Generic names in Magnaporthales

Abstract

The order Magnaporthales comprises about 200 species and includes the economically and scientifically important rice blast fungus and the take-all pathogen of cereals, as well as saprotrophs and endophytes. Recent advances in phylogenetic analyses of these fungi resulted in taxonomic revisions. In this paper we list the 28 currently accepted genera in Magnaporthales with their type species and available gene and genome resources. The polyphyletic Magnaporthe 1972 is proposed for suppression, and Pyricularia 1880 and Nakataea 1939 are recommended for protection as the generic names for the rice blast fungus and the rice stem rot fungus, respectively. The rationale for the recommended names is also provided. These recommendations are made by the Pyricularia/Magnaporthe Working Group established under the auspices of the International Commission on the Taxonomy of Fungi (ICTF).

Introduction

Magnaporthales (Sordariomycetes, Ascomycota) contains important pathogens of cereals and grasses, including the rice blast fungus Pyricularia oryzae (Magnaporthe oryzae), the take-all pathogen of cereals Gaeumannomyces graminis, the rice stem rot pathogen Nakataea oryzae (Magnaporthe salvinii) and the summer patch pathogen of turf grass Magnaporthiopsis poae (Cannon 1994, Thongkantha et al. 2009). To date, about 200 species have been described in Magnaporthales, of which approximately 50% are pathogens of domesticated and wild monocotyledons.

The rice blast fungus has conidial (asexual) and ascosporic (sexual) morphs and the rice stem rot fungus produces sclerotial (asexual), conidial (asexual) and ascosporic (sexual) morphs. Historically, such pleomorphy added to the difficulty in resolving taxonomic and nomenclatural problems associated with these species.

Recent advancement in gene, transcriptome and genome sequencing of Magnaporthales fungi resulted in robust phylogenies, which correspond well with the pathogenicity, ecology and biology of these species. However, the phylogenies conflict with certain traditional generic concepts based on morphology. Magnaporthe and Gaeumannomyces, for example, were shown to be polyphyletic. Taxonomic revisions have been carried out for some of these taxa in recent publications (Luo & Zhang 2013, Klaubauf et al. 2014, Luo et al. 2015a). In this paper, we list 28 accepted genera in Magnaporthales and provide the rationale for the recommended genera if there is competition.

A list of accepted generic names in Magnaporthales, with the type species, is compiled in Table 1 including references that serve as the basis for recognizing these genera. This follows approval of their usage by the Pyricularia/Magnaporthe Working Group, without prejudice. Cases that require action to be approved by the Nomenclature Committee for Fungi (NCF) are indicated by an asterisk in that Table.

Table 1 Accepted generic names in Magnaporthales with type species, number of species in each genus, and gene or genomic resources. Names needing approval are indicated by an asterisk (*).

Recommendations

(A) = a name typified by an asexual morph, (S) = a name typified by a sexual morph.

Use Nakataea Hara 1939 (A) rather than Magnaporthe R.A. Krause & R.K. Webster 1972 (S)

Cattaneo (1876) first named the rice stem rot pathogen as Sclerotium oryzae based on the sclerotial morph. In the same paper he also described Leptosphaeria salvinii, which was later recognized as the sexual morph of the same fungus (Tullis 1933). Hara (1939) named the conidial morph of the species Nakataea sigmoidea. Krause & Webster (1972) then established the new generic name Magnaporthe, typified by the ascosporic morph, to accommodate the rice stem rot pathogen as Magnaporthe salvinii, as necessitated by the nomenclatural rules then in force. Sclerotium and Leptosphaeria currently belong in Basidiomycota and Dothideomycetes respectively (Xu et al. 2010), and are therefore not applicable to this sordariomycetous species. Nakataea and Magnaporthe are congeneric and their type species, Nakataea sigmoidea and Magnaporthe salvinii, refer to the same species (Krause & Webster 1972). Subsequent to the ending of the separate naming of morphs of the same fungus species in 2011, under Art. 59.1 of the International Code of Nomenclature for algae, fungi, and plants (ICN; McNeill et al. 2012), Luo et al. (2013) made a new combination for the rice stem rot fungus as Nakataea oryzae, using the oldest legitimate generic name and species epithet. Those authors did not, however, formally propose the rejection or suppression of the later sexually typified names as currently required by Art. 57.2. We note that it is proposed that this requirement be eliminated from the ICN in 2017 (Hawksworth 2015), but it is currently in force.

Use Pyricularia oryzae Cavara 1892 (A) rather than Magnaporthe oryzae (Catt.) B.C. Couch 2002 (S)

In 1880, Saccardo established the generic name Pyricularia based on the asexually typified P. grisea on crabgrass. The rice isolates were designated as P. oryzae in 1892 by Cavara, which now is known as the rice blast fungus. Since then, over 50 species have been listed as Pyricularia that cause blast diseases of monocotyledonous plants.

The sexual morph of Pyricularia was irst observed in 1970 from laboratory crossing experiments and believed to belong to Magnaporthe because of the similarity in ascospore morphology (Hebert 1970, Barr 1977, Couch & Kohn 2002). However, recent phylogenetic and phylogenomic analyses demonstrated that the sexually typified genus Magnaporthe was polyphyletic. The rice blast fungus is not congeneric with the type species of Magnaporthe, M. salvinii, and the placement of the rice blast fungus in Magnaporthe was based on an incorrect morphological identiication (Zhang et al. 2011, Luo & Zhang 2013, Luo et al. 2014, Murata et al. 2014, Luo et al. 2015a). This is not a nomenclatural issue because the generic names Magnaporthe and Pyricularia are not congeneric and so do not compete for priority.

Pyricularia and Magnaporthe are currently both widely used generic names, and the rice blast fungus is an economically and scientiically important species that deserves much caution. The Pyricularia/Magnaporthe Working Group has considered the possibility of conserving the name Magnaporthe over Pyricularia. However, such conservation would require a change in the type species of the genus Magnaporthe, and would cause numerous name changes for those species currently placed in Pyricularia.

The asexually typified generic name Pyricularia is the correct name for the rice blast fungus, which corresponds well with pathogenicity and ecological and evolutionary features. The name Pyricularia oryzae should therefore be used for the rice blast fungus. The synonym Magnaporthe oryzae, can nevertheless continue to be mentioned in publications as “Pyricularia oryzae (syn. Magnaporthe oryzae)”. This practice will help to bridge a potential gap in the literature and knowledge for this important species.

Use Clasterosporium Schwein. 1832 (A) rather than Clasterosphaeria Sivan. 1984 (S)

The generic name Clasterosphaeria, typified by C. cyperi, was established for the sexual morph of Clasterosporium cyperi and includes only two names. The generic name Clasterosporium based on C. caricinum includes 158 names, many of which have been placed in other genera. Whether or not Clasterosphaeria cyperi is congeneric with Clasterosporium caricinum is not known, although this seems likely given that both occur on Cyperaceae. If this is the case, use of the older, more commonly used generic name Clasterosporium is recommended.

Use Gaeumannomyces Arx & D.L. Olivier 1952 (S) rather than Harpophora Gams 2000 (A)

The generic name Gaeumannomyces, typified by G. graminis, has long been used for the cause of take-all of wheat disease (Walker 1972, 1980). Harpophora was established for phialophora-like species that were known to be asexual morphs related to Gaeumannomyces and Magnaporthe but did not produce a sexual morph (Gams 2000). With the change to one name, Harpophora based on H. radicicola is to be considered a synonym of Gaeumannomyces, based on phylogeny (Luo et al. 2015b). Given the greater number of species, priority, and numerous reports, we see no reason not to use the irst published name, Gaeumannomyces.