Taxonomy

The taxonomy of the DED fungus has been subject to many changes. A recent overview of the classification of the genus Ophiostoma (including its asexual stages) within the division Ascomycota (kingdom Fungi) is given in Fig. 4 {[515]}.Ophiostoma sp. belong to the class Pyrenomycetes, the order Ophiostomales, and the genus Ophiostoma, which comprises more than a hundred species {[197],[678]}. The genus Ophiostoma is often described as a section of the Ceratocystis sensu lato complex. Van Wyk et al. {[512]} introduced this complex in 1990 to refer to the related genera Ophiostoma, Ceratocystis sensu stricto, and Ceratocystiopsis. However, recent comparison of ribosomal DNA sequences indicates that Ophiostoma and Ceratocystiopsis are most likely synonyms, while Ophiostoma and Ceratocystis s.s. are only distantly related (Fig. 4 {[191],[211],[218]})

Due to the presence of both chitin and cellulose – rather than only chitin – in the cell wall of species that belong to the genus Ophiostoma, this genuscan be considered as a special group within the Ascomycetes {[219]}.

The genera used for asexual stages (anamorphs) of higher fungi are form-genera or pseudo-taxa. Form-generic names apply only to one state or morph represented by their type. If there is more than one state or morph, form-genera do not apply to the whole organism. Besides a teleomorph (sexual or perfect stage), a single species may have anamorphs referring to two or more form-genera {[506]}. The genus Ophiostoma is mainly associated with the anamorph genera Graphium, Leptographium, Sporothrix, Hyalorchinocladiella, and Knoxdaviesia {[155]}. The genera Sporothrix and Graphium commonly occur in the same species of Ophiostoma {[40]}. Besides the ascospores (sexual spores), Ophiostoma sp. produce three different anamorph spore types: Graphium (Pesotum)-type spores, Sporothrix (Cephalosporium)-type spores, and yeast-like spores {[311],[362],[513]}.

Most species within the genus Ophiostoma are weak parasites or saprophytes. However, even such less pathogenic species as O. ips, O. minus, O. penicillatum, and O. querqus (O. piceae)  can cause severe economic losses as inciters of blue stain in harvested timber. Many Ophiostoma species have bark beetle associations. So far, in addition to the earlier identified Ophiostoma ulmi, two species within the genus Ophiostoma have been reported to cause DED: O. novo-ulmi Brasier and O. himal-ulmi Brasier {[139],[321]}. Ophiostoma ulmi sensu lato (s.l.) collectively refers to these three related species {[210]}. The aggressive O. novo-ulmi occurs as two separate races, i.e., the Eurasian (EAN) and the North American (NAN) {[513]}