Ceratocystis species: Taxonomic challenges in a group of pathogens of increasing global importance

Jolanda Roux¹, Brenda D. Wingfield² & Michael J. Wingfield¹

¹Department of Microbiology and Plant Pathology, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, South Africa; ²Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, South Africa; jolanda.roux@fabi.up.ac.za

The genus Ceratocystis includes numerous pathogens of plants, important to both agriculture and forestry. During the course of the past two decades, the number of reports of tree diseases caused by species in the C. fimbriata complex, as defined by phylogenetic inference, has increased more than three-fold (Roux & Wingfield 2009).  Ceratocystis acaciivora in Asia, C. albifundus in Africa, C. fimbriata sensu lato and C. cacaofuenesta in South America, C. manginecans in the Middle East and C. platani in Europe, for example, all result in devastating diseases of trees. These species are easily moved in soil, on plant material and with the insects with which they are associated, and together with their often wide host range and ability to undergo host shifts, they represent a very significant global quarantine threat. Yet, the study of this group of pathogens is encumbered by a lack of ideal markers to clearly define species boundaries.

The name Ceratocystis was first applied to a pathogen of sweet potato in the USA, C. fimbriata, in the late 1800s.  Since its first description, the taxonomy of the genus and its type species has undergone several revisions. For many years the taxonomy of especially Ceratocystis and Ophiostoma species was confused, due to their morphological similarities.  This ultimately resulted in the term ophiostomatoid fungi being applied to species of Ceratocystis and Ophiostoma, arguably resulting in increased taxonomic confusion; ironically, despite the application of DNA sequence data showing that they reside in different orders of fungi, the Microascales and Ophiostomatales (De Beer et al. 2013).

 Ceratocystis resides in the Ceratocystidaceae as part of the Microascales (Reblova et al. 2011).  However, both generic and species boundaries for fungi in this family are being subjected to ongoing revision.  As part of this extensive revision, Ceratocystis will in future be applied only to species in the phylogenetic clade that accommodates C. fimbriata (Wingfield et al. 2013).

At the species level, host range, morphology, mating studies, DNA sequence comparions and population genetic markers have been used to resolve boundaries for taxa in the C. fimbriata complex.  While the boundaries remain unresolved at the present time, there is substantial evidence to show that there are many different species in the C. fimbriata complex.  Some of our recent genomics studies have shown that there are multiple forms of the ITS gene region in some species of C. fimbriata s.l (Naidoo et al. 2013). There is also emerging evidence for hybridisation between species, which is not surprising given that many species occur in comnon niches and have been moved widely around the world.  For this reason, it is likely that some species names currently being used will be reduced to synomymy while others will be shown to represent species complexes.  The genomes of the many species in the C. fimbriata complex, now available to us, are already providing new and useful markers, for both taxonomic and biological studies.  These will ultimately bring an increased understanding of this important group of pathogens and hopefully also facilitate steps to reduce their global spread.

References

De Beer ZW, Seifert KA, Wingfield MJ (2013). The ophiostomatoid fungi: their dual position in the Sordariomycetes. In: The Ophiostomatoid Fungi: Expanding Frontiers (Seifert KA, ZW De Beer and MJ Wingfield, eds). CBS Biodiversity Series 12. CBS, Utrecht, The Netherlands: 1-19.

Naidoo K, Steenkamp ET, Coetzee MPA, Wingfield MJ, Wingfield BD (2013).  Concerted evolution in the ribosomal RNA cistron.  PLOS One 8, e59355.doi:10.1371/journal.pone.0059355

Réblová M, Gams W, Seifert KA (2011). Monilochaetes and allied genera of the Glomerellales, and a reconsideration of families in the Microascales. Studies in Mycology, 68, 163–191. doi:10.3114/sim.2011.68.07

Roux J, Wingfield MJ (2009). Ceratocystis species: Emerging pathogens of non-native plantation Eucalyptus and Acacia species. Southern Forests 71, 115-120.

Wingfield BD, Van Wyk M, Roos H, Wingfield MJ (2013). Ceratocystis: emerging evidence for discrete generic boundaries. In: The Ophiostomatoid Fungi: Expanding Frontiers (Seifert KA, ZW De Beer and MJ Wingfield, eds).  12. CBS, Utrecht, The Netherlands: 57–64.

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