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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 µm diam x 250-400(-600)
µm high.
Ostioles lower than the stromatal surface, at times, on mature
stromata, surrounded by a ring of white substance 40-60 µm diam.
Asci 105-170 µm total length x 7-10(-13) µm broad, the spore
bearing-parts 65-108 µm long, the stipes 35-90 µm long, with apical ring
discoid, amyloid, 0.8-1.5 µm high x 2.8-3.5 µm broad.
Ascospores brown to dark brown, ellipsoid-inequilateral with
narrowly rounded ends, 11-16 x 5-8 µm ( M = 13.2 x 5.8 µm), (on
Carpinus betulus 9.5-12.2 x 4-6.2 µm, M = 11.4 x 5.2 µm), 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 µm. Conidia ellipsoid, hyaline, 3.5-5.5 x
2-3.5 µm. 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 : Ariège (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. Côte d'Or (21): Etevaux, Forêt
de Saint Léger, 04 Oct. 1990, P. Leroy 901004.01, on Carpinus
betulus. Haute Garonne (31): Forêt de Bouconne, 29
Mar. 1997, JF-97041, on Carpinus betulus. Indre et Loire
(37): Château-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, Forêt domaniale, carrefour de
l'Etoile, 16 Sept. 2001, P. Leroy 01377E, on Carpinus
betulus. Pyrénées Atlantiques (64): Mauléon, Hôpital
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 µm 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 µm versus 13.2 x 5.8 µm), while a
collection on Alnus glutinosa shows larger ascospores (13.7 x 7.3
µm).
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 µm. 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
Mühlbauer 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
Mühlbauer 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.
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