Amy Y. Rossman Systematic Mycology & Microbiology Laboratory, USDA-Agricultural Research Service, Beltsville, MD Mary E. Palm-Hernández Molecular Diagnostics Laboratory, USDA-Animal and Plant Health Inspection Service, Beltsville, MD Systematics of Plant Pathogenic Fungi: Why It Matters Systematics is the study of biological di- comparison of portions of the genome are with that name suggests that this organism versity; more specifically, it is the science also used to characterize fungi, especially is more likely to decay dead cellulosic that discovers, describes, and classifies all in determining species concepts and rela- material. organisms. Taxonomy, nomenclature, and tionships among fungi at all levels ranging As an example of scientific names phylogeny are all part of systematics. Fun- from population genetics to the phylogeny changing to reflect increased knowledge, gal systematic studies result in the discov- of major groups of fungi as well as fungal- one can examine a fungal pathogen caus- ery and description of fungi, the principles like organisms. ing root rot of woody plants known for of nomenclature guide the naming of or- many years as Armillaria mellea (Vahl:Fr.) ganisms, and phylogenetic studies contrib- What’s in a name? Karst., which has the common names in ute to the classification of taxa into geneti- Names are the means by which informa- English of honey mushroom, shoestring, or cally related groups. A taxon (pl. taxa) is a tion is communicated about an object, in bootlace fungus (Fig. 1). Decades ago, A. taxonomic group at any rank, e.g., order, this case an organism. The name of an mellea was considered to be just one ubiq- family, genus, species, and subspecies organism may be a common name or sci- uitous species infecting many different among others. entific name. Common names of organ- hosts (60). Hints of the fact that A. mellea Systematics is a dynamic science. As isms can vary considerably from place to was a species complex came from mating systematists obtain new data about fungi, place and among different languages and experiments in which several groups were they use that to more accurately determine are therefore much less precise than scien- shown to be genetically isolated by a com- the concept of a taxon and relationships tific names. Scientific names are used to plex mating system (3,67,68). More re- among taxa. When new relationships are accurately define an organism or set of cently, molecular sequence data have con- discovered or old relationships are found organisms and to communicate about firmed the existence of the groups defined to be incorrect, systematists must account them. As systematic scientists learn more by mating studies (17,18). Upon careful for those discoveries. Necessarily, this new about each species or other taxon and the examination of specimens representing the knowledge may result in changes of scien- relationships among them, scientific names different groups, morphological characters tific names. Centuries ago, fungal speci- change to reflect this increased knowledge. were discovered that reflected the distinc- mens were described simply by looking at Based on the knowledge associated with tiveness of the groups. them with the unaided eye, and then mac- the name, it is possible to predict the be- As a result of scientific advances in sys- roscopically using a ×20 hand lens. This havior or biology of that organism. For tematics, at least eight species in North approach was followed by the use of the example, if one isolates and identifies a America and five species in Europe are compound light microscope that could Phytophthora from woody plant material, recognized that would previously have magnify the image of structures up to the name Phytophthora suggests a poten- been called by one name, Armillaria ×1,000. In the late twentieth century, the tially destructive plant pathogen. On the mellea (2). Combining morphological, scanning electron microscope allowed the other hand, if a Chaetomium is found, one biological, and genetic data, narrow spe- close observation of external features of can predict that this fungus is unlikely to cies have been defined that reflect knowl- morphological structures such as ascospore cause a disease, and knowledge associated edge including the biology of each species ornamentation, while the transmission electron microscope led to the discovery of internal organelles and structures of cells. All of these tools for observing specimens are still useful. Today, sequencing and As allCorresponding author: Amy Y. Rossman, Systematic Mycology & Microbiology Laboratory, USDA-Agricultural Research Service, Rm. 304, B011A, 10300 Baltimore Ave., Beltsville, MD 20705 USA; E-mail: [email protected] doi:10.1094/ PDIS-92-10-1376 This article is in the public domain and not copy- Fig. 1. Species of Armillaria. A, Armillaria mellea sensu stricto, associated with Fagus rightable. It may be freely reprinted with custom- grandifolia (beech), North Carolina, Schenck Forest, October 1998. B, Armillaria ary crediting of the source. The American Phyto- ostoyae on Quercus coccinea (scarlet oak), North Carolina, Bent Creek, October 1998. pathological Society, 2008. Photos by Larry F. Grand, North Carolina State University. 