Genus Hypoxylon
Key to Taxa of Hypoxylon
Accepted Taxa
List of Names

Yu-Ming Ju
Michael J. Adams

The definition and delimitation of Hypoxylon has differed among mycologists from time to time. When Hypoxylon is used herein, it refers to Hypoxylon sensu stricto which is described in detail in GENERIC DESCRIPTION. When appropriate, Hypoxylon as interpreted by other mycologists is followed by the word sensu and the mycologist's name(s).

Hypoxylon Bull. is conserved against the earlier homonyms, Hypoxylon Mentz ex Adanson and Hypoxylon Jussieu, and the earlier synonym, Sphaeria Haller. Hypoxylon Mentz ex Adanson, Hypoxylon Jussieu, and Sphaeria Haller are lectotypified by "Hypoxylon Mentz. t. 6" [= Xylaria polymorpha (Pers.: Fr.) Grev.], Hypoxylon cornutum (Hoffm.) Mérat [= Xylaria hypoxylon (L.: Fr.) Grev.], and Sphaeria rubra, fragi similis Haller [= H. fragiforme (Pers.: Fr.) J. Kickx fil.], respectively (see Donk, 1964; Greuter, 1988). After Bulliard (1791), application of the name Hypoxylon was largely abandoned in the mainstream of mycology in favor of Sphaeria Haller for half a century (e.g., Persoon, 1801; Fries, 1823; Schweinitz, 1822, 1832).

Persoon (1801) distributed seven species of Hypoxylon (nos. 12-18) in his second section, Periphaericae, of Sphaeria. Schweinitz (1822) followed Persoon in placing Hypoxylon taxa in Sphaeria, assigning seven of them (nos. 13-19) to section Periphericae and two (nos. 45, 46) to section Compressae, and one, Sphaeria truncata Schwein. (no. 174), to section Simplices. Fries (1823) continued using Sphaeria in which six (nos. 19-24) Hypoxylon taxa were listed under tribe Pulvinatae of section Periphericae, two (nos. 37, 38) under tribe Connatae of section Periphericae, and Sphaeria truncata (no. 282) under tribe Byssisedae of section Superficiales. Fries (1828) updated his previous system of Sphaeria and assigned several newly described taxa to his subgeneric classification. It is noteworthy that Fries (1828) placed Sphaeria annulata Schwein. (no. 24) in tribe Pulvinatae and Sphaeria marginata Schwein. (no. 53) in tribe Glebosae (see NOTES on Hypoxylon annulatum and H. truncatum). This system was soon adopted by Schweinitz (1832) who distributed 19 Hypoxylon taxa in four tribes of Sphaeria: one taxon (no. 1166) in Poronia; ten (nos. 1169-1174, 1176-1177, 1179-1180) in Pulvinatae; seven (nos. 1193-1194, 1204, 1209-1211, 1214) in Connatae; and Sphaeria truncatula Schwein. (no. 1501) in Byssisedae.

In spite of the then-prevailing usage of Sphaeria to cover Hypoxylon taxa, several authors such as Greville, Kickx, Montagne, and Westendorp used Hypoxylon to include various fungi of which only a small proportion belong to Hypoxylon.

Fries (1849) was credited by Miller (1961) as the founder of Hypoxylon sensu J. H. Miller. However, this is arguable. It appears that Nitschke (1867) was more instrumental in defining the genus towards Miller's sense (see later). Hypoxylon sensu Fries (Fries, 1849), although more advanced than its predecessors in including a higher percentage of Hypoxylon taxa, was still quite heterogeneous. While reinstating the generic name Hypoxylon, Fries (1849) listed 25 taxa of Hypoxylon in Summa Vegetabilium Scandinaviae. Approximately one-third of these taxa can not be accommodated in Hypoxylon sensu J. H. Miller, including two Camarops P. Karst., one Cercophora Fuckel, two Daldinia Ces. & De Not., one Lopadostoma (Nitschke) Traverso, one Valsaria Ces. & De Not., and one dothidealean fungus. Three subgeneric groups, which were apparently derived from those tribes of his Sphaeria in the Systema (Fries, 1823), appeared in Fries' 1849 work, using only the gross morphology of stromata to distinguish them. Glebosae contains taxa of Biscogniauxia, Camarops, Lopadostoma, and Ustulina Tul. & C. Tul.; Pulvinatae contains taxa of Daldinia, Valsaria, and the pulvinate taxa of Hypoxylon; and Effusae contains the effused taxa of Hypoxylon. Montagne's subgenus Bacillaria (Montagne, 1840) was segregated from Hypoxylon by Fries (1849) who erected a new genus Camillea Fr. to accommodate it.

Tulasne and Tulasne (1863) recognized the bipartite nature of stromata in three xylariaceous fungi, and erected a new genus Nummularia Tul. & C. Tul. to accommodate them. They are N. bulliardi Tul. & C. Tul. [º Biscogniauxia nummularia (Bull.: Fr.) Kuntze], N. discreta (Schwein.) Tul. & C. Tul. [= Biscogniauxia marginata (Fr.: Fr.) Pouzar], and N. dryophila Tul. & C. Tul. [º Obolarina dryophila (Tul. & C. Tul.) Pouzar]. Moreover, they found that the anamorphs of Nummularia, unlike those of Hypoxylon which are produced on the surface of stromata, are borne between a stromatal layer that dehisces with the overlying bark, which they termed as "fleshy and variously swollen stratum", and an underlying conidium-bearing stromatal layer, which they termed "crust". In the same work (Tulasne and Tulasne, 1863), they erected another new genus, Ustulina, because its conidiophores are arranged in palisades as in Xylaria Hill ex Schrank rather than Hypoxylon. Their decision to keep Nummularia and Ustulina generically distinct from Hypoxylon, although not favored by Miller (1928, 1961), has been widely recognized by most mycologists. However, Tulasne and Tulasne (1863) still kept fungi usually considered to be Daldinia in Hypoxylon. Daldinia was established in the same year by Cesati and De Notaris (1863), based on Sphaeria concentrica Bolton: Fr. which up to that time had been commonly accepted as Hypoxylon concentricum (Bolton: Fr.) Grev.

Nitschke (1867) described 26 species of Hypoxylon with great detail in Pyrenomycetes Germanici. Except for including one species of Camarops [in section Bolinia Nitschke] and one species of Daldinia [in section Daldinia (Ces. & De Not.) Nitschke], Hypoxylon sensu Nitschke is within the generic concept of Miller's Hypoxylon (1961). It is fair to say that Nitschke's contribution modernized Hypoxylon. Nitschke (1867) followed the Tulasne brothers in separating Ustulina and Nummularia from Hypoxylon as well as in including Daldinia in Hypoxylon. His subgeneric classification system for Hypoxylon, which is as admirable as his generic concept of Hypoxylon, was later incorporated into Miller's system (1928, 1961). Nitschke (1867) used the ostiolar character in combination with the relative position between stromata and substrates to distinguish three sections, III, IV, and V. Section III, Euhypoxylon, contains taxa with umbilicate ostioles. Section IV, Epixylon, contains taxa with papillate ostioles and superficial stromata. Section V, Endoxylon, contains taxa with papillate ostioles and more or less immersed stromata. In section Epixylon, Nitschke (1867) included an Italian species H. michelianum Ces. & De Not. which was then the only known European Hypoxylon with annulate discs surrounding the papillate ostioles.

Saccardo (1882a) dispersed Hypoxylon taxa among the first three sections of his Hypoxylon, namely, Euhypoxylon, Placoxylum, and Endoxylon, and Kretzschmaria taxa in the fourth section of his Hypoxylon, Coenopus. Saccardo's 1882 system is a compromise between Fries' and Nitschke's. He used both the gross morphology of stromata, as used in Fries' system, and the relative position between stromata and substrates, as used in Nitschke's system, to differentiate the three sections containing Hypoxylon taxa. However, the ostiolar character was not used by Saccardo (1882a). Meanwhile, Saccardo (1882a) raised Nitschke's section Bolinia to the generic level, and recognized Daldinia, Nummularia, and Ustulina as distinct genera. Bolinia (Nitschke) Sacc. was treated as a synonym of Camarops P. Karst. by Nannfeldt (1972).

Cooke (1883b) intended to compile the described Hypoxylon names up to that time. He published some named but unpublished specimens left by M. J. Berkeley. He also amended some Hypoxylon descriptions by adding ascospore sizes that were not included when first published. He also excluded some Hypoxylon names and referred them to other taxonomic positions. He basically followed Fries' concept in classifying Hypoxylon into four sections, Macroxylon, Sphaeroxylon, Clitoxylon, and Placoxylon, but adopted Nitschke's section Endoxylon for those taxa with immersed stromata. For Saccardo's section Coenopus, Cooke (1883a, 1883c) established a new genus Rhopalopsis Cooke. Immediately thereafter, Saccardo (1883) reinstated the genus Kretzschmaria Fr. and synonymized Rhopalopsis with it.

Cooke's system (1883b) was soon adopted by Ellis and Everhart (1888) and Saccardo (1891). On the other hand, Winter (1887) and Traverso (1906) continued using Nitschke's system in subdividing most Hypoxylon taxa into three subgenera, Euhypoxylon, Epixylon, and Endoxylon. Up to this point, most students of pyrenomycetes seemed to have reached a consensus in not including Daldinia, Nummularia, and Ustulina in Hypoxylon.

Petch (1924) critically restudied the xylariaceous fungi, including Hypoxylon, that were described by Berkeley and Broome (1873) from Sri Lanka, formerly Ceylon. Petch (1924) also described several new taxa of Hypoxylon and other Xylariaceae of that country. Not only did he give precise descriptions of the taxa that were dealt with in his study, but he also extensively tested them with a KOH solution and an ethanol solution, and recorded colors of the extracted pigments. This is probably the first time that chemical tests had been systematically applied to Hypoxylon.

Shear (1928, 1938, 1941, 1945, 1947) was one of the first mycologists who intended to do a worldwide monographic study of the genus Hypoxylon. Although he finally gave up due to ill health, his detailed accounts of a number of Hypoxylon taxa, especially their type and/or authentic specimens, are still highly valuable.

Rick (1931) basically republished Theissen's treatments (e.g., 1908a, 1908b, 1909) on Brazilian Hypoxylon. However, he had his own idea in subdividing Hypoxylon. He listed seven sections, Marginatum, Pulvinatum, Glomerulatum, Effusum, Rubiginosum, Serpens, and Hypodiscus. Unfortunately, he did not explain how they were separated, and his subgeneric system is thus regrettably inoperable.

The late J. H. Miller started publishing on Hypoxylon in 1928. Miller (1928) recognized four groups in Hypoxylon. In spite of the differences of assigning various species to their subgeneric taxa, Miller's first and third groups in general correspond to Nitschke's sections Euhypoxylon and Endoxylon, respectively. Miller's second group roughly corresponds to Nitschke's Epixylon; both Miller and Nitschke included taxa with papillate ostioles and with or without annulate ostiolar discs in their corresponding subgeneric taxa. Miller disposed of applanate forms of Nummularia in his fourth group of Hypoxylon. He was uncertain about the disposition of cupulate forms of Nummularia, as exemplified by N. discreta. He wrote that N. discreta "appears to be intermediate between the third and fourth series, but it has affinities for the genus Anthostoma". It should be noted that Miller's definition (1928) of Anthostoma, which was proven to be diatrypaceous (Rappaz, 1992), is essentially the same as Lopadostoma. Later, Miller (1932) emended Nummularia and selected N. discreta as the type species. The problems that are involved with Nummularia were discussed by Pouzar (1979).

In his final monumental study of Hypoxylon, Miller (1961) modified his previous scheme of 1928, and proposed four sections for Hypoxylon: Hypoxylon, Papillata, Annulata, and Applanata. Section Papillata was further subdivided into two subsections, Papillata and Primocinerea. Miller (1961) gave a relatively broad definition to the genus. Nonetheless, he definitely excluded Daldinia, Camarops, cupulate forms of Biscogniauxia, and erect forms of Camillea from Hypoxylon. Totally, 120 taxa were recognized in his monograph.

One of the most troublesome parts of Hypoxylon sensu J. H. Miller involves H. rubiginosum and its segregates. Miller (1928) considered that the stroma of H. rubiginosum "seems to be the most variable of all the Hypoxylon species". He also considered that the variation of stroma "is entirely plastic under different environmental conditions". This concept seems not to have changed for the rest of his life. This was reflected by the continuously increasing length of the lists of synonyms under H. rubiginosum over time (Miller, 1930, 1934, 1942, 1961). Finally, Miller (1961) concluded that H. rubiginosum is "the most common species in the world and the most variable". This broad concept made it easy to identify specimens as H. rubiginosum but, on the other hand, also blurred the boundaries between H. rubiginosum and other Hypoxylon. His treatments of H. serpens, H. truncatum, and H. nummularium suffer the same type of problem. Rogers (1979) pointed out that there is a variety of germ slit types in Miller's H. serpens. Later, this complex problem was further pursued by Pouzar (1985a, 1985b) mainly for European taxa, and by Petrini and Rogers (1986) for most published taxa that were involved.

Martin (1967a, 1967b, 1968a, 1968b, 1969a) modified Miller's 1961 system, and proposed four sections: Euhypoxylon, Annulata, Papillata, and Entoleuca. Martin's sections Euhypoxylon (1969a), Annulata (1968b), and Papillata (1968b) roughly correspond to Miller's sections Hypoxylon, Annulata, and subsection Papillata of section Papillata, respectively. In his section Entoleuca, not only did Martin (1967b, 1968a) include those taxa in Miller's section Papillata, subsection Primocinerea, but he also included taxa from Coniochaeta (Sacc.) Cooke, Rosellinia De Not., and penzigioid taxa of Xylaria. Martin (1969b) then transferred those taxa in Miller's section Applanata along with taxa of Camarops to his new section Innata of Numulariola House [º Biscogniauxia] where he also disposed of taxa of Camillea, as circumscribed by Dennis (1957), under his section Camillea. Martin (1970) excluded Ustulina from Hypoxylon but considered it to be a synonym of Kretzschmaria.

Pouzar (1979, 1986) reinstated the genus Biscogniauxia to which those dark-ascospored taxa of Miller's Hypoxylon section Applanata were assigned. Biscogniauxia is a nomenclatural synonym of Nummularia Tul. & C. Tul., the latter name being a later homonym of Nummularia Hill and, hence, illegitimate. Pouzar's decision of reinstating Biscogniauxia was subsequently accepted by Callan and Rogers (1986), Petrini and Müller (1986), Granmo et al. (1989), and others. Later, Whalley et al. (1990a) transferred those taxa with appendaged ascospores that Miller (1961) had placed in Hypoxylon section Applanata to Biscogniauxia. Recently, Laessøe et al. (1989) monographed Camillea into which they transferred the light-ascospored taxa of Miller's Hypoxylon section Applanata.

Pouzar (1985a, 1985b) transferred some taxa of Miller's Hypoxylon section Papillata, subsection Primocinerea, primarily those taxa close to Hypoxylon serpens, into the genus Nemania. On the other hand, Petrini and Rogers (1986) and Petrini and Müller (1986) preferred to retain these taxa tentatively in Hypoxylon until the limits among these hypoxyloid genera could be straightened out. We accept Nemania at present for a small number of taxa closely related to H. serpens.

The circumscription of the genus Hypoxylon is now restricted to roughly include Miller's Hypoxylon section Hypoxylon, section Annulata, and section Papillata, subsection Papillata. For most taxa of Miller's Hypoxylon section Papillata, subsection Primocinerea, we prefer to leave them in Hypoxylon for the time being until their taxonomic positions are eventually sorted out.