. Leroy 901004.01, on Carpinus betulus. Haute Garonne (31): Fort de Bouconne, 29 Mar. 1997, JF-97041, on Carpinus betulus. Indre et Loire (37): Chteau-Renault, Morand, 13 Sept.1987, P. Leroy 870913.03, on Carpinus betulus; Ballan, Bois de la Motte, 11 Nov. 1987, P. Leroy 871111.02, on Carpinus betulus. Loiret (45): Russy, Fort domaniale, carrefour de l'Etoile, 16 Sept. 2001, P. Leroy 01377E, on Carpinus betulus. Pyrnes Atlantiques (64): Maulon, Hpital Saint Blaise, Les Barthes, 27 Oct. 1998, JF-98146, on Prunus spinosa.

Notes: When growing on bark, H. fuscum is highly distinctive in its small-sized, rounded, pulvinate, purple stromata. Hypoxylon porphyreum is a rare taxon with a boreal distribution which is much like H. fuscum in macroscopic and microscopic features. The main differences in the field are its occurrence on Quercus and its slightly more massive stromata.

When effused on decorticated wood H. fuscum may be confused in the field with other members of Hypoxylon having a purplish brown stromatal surface, i. e., H. carneum, H. macrocarpum, H. macrosporum, H. petriniae, H. perforatum and H. vogesiacum.

Hypoxylon carneum differs in having livid violet KOH-extractable pigments, H. petriniae in having orange KOH-extractable pigments and H. vogesiacum in having none or purplish grey (127-128) KOH-extractable pigments. Moreover they differ from H. fuscum in ascospore size range, shorter for the both former, much longer for the latter.

Hypoxylon macrosporum differs from H. fuscum in its arctic-alpine distribution, its host specificity for Salix, its often prominent ostioles and its much larger ascospores.

Hypoxylon macrocarpum differs mainly from H. fuscum in having much larger stromata with a metallic shine, smaller ascospores and in giving a strong aromatic smell to the substrate.

Hypoxylon perforatum differs from H. fuscum in having ostioles usually conspicuously surrounded by a ring of white substance 60-120 m diam, smaller ascospores and short-stipitate asci.

In any case, microscopic observation of the sigmoid germ slit of at least a few ascospores will bring a decisive confirmation to the identification of H. fuscum.

Ascospore size range of H. fuscum has been discussed by L.E. Petrini and O. Petrini (1987) and by Ju and Rogers (1996). The former proposed a statistical approach to this problem, suspecting a correlation between ascospore size and substrate. They pointed out that collections made on Carpinus betulus have clearly smaller ascospores, while ascospores of specimens collected on Alnus viridis appear larger than on other hosts, but they concluded that, in absence of other teleomorphic or anamorphic differences, a varietal subdivision was not appropriate for the different ascospore size ranges overlap. In fact, our observations agree with these results for, in our study, the five collections of H. fuscum on Carpinus betulus show smaller ascospores (averaging 11.4 x 5.2 m versus 13.2 x 5.8 m), while a collection on Alnus glutinosa shows larger ascospores (13.7 x 7.3 m).

On the other hand, Ju and Rogers (1996), considering H. fuscum in its worldwide distribution, gave the very wide ascospore size range 8-20 x 4-8 m. In their opinion, this situation will probably lead to a further splitting of this species. They also consider H. fuscum as restricted to the Betulaceae in the North temperate zone, and less host-specific in tropics, but our observations broaden the host range of H. fuscum in Europe.

Investigatilon of secondary metabolites of H. fuscum by Mhlbauer et al.(2002) and Quang et al. (2003b) showed that BNT is the prevailing metabolite, along with azaphilones daldinal A and daldinins C, E and F, and lack of mitorubrin and derivatives. Daldinal A and daldinin C are also known from Daldinia, while the two latter are only known from H. fuscum and H. fuscopurpureum.

Three deviating collections of H. fuscum on Alnus and Betula from France and Germany with pale purplish grey (127) granules and KOH-extractable pigments and small ascospores are currently under investigation (Stadler, pers. comm.). Additional material with similar features from other hosts or locations would be welcome.

In a recent evaluation of relationships among xylariaceous genera based on DNA sequencing, Triebel et al. (2005) failed to find really conclusive results about 11 investigated sequences of H. fuscum. Their results do not support the host-specific variations in ascospore size observed by Petrini & Petrini (1987) and Mhlbauer et al.(2002), and rather point toward "a geographical pattern in the genetic variability within H. fuscum." Hopefully, the combination of morphological, chemical and molecular investigations on material from all parts of the world should eventually shed some light on this pending question.