1376 Plant Disease / Vol. 92 No. 10 (9,14,33). For example, Armillaria tional Code of Zoological Nomenclature on adaptation to a different host. Although luteobubalina Watling & Kile occurs only governs the naming of animals, while bac- this designation is recognized by the in Australia, where this fungus is a primary teria and other prokaryotic organisms are ICBN, its use is not governed by those pathogen of Eucalyptus causing decline named according to the International Code rules. and death especially in plantations (36). of Nomenclature of Bacteria. The first time a scientific name is men- Now recognized as distinct, this species The International Code of Botanical No- tioned in a research article, the author(s) was recently reported from Chile (17). menclature has been developed over many who initially described a taxon, often a Another segregate species, Armillaria decades based on principles that have re- species, is associated with that name, as is ostoyae (Romagn.) Herink, occurs in the mained relatively stable since about the the author(s) who may later transfer that Northern Hemisphere (Fig. 1B). Making 1950s. These basic principles are: (i) the name to another genus. As an example, we the distinction between these species using first scientific name applied to a species or will use Neonectria coccinea (Pers.:Fr.) their scientific names is essential for pre- other taxon has priority; (ii) each name Rossman & Samuels, cause of beech bark venting the movement of A. luteobubalina must be based on a type that represents canker in Europe. This name was origi- to South Africa or spreading A. ostoyae to that entity. In the case of a species, the nally described as Sphaeria coccinea New Zealand. If a pest risk assessment type is a specimen; for a genus, the type is Pers.:Fr. by Persoon in 1800 (49) and then were based on A. mellea as it was defined a species; for a family, the type is a genus, listed in Fries’s 1823 Systema Myco- 50 years ago as one cosmopolitan species, etc.; (iii) a name must be published in a logicum (27). Because this name is men- it is possible that conifers would be moved specific manner, i.e., with a Latin descrip- tioned by Fries (27), it is sanctioned ac- from the Northern Hemisphere to New tion, a type designated, the publication cording to the ICBN. This sanctioned Zealand, not knowing that this posed the widely distributed, among other require- status is denoted by the “:Fr.” and, because risk of introducing a pathogenic species ments; and (iv) a species can have only one it is sanctioned, this name has priority over not known to be present in that country. correct scientific name except in the case other names for this species published Species previously recognized as Armil- of fungi that have alternate states (Article prior to Fries (27). According to the ICBN, laria mellea sensu lato (in the broad sense) 59, ICBN). The ICBN also governs how to Fries’s major publications are considered are now known as precisely defined spe- transfer a species name from one genus to the starting point for the nomenclature of cies with names that communicate their another as the concept of the genus or ascomycetes and certain other groups of biological differences, including host species changes, as well as many more fungi. Later, Fries (28) revised his concept range, pathogenicity, and geographic dis- details about the naming of plants and of Sphaeria, a genus that initially included tribution. fungi. Changes are made to the ICBN all perithecial ascomycetes, and placed this following much discussion, debate, and a species in Nectria, a genus that included What rules govern the scientific vote at the Nomenclature Session during fleshy, colored, uniloculate, perithecial names of fungi? the International Botanical Congresses ascomycetes. With that taxonomic deci- Nomenclature is the branch of systemat- held every six years. Although the details sion, the scientific name of this species ics that determines the correct scientific and complexity of the ICBN have changed became Nectria coccinea (Pers.:Fr.) Fr. name for a taxon. The naming of fungi is over time, the basic principles have re- The abbreviation Fr. outside the parenthe- governed by the International Code of mained the same. The stability of scientific ses refers to the author who placed the Botanical Nomenclature (ICBN) (42) be- names of plants and fungi is facilitated species name in that genus. Later, when cause of the historical assumption that through application of the ICBN to issues Rossman et al. (54) placed this species in fungi and plants were closely related. We of nomenclature. the genus Neonectria as N. coccinea now know fungi in the traditional sense Each scientific name for a species con- (Pers.:Fr.) Rossman & Samuels, their comprise a diverse range of organisms sists of two or more portions, minimally, a names were placed outside the parenthe- including true Fungi (Kingdom Fungi), genus and a species epithet.