mammatum (Wahlenberg: Fr.) P. Karst., long cited as H. mammatum
(Wahlenberg) J. H. Miller, is a serious pathogen of Populus tremuloides
Michx. and a number other species of Populus L., Alnus Mill., Salix
L., Betula L., and other genera. Reference to Miller (1961) shows a number
of names placed into synonymy under H. mammatum. Some of
these, including H. pruinatum (Klotzsch) Cooke, H.
morsei Berk. & M. A. Curtis, and H. blakei
Berk. & M. A. Curtis, have been considered as host-specific species. French et al.
(1969) cross-inoculated isolates from Populus, Alnus, and Salix
species among species of these three genera and showed that some degree, perhaps high, of
host specificity exists. However, dimensions of ascospores and perithecia among
collections from various hosts did not indicate distinctive morphological differences and
they thus concluded that Miller (1961) was justified in his concept of H. mammatum
(French et al., 1969). They believed that further work might indicate pathological
variants or formae speciales within H. mammatum
(French et al., 1969). Based on this study and our own observations we presently accept
Millers concept (1961) of the species. However, we are convinced that H.
mammatum is not a Hypoxylon
Bull. as understood by us. It has been convenient to refer to the fungus as a Hypoxylon
because of its long acceptance in the genus and because of its formal treatment in
Millers monograph (1961). Hypoxylon, as comprehended by Miller (1961),
is a heterogenous assemblage. In recent years large numbers of taxa have been removed from
Hypoxylon to other genera, including Biscogniauxia Kuntze (Pouzar,
1979), Camillea Fr. (Laessøe et al., 1989), and Nemania Gray
(Pouzar, 1985a). There remain, however, other taxa that should be removed from Hypoxylon.
During the course of preparing a revision of Hypoxylon sensu stricto,
we have examined hundreds of type, authentic, and other specimens of putative Hypoxylon
species. We concluded that Hypoxylon should be restricted to taxa having all
or most of the following features: 1. stromata that are colored at some stage of their
development and yield a colored pigment --- characteristic for the taxon --- in potassium
hydroxide; 2. a germ slit on the convex side of the ascospore; 3. an immature ascospore
that does not bear a cellular appendage; 4. an ascus apical apparatus that is usually
broader than tall and often reduced to a disc or plate; 5. an ascospore perispore that
dehisces in KOH; 6. a stroma that is waxy to woody or with a carbonaceous layer that
surrounds perithecia in section Annulata
(Ju and Rogers, 1996); 7. and an anamorph referable to Nodulisporium Preuss.
Many of the taxa that we accept in Hypoxylon have all of these features and
all accepted taxa have most of them. Moreover, most Hypoxylon species appear
to be, at most, weak parasites. The most variable interspecific character is the
dehiscence of the ascospore perispore in KOH. We accept H.
fragiforme (Pers.: Fr.) J. Kickx fil. as the type species of the genus.
Hypoxylon mammatum is not a Hypoxylon in the above definition of the genus. It has a very carbonaceous stroma that overlies and partially encases mature perithecia (Rogers and Berbee, 1964); it lacks potassium hydroxide-extractable pigments. The ascospore germ slit is usually on the concave side of ascospores that are inequilateral; the perispore is rudimentary and not removed in potassium hydroxide. The immature ascospore bears a cellular appendage. The ascus apical apparatus is massive, conspicuously higher than broad. The anamorph in nature is usually borne upon bark-rupturing pillars or specialized synnemata (Rogers and Berbee, 1964) and is referable to Geniculosporium Chesters & Greenhalgh. Griffin et al. (1992) have pointed out that Nodulisporium and Geniculosporium morphologies can be found in cultures of H. mammatum, but indicated that the latter was more prevalent. Distinctions between Geniculosporium and Nodulisporium are not always absolute, as noted by Greenhalgh and Chesters(1968). We accept Geniculosporium as the anamorph of H. mammatum, as do Petrini and Müller (1986) and others. Geniculosporium is correlated with other genera that have features in common with H. mammatum and seem strongly related to it, e.g., Nemania and Rosellinia De Not.
A fungus very similar to H. mammatum was described as Entoleuca callimorpha Syd. (Sydow and Petrak, 1922). Examination of type material of this fungus convinces us that H. mammatum could and should be transferred to Entoleuca Syd. Pouzar (1985a) suggested the availability of Entoleuca for H. mammatum and Laessøe and Spooner (1994) and Laessøe (1994) agreed, but considered Entoleuca to be a synonym of Rosellinia. They did not, however, examine type material of E. callimorpha. Our examination convinces us that Entoleuca is not a Rosellinia and, indeed, is worthy of generic status.
We do not consider H. mammatum to be a Rosellinia for the following reasons. First, the development differs from that of Rosellinia species. In most Rosellinia species perithecial stromata develop in a subiculum that sometimes also bears the anamorph. In Rosellinia mirabilis (Berk. & Broome) Y.-M. Ju & J. D. Rogers [º Astrocystis mirabilis Berk. & Broome] the teleomorph develops beneath the Acanthodochium Samuels, J. D. Rogers, & Nagas. state, displacing it (Diehl, 1925; Ju and Rogers, 1990). In a number of other Rosellinia species the teleomorph is associated with coremia as well as with a subiculum. In Hypoxylon mammatum, however, no subiculum is present. The teleomorph is initiated in the bases of specialized coremioid pegs that break bark overlying them (Rogers and Berbee, 1964). As far as we are aware there is not a comparable developmental sequence in Rosellinia. Moreover, stromata of H. mammatum usually contain more than one perithecium, whereas a cardinal defining character of Rosellinia, in our opinion, is the uniperitheciate stroma. Finally, Hypoxylon mammatum is a virulent bark pathogen of apparently vigorous trees, whereas most Rosellinia species seem to be facultative parasites of roots of stressed or senescent hosts. We do not have detailed developmental data on Entoleuca callimorpha, but the general pattern seems identical to H. mammatum. We thus consider H. mammatum to represent the second species of Entoleuca (see later). Placing H. mammatum into Entoleuca allows the use of an extant genus and, thus, does not require the erection of a new genus. If fresh collections of E. callimorpha were to become available and be shown to be biologically different from H. mammatum, it is our opinion that a new genus would be required for the latter.
The affinities of Entoleuca are with Nemania S. F. Gray, Ustulina Tul. & C. Tul., Stilbohypoxylon Henn., Penzigia Sacc., and Rosellinia. These genera are characterized by white stromatal flesh that initially encases the perithecia, its development varying from scanty to robust among various taxa. In many taxa the white flesh darkens or deteriorates during perithecial maturity. The anamorph types of all of these genera (including those Penzigia species that should be reassigned to Xylaria Hill ex Schrank) are Geniculosporium or with Geniculosporium-like features.