The Cryphonectriaceae is a recently established family in the Diaporthales and