Species in the genus Calonectria (anamorph: Cylindrocladium) are euascomycetes in the order Hypocreales and are important pathogens of a wide range of plant hosts globally. At the outset, this thesis considers the literature pertaining to species of Calonectria and especially the importance of the biological, morphological and phylogenetic species concepts on the taxonomy of this group. It is clear that DNA sequence comparisons have revolutionised the taxonomy of Calonectria and literature also highlights the importance of a polyphasic approach to species identification. Studies in this thesis treat a number of forest nursery disease problems caused by Calonectria spp. and new species are consequently described based on DNA sequence comparisons, morphological characteristics and sexual compatibility tests. As a consequence several cryptic species were also identified in the genus. Therefore, a multigene genealogy was constructed for all Calonectriaspp. for which cultures were available and shown to group together in 13 subclades also supported by morphological similarities. As a consequence all Cylindrocladium spp. were circumscribed to the genus Calonectria, regardless whether the teleomorph state was present or not, based on new nomenclature regulations stated in Article 59.