Hypoxylon howeianum

              

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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 m diam x 300-500 m high.

Ostioles lower than the stromatal surface or at the same level, black.

Asci 93-144 m total length x 4.5-6.5 m broad, the spore bearing-parts 48-56 m long, the stipes 45-88 m long, with apical ring discoid, amyloid, 0.5-0.7 m high x 1.5-2 m broad.

Ascospores brown, ellipsoid-inequilateral, 6.8-8.8 x 3.4-4.8 m (M=7.6 x 3.8 m), 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 m diam, of dark brown hyphae 5 m wide, radiating from the base of young stromata. Conidiogenous cells 13-20 (-25) x 3-4 m. Conidia ellipsoid, 4-4.5(-6) x 2.5-3.5 m. 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 Mller (1986).

Known distribution: worldwide.

Specimens examined: FRANCE, Arige (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, Fort communale, 03 Dec. 2000, on Eucalyptus sp. Hrault (34): Bdarieux, 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 m). 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 (Mhlbauer 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 m).