Fusarium taxonomy: a hot debate
Fusarium is fungal genus which includes many species that are important plant pathogens but whose delimitation has been subject to much debate among mycologists.
Fusarium or not Fusarium?
Within the context of the application of the new ‘One fungus One name’ rules, the question of conserving the anamorph name Fusarium over teleomorph names was discussed. In 2013 and in a letter to the editor of the journal Phytopathology, a large number of Fusarium specialists (Geiser et al., 2013) supported the view that the name Fusarium should be kept and that teleomorph names should be abandoned.
However, while reviewing the family Nectriaceae, it was proposed in 2015 to split Fusarium into 7 genera: Albonectria, Bisifusarium, Cyanonectria, Fusarium (species belonging to the Gibberella clade), Geejayessia, Neocosmospora (including the agriculturally important Fusarium solani species complex), Rectifusarium (Lombard et al., 2015). These changes were reflected in taxonomic databases (e.g. MycoBank).
In 2021, 166 scientists from 30 countries signed another letter to the editor of the journal Phytopathology (Geiser et al., 2021) to voice their disagreement with this change. They considered that: ‘although the generic concepts proposed by Lombard et al. (2015) are monophyletic and nomenclaturally valid, they fail on the practicality criterion because they exclude species with a longstanding place in genus Fusarium’. In addition, they queried a number of scientific results used to substantiate the subdivision of Fusarium.
This opinion is contested by 127 scientists from 43 countries in Crous et al. (2021), arguing that: ‘the generic treatment of Fusarium by Geiser et al. (2013, 2021), produced an ill-delimited genus without clear synapomorphies, as fusarium-like macroconidia are strongly polyphyletic within Nectriaceae and also occur outside their very broadly circumscribed Fusarium concept. We argue that a narrower concept of genera with a clear, unique combination of features is needed for the majority of fusarioid species.’.
Neocosmospora and other genera were also supported in a list produced by the International Subcommission for the Taxonomy of Phytopathogenic Fungi.
Case of Panama disease of banana
In 2019, it was proposed to consider Fusarium oxysporum f. sp. cubense Tropical race 4 (Foc TR4) as a distinct species called Fusarium odoratissimum (Maryani et al., 2019). However, this proposal has been questioned by Torres-Bedoya et al. (2021) who considered that this change was premature and not sufficiently substantiated. In addition, it was noted that F. odoratissimum is not a synonym of Foc TR4 as at least one isolate does not belong to Fusarium oxysporum f. sp. cubense (but to another forma specialis).
EPPO Secretariat note
In 2019, Summerell stated that ‘it is critically important that Fusarium has a stable taxonomy with well-defined generic and species concepts that allow practitioners diagnosing diseases, identifying these fungi, and developing management strategies the confidence to make decisions about the identity of the species they are interested’. As expressed by Crous et al. (2021): ‘… all scientists working with Fusarium desire a stable taxonomic system, and all agree that taxonomic changes should be made with the aim of promoting stability’.
While recognizing the difficulties in delimiting taxon boundaries, being a user of taxonomy in all its Standards, documents and databases, the EPPO Secretariat would exceptionally take this opportunity to make a plea for a more stable and robust taxonomy that is essential to ensure correct communication and avoid confusion. This remark applies not only to Fusarium but to many other taxa that are of importance in plant health.
For example and in relation to the management of EPPO Codes and their associated scientific names in the EPPO Global Database, the EPPO Secretariat would like to emphasize that it thrives to: 1) assign a unique code to each taxon; 2) associate relevant scientific names to this unique code and select the preferred scientific name based on available taxonomic sources; and 3) when the preferred name is modified, make the necessary changes to the EPPO Global Database. For the moment, the proposal to transfer some Fusarium species to Neocosmospora and other genera has been followed in the EPPO Global Database. These species still have their Fusarium names as synonyms and have necessarily kept the same EPPO Code. The preferred names of individual species will be adjusted when the controversy is resolved.
Sources
Crous et al. (2021) Fusarium: more than a node or a foot-shaped basal cell. Studies in Mycology 98, 100116, 184 pp.
Geiser DM et al. (2013) Letter to the Editor. One Fungus, One Name: defining the genus Fusarium in a scientifically robust way that preserves longstanding use. Phytopathology 103(5), 400-408.
Geiser DM et al. (2021) Letter to the Editor. Phylogenomic analysis of a 55.1-kb 19-gene dataset resolves a monophyletic Fusarium that includes the Fusarium solani species complex. Phytopathology 111(7), 1064-1079. https://doi.org/10.1094/PHYTO-08-20-0330-LE
International Subcommission for the Taxonomy of Phytopathogenic Fungi. List of plant pathogenic fungi 18 January 2021. Available at https://www.fungaltaxonomy.org/subcommissions
Lombard L, van der Merwe NA, Groenewald JZ, Crous PW (2015) Generic concepts in Nectriaceae. Studies in Mycology 80, 1989-245. https://doi.org/10.1016/j.simyco.2014.12.002
Maryani N, Lombard L, Poerba YS, Subandiyah S, Crous PW, Kema GHJ (2019) Phylogeny and genetic diversity of the banana Fusarium wilt pathogen Fusarium oxysporum f. sp. cubense in the Indonesian centre of origin. Studies in Mycology 92, 155-194. https://doi.org/10.1016/j.simyco.2018.06.003
Summerell BA (2019) Resolving Fusarium: current status of the genus. Annual Reviews 57, 15.1-15.17.
Torres Bedoya E, Bebber D, Studholme DJ (2021) Taxonomic revision of the banana Fusarium wilt TR4 pathogen is premature. Phytopathology 111(12), 2141-2145. https://doi.org/10.1094/PHYTO-03-21-0089-LE