ECOLOGY AND HOST-FUNGUS RELATIONSHIPS
Biscogniauxia taxa are exclusively
parasites of dicotyledonous angiosperms. The most recent host-fungus index for the USA
lists 5 taxa on various hosts (Farr et al., 1989). Some other taxa--not all of which are
considered herein to be Biscogniauxia--are listed under Nummularia;
hosts are dicots. Collection data suggest host preferences, if not host specificity, among
Biscogniauxia taxa. In Great Britain (and probably throughout Europe) B.
nummularia (Bull.: Fr.) Kuntze is usually encountered on Fagus
spp. (Cannon et al., 1985). Biscogniauxia
marginata (Fr.: Fr.) Pouzar [classically known as Nummularia
discreta (Schwein.) Tul. & C. Tul.] is often, but not exclusively,
encountered on trees of family Rosaceae. It causes a classic canker disease called
nail-head, a reference to the more or less flat-topped stromata; this is also often called
blister disease (Hepting, 1971). Biscogniauxia
atropunctata (Schwein.: Fr.) Pouzar [as Hypoxylon] is said
to cause a bark disease of Carya, Fagus, and Quercus
species in northern Georgia; it is suspected that lack of vigor predisposes trees to
invasion by the fungus (Hepting, 1971). Bassett et al. (1982) implicated the same fungus
as causing cankers on oaks in Arkansas. Bassett and Fenn (1984) showed experimentally that
girdling of healthy-appearing Quercus spp. trees resulted in the
production of B. atropunctata stromata on 70% of white oaks
(subgenus Leucobalanus) and 77% of black (red) oaks (subgenus Erythrobalanus)
within 5 months. They concluded that fruiting occurred following girdling by stimulating
the activity of mycelium from latent infections. Tainter and Gubler (1973) believed that
lowered levels of survival of the oak wilt fungus [Ceratocystis fagacearum
(Bretz) Hunt] in oaks resulted from the rapid decay of sapwood by B. atropunctata.
mediterranea (De Not.) Kuntze has been implicated in the death and
destruction of Quercus robur in Portugal and North Africa
(Barbosa, 1958; Malenšon and Marion, 1951). Roger (1953) mentions two taxa [as Nummularia]
on Hevea, considering them to be "semi-parasites."
We suspect that all Biscogniauxia species are at least weak pathogens, invading hosts that are weakened by drought, etc. and fruiting when the host is highly weakened or dead. These fungi could be classified as facultative saprophytes, i.e., spending most of their cycles as parasites, but capable of persisting for periods on dead material. Experience in Hawaii (JDR) and elsewhere indicates that dying or dead branches on many trees have evident stromata of a Biscogniauxia species and often a Eutypella species. Once branches fall to the ground, however, they are invaded by basidiomycetes-often a number of species (R. W. Gilbertson, personal communication)-, but additional xylariaceous inhabitants seem infrequent. We take this to mean that Biscogniauxia (and Eutypella) are parasites that persist on dead material that was fallen to the ground, but probably do not invade such material. Indeed, once material has fallen these taxa probably cannot compete with climax rotters, mostly basidiomycetes. Specific parasite-host combinations noted in the Hawaiian Islands are Biscogniauxia capnodes (Berk.) Y.-M. Ju & J. D. Rogers on Metrosideros polymorpha Gaud., Acacia koa Gray, Eucalyptus sp., Acacia sp.; B. citriformis (Whalley, Hammelev, & Taligoola) Van der Gucht & Whalley on Casuarina equisetifolia L. ex J. R. & G. Forst., and numerous other unidentified decaying branches; B. uniapiculata (Penz. & Sacc.) Whalley & Laess°e on Metrosideros polymorpha, Acacia koa, Eucalyptus sp., Aleurites moluccana (L.) Wild., and Psidium cattleianum Sabine. Whalley (1993) and Rogers (1979) cite other examples in Biscogniauxia and other xylariaceous genera. Reference to Hepting (1971) indicates that Biscogniauxia species [as Hypoxylon and Nummularia] are not generally regarded as serious pathogens of forest trees.
All experimental and observational evidence indicates that Biscogniauxia species are adapted to dry, or at least seasonally dry, habitats. Development within bark probably buffers developing stromata from drying. Sufficient water for the activities of these fungi in dead or drying trees is probably via the decomposition of cellulose with the consequent production of water molecules.