Introduction to Biscogniauxia



Biscogniauxia has long been known as Nummularia Tul. & C. Tul., until Miller (1961) placed most of its members in his section Applanata of Hypoxylon. The current concept of Biscognauxia was defined by Pouzar (1979, 1986) and resumed by Ju, Rogers, San Martin & Granmo (1998).

Biscogniauxia is a xylariaceous genus represented in Europe by ten known species, five of which having applanate stromata and low margins, i.e., B. anceps, B. cinereolilacina, B. granmoi, B. mediterranea and B. nummularia, and thus being likely to be confused with Hypoxylon. The five other species, i.e., B. dennisii, B. marginata, B. querna, B. repanda and B. simplicior differ in having more or less cupulate (concave) stromata with conspicuously raised margins.

Biscogniauxia shares with Hypoxylon discoid ascal apical rings and Nodulisporium-like anamorphs as defined in Ju & Rogers (1996). It is separated from Hypoxylon primarily in having bipartite stromata with an outer stromatal layer which disappears on mature stromata, and in lacking KOH-extractable pigments. Moreover, the stromatal carbonization is important in Biscogniauxia while it is weak in Hypoxylon section Annulata and usually absent in Hypoxylon section Hypoxylon (Ju & Rogers, 1996, Ju et al., 1998). The outer dehiscent stromatal layer is often overlooked as it is only present on young stromata, remnants of this layer may be found at margins of mature stromata, but are usually inconspicuous. It is noteworthy that in Biscogniauxia, the natural anamorph develops on young stromata either between the outer layer and the stromatal surface, before the outer layer dehisces (Ju et al.,1998), or on the upper side of the dehiscing layer and on stromatal margins (cf. illustrations of B. simplicior).

In the field, applanate stromata of Biscogniauxia also are easily confused with those of Diatrype Fr. (Diatrypaceae) and Graphostroma Piroz. (Graphostromataceae). In Diatrype, stromata differ in lacking carbonaceous tissue and in having a well-developed interperithecial tissue (entostroma) of usually white granules; moreover, asci are typically long-stipitate, ascospores are yellowish and allantoid, and conidia are scolecosporous (anamorph Libertella Desm.). Graphostroma is somewhat intermediate between the Xylariaceae and the Diatrypaceae through a Nodulisporium-like anamorph recalling the Xylariaceae and light-coloured suballantoid ascospores recalling the Diatrypaceae.

Biscogniauxia species develop in bark of trees and shrubs, especially on dead or dying branches, more rarely on trunks, and are suspected to be at least weak pathogens (Ju et al., 1998). Therefore they are to be searched for on dead branches still attached to the trunk, and specimens of Biscogniauxia found on dead branches lying on the ground are usually old and their perithecia are empty, unless the branches have broken off recently.

Identification of Biscogniauxia mainly relies on stromatal characters and ascospore size and morphology. Identification of host is a decisive feature in several species which are apparently host-specific, i.e., B. cinereolilacina specific to Tilia sp., B. granmoi specific to Prunus sp., B. nummularia specific to Fagus sylvatica and B. simplicior specific to Rhamnus catharticus.

For synonyms, see Ju et al., 1998.