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Hypoxylon howeianum Peck.
Stromata hemispherical to nearly spherical, unfrequently coalescent,
with conspicuous to inconspicuous perithecial mounds, 3-8 (-15) mm diam x
2-6 mm thick; surface rust (39), bay (6) to dark brick (60); orange red granules beneath
surface, with KOH-extractable pigments orange (7) to rust (39), white granules between
perithecia when young; the tissue below the perithecial layer black, up to
5 mm thick, extending upwards between the perithecia.
Perithecia obovoid, 200-400 µm diam x 300-500 µm high.
Ostioles lower than the stromatal surface or at the same level,
black.
Asci 93-144 µm total length x 4.5-6.5 µm broad, the spore
bearing-parts 48-56 µm long, the stipes 45-88 µm long, with apical ring
discoid, amyloid, 0.5-0.7 µm high x 1.5-2 µm broad.
Ascospores brown, ellipsoid-inequilateral, 6.8-8.8 x 3.4-4.8 µm
(M=7.6 x 3.8 µm), with faint germ slit spore-length; perispore dehiscent
in 10% KOH, smooth.
Anamorph in nature present in young stages, buff (45) to honey
(64), synnematous, growing on
very conspicuous dark brown chords 150-200 µm diam, of dark brown hyphae 5
µm wide, radiating from the base of young stromata. Conidiogenous cells
13-20 (-25) x 3-4 µm. Conidia ellipsoid, 4-4.5(-6) x 2.5-3.5 µm.
Conidiogenous structure is Nodulisporium-like.
Habitat: saprophyte on bark or decorticated wood of branches
of various deciduous trees. Usually found on recently dead wood, more
rarely on rotten wood. Frequent on Corylus avellana, Alnus
glutinosa, Salix caprea and Quercus robur. Also
occasionally found during this study on Cydonia oblonga, Betula
pendula, Fagus sylvatica, Fraxinus excelsior,
Eucalyptus sp., Prunus avium, Quercus ilex and
Rosa canina. Also reported from Fagus by
Petrini and Müller (1986).
Known distribution: worldwide.
Specimens examined: FRANCE, Ariège (09): Rimont,
Las Muros, 10 Jul. 1996, JF-96050, on Quercus robur; Rimont, Las
Muros, ruisseau de Peyrau, 11 Aug. 2000, JF- 00155, on Corylus
avellana; Rimont, Las Muros, 01 Sept. 2000, JF-00179, on
Fraxinus excelsior; Rimont, Saurine, 29 Jun. 2001, JF-01126,
on Betula pendula; Saint Quirc, Forêt communale, 03 Dec.
2000, on Eucalyptus sp. Hérault (34):
Bédarieux, tunnel du Col du Buis, 23 Oct. 2001, JF-01244, on Quercus
ilex.
Notes. Hypoxylon howeianum is a common saprophyte, often
growing in large colonies on dead branches of various deciduous trees. It
is characterized by the combination of the following features: stromata
hemispherical, rust-coloured, with conspicuous perithecial mounds and
black ostioles opening lower than the stromatal surface, with perithecia
surrounded by a white to greyish tissue in young stages and lying on a
thick black basal tissue, small-sized ascospores and ochraceous anamorph
growing on radiating hyphal chords.
In the field, H. howeianum can be confused with the very similar
H.
fragiforme which differs in being usually restricted to
Fagus, while the former is plurivorous. Moreover, the synnemata of the
anamorph of H. howeianum are, when present,
very distinctive, and microscopically H.
fragiforme is readily distinguished from H. howeianum in
having larger ascospores (10-15 x 5-6.5 µm). Several recent collections
of both species growing mixed either on Fagus or on Quercus prove
confusions can easily be made when the anamorph is not present.
Moreover, both H. howeianum and H. fragiforme belong to the
same chemotype, characterized by mitorubrin and derivatives, and orsellinic
acid
(Mühlbauer et al., 2002;
Hellwig et al., 2004;
Stadler et al., 2004b).
Hypoxylon
commutatum is a poorly known species, only known from the type
collection (Nitschke, 1867),
which can have hemispherical rust-coloured
stromata. It differs from H. howeianum in lacking ascal apical ring
and in having larger ascospores (9-11 x 4.5-5.5 µm).
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