STUDIES of PYRENOMYCETES IV. NECTRIA (Part 1)

Issued 30 December 1959 Mycological Papers, No. 73 STUDIES OF PYRENOMYCETES IV. NECTRIA (part 1) By C. BOOTH Commonwealth Mycological Institute HIS paper deals with the life histories of the British Nectria species. In Ta further paper now in preparation the predominantly tropical Nectria species will he described. The name Nectria was first proposed by Pries (1825). Under 'Order 1 Sphaeriacei; Sub-order Sphaerini; genus Hypocrea' he erected the following four sections: A. Cordyceps; B. Hypocrea; C. Hypomyce; D. Nectria; the latter with 'Perithecia superficialia subiculo tomentoso imposita aut omnino libera'. It should be noted that this diagnosis must include certain Hypomyces species together with the discrete astromatic Nectria spp. De Notaris (1844) accepted the Fries classification but changed the name to Hypocreacei; he suggested that the four sections proposed by Fries should be regarded as distinct genera. However, it remained for Fries (1849) to erect Nectria as a distinct genus when he gave the following diagnosis: ' Peritheciis liberis, membranaceis (neque corticatis nec pruina velatis) flaccidis, laete colora- tis, papilla pallida, nucleo fluxiti pallido guttae 1, floccorum alborum instar expulsae, asci 8-sporis sporisque pellucidis.' Thus, as Fries conceived the genus, it is based on structure, colour, and texture of the perithecmm. In a footnote he states that it includes species whose asci are variable and whose ascospores range from simple to filiform. We owe our modern conception of the genus to Saccardo (1878) who restricted Nectria to species with 1-septate hyaline ascospores and gave the following diagnosis: 'Perithecia erumpenti-superficialia v. superficialia, caespitosa v. dis- creta, saepe stromate pulvinato, primitus conidiophoro insidentia, rosea v. laete colorata. Asci cylindracei v. fusoidei, initio infra apicem coarcitati. Sporidia ellipsoidea v. oblonga, 1-sepatata, hyalina, loculis typice non secedentibus.' Saccardo segregated the diverse species which had been placed in the genus Nectria on the artificial but usually workable basis of spore septation, shape, and colour. The exception to this is the genus Sphaerostilbe, where he followed Tulasne and Fuckel in basing the genus on the presence of the conidial fructifica tion. Within his separation on spore characters, Saccardo also recognized the Friesian distinctions of position of perithecia with reference to the stroma and substratum. This, of course, led to the erection of the many new genera in the Hypocreaceae described by Saccardo. In spite of this segregation, when Saccardo (1883) compiled Nectria in the Sylloge Fungorum he accepted 145 species, which he arranged in the following subgenera: 1. Eu-Nectria: perithecia caespitose on a stroma; glabrescent. 2. Dialonectria: perithecia subdiscrete; glabrescent. 3. Hyponectria: perithecia glabrous but on a byssoid subicle. B 8165 A 4. Lepidonectria: peritheoia scaly. 5. Lasionectria: peritheoia minutely pilose. 6. Cryphonectria: peritheoia subimmersed in a crustose stroma. 7. Cosmospora: ascospores verrucose, rufesoent. 8. Species minus cognitae. Since 1883, mycologists have accepted Saccardo's classification with minor alterations and additions. Cooke (1884) raised Dialonectria to generic rank in his Synopsis Pyrenomycetum and this genus has been frequently used in Britain. The rest of Cooke's classification has been largely ignored. Seaver (1909) reverted to a refinement of the Fries method. He recognized the two families Hypocreaceae and Neetriaceae. The latter he divided into two tribes: 1. Nectrieae—superficial or erumpent non-stromatic peritheoia. 2. Creonectrieae—superficial caespitose peritheoia formed on a more or less conspicuous conidial stroma. Seaver's Nectria corresponds to Cooke's Dialonectria and Creonectria to the genus Nectria. The fundamental objection to Seaver's classification was pointed out by Theissen and Weese, who emphasized that the presence or absence of a stroma is a character which varies in some species with the nature of the substratum on which it develops. In Nectria coccinea the stroma is absent when the peri theoia develop on hard wood but well developed when they occur on bark. Most of the available evidence supports the view that the archicarp arises in a stroma, which in the mature perithecium may be represented only by the outer region of the perithecial wall, or may be well developed. Theissen (1911), identifying Nectria species from Brazil, went further than Saccardo in using spore characters as a basis of separation within the genus, for he proposed three sections based on the appearance of the outer ascospore wall: A. Leiosporae—spores smooth, not lineated or warted. B. Ehabdotosporae—spore membrane longitudinally striate. C. Cosmosporae—spore membrane warted. Saccardo's classification is based on the compilation of the descriptions of the Nectria species published at that time and cannot in any sense be regarded as critical. Nectria species cannot be readily distinguished on the Saccardoan principles of size and shape of the peritheoia, asci, and ascospores. A usable classification must bring together related species which have been sufficiently well described to make their identification and separation possible. The application of Theissen's separation as a primary division of the British Nectrias results in cutting across the natural groups, and brings together species which have only the ornamentation of the spore wall in common. The modern approach to the classification of Nectria has progressed along two lines. The first was begun by Hohnel and Weese, working in Austria. They began a revision of Nectria at the species level and this work resulted in a large number of species names being reduced to synonymy. Weese made a marked contribution to the taxonomy of this group when he stressed the structure of the perithecial wall as a basis of separation within the genus. He was the first to emphasize the characteristic pseudo-palisade structure of the Mammoidea group. Unfortunately, Weese never published any comprehensive classification based on his work, and his publications remain fragmentary and scattered. The first attempts to base a classification on perithecial wall characters were indecisive, but Miss Dingley (1951) was more successful when she used the perithecial wall structure as a primary character in the separation of the New Zealand Nectria spp. The second line of approach to a modern classification of Nectria has been from a study of their conidial states. These were first mentioned by Tulasne, whose remarkable work on the pleomorphic nature of fungi was published in 1861-5. Fuckel (1870) also frequently mentioned the conidial states in his descriptions of Nectria spp. Both these workers based their evidence for the conidial state on the association of conidia and peritheoia. The first serious attempt to obtain cultural evidence of the conidial states of Nectria species was made by Brefeld and Tavel (1891); they described the conidia, at least the microconidia, from ascospore isolations of more than twenty Nectria spp. However, the greatest progress in this line of approach was made by Wollen- weber (1911-43). He spent many years on the monographic investigations of Fusarium species and in the course of this work he elucidated the life-histories of a large number of Nectria spp. The emphasis of Wollenweber's work was on the conidial state and he paid comparatively little attention to the peritheoia. In 1926 he proposed a natural classification based on the morphology of the fungus as a whole, but in which the primary divisions were based on the struc ture and form of the conidial state. The conidial states of the British Nectria spp. have been placed in the follow ing imperfect genera: Acrostalagmus, Cephalosporin, Ciliciopodium, Cylindro- carpon, Fusarium, Gliocladium, Myrofhecium, Sphaeropsis, Stilbella, Stilbum, Tubercularia, Verticillium, Zythia, and Zythiostroma. These genera are normally assigned to the following four families in the fungi imperfecti: Mohiliaceae, Nectrioidaceae, Stilbellaceae, and Tuberculariaceae. When viewed from this aspect these genera appear very diverse, but, in fact, a very close inter-relationship exists and there is no clear line of demarcation between them. In the first place, they all have phialospores. In the genera with hyaline aseptate conidia the genera are based on the way the phialides are borne. In the simplest case, where the conidiophores are single lateral phialides on the mycelium, they are placed in the genus Gephalosporium. Where the conidiophores branch before bearing the phialides, we have the genus Gliocladium, unless the phialides are in whorls at each node of the conidiophore, in which case the species is assigned to the genus Verticillium. Where the branched conidiophores are aggregated on the surface of a stroma a sporodochium is formed as in Tubercularia; if the sporodochium is stalked, and therefore referred to as a synemma, we have Stilbum or Stilbella. Even in the conidial state of Nectria cinnabarina {Tubercu laria vulgaris) the first phialides form below the surface and the outer layers of the stroma are then ruptured. If these outer layers persist, then a pycnidium is formed as found in the genus Zythia or Zythiostroma. Other genera owe their status to the morphology of the conidium. In Myro- thecium they are dark. In Fusarium and Cylindrocarpon, although still remaining hyaline, they are elongate cylindrical and form transverse septa. The use of conidial names should

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