The genus Hypoxylon is delimited by
four main characters: one, anamorphs Nodulisporium-like--the primary
criterion for recognizing a xylariaceous fungus as a member of Hypoxyloideae; two,
stromata unipartite; three, stromatal tissue below the perithecial layer solid and
homogeneous; and four, stromata not upright, i.e., with the height less than the length
and the breadth. Hypoxylon differs from genera in Xylarioideae such as Nemania
which has Geniculosporium-like anamorphs; from Biscogniauxia and Camillea
which have bipartite stromata; from Daldinia
which has concentric rings; from Entonaema A. Möller which has a hollow
cavity lined with gelatinous tissue; and from Rhopalostroma D. Hawksworth
which has stipitate stromata.
Some other characters are useful in suggesting Hypoxylon. Unfortunately, these characters are not universally possessed by taxa of Hypoxylon, and therefore their usefulness in defining Hypoxylon is limited. These characters include the dehiscent perispore, the flat ascal apical ring, the germ slit on the convex side of the ascospore, the waxy stromatal tissue, and the KOH-extractable stromatal pigments. For example, there are more than thirty taxa of Hypoxylon in which the perispore is not easily dehiscent, and many taxa have nearly equilateral ascospores which make the position of the germ slit-whether it is on the convex or the flattened side-impossible to assess.
Hypoxylon is a cosmopolitan genus, but has its highest diversity in the tropics and subtropics. Approximately two-thirds of both taxa in section Hypoxylon and taxa in section Annulata can be found in the tropics and subtropics. Many of them are weak to damaging pathogens. The majority of them are usually found fruiting on dead wood, probably following a "silent" invasion of living host tissue. Some Hypoxylon taxa seem highly host specific with regard to species, genera, or families. Others seem to have wide substrate ranges. It must be admitted, however, that the hosts of most tropical taxa have not been identified and, thus, the matter of host specificity is mostly conjectural. Recently, Petrini and Petrini (1985) reported a number of Hypoxylon taxa living endophytically in living plants. This subject is discussed under SPECIATION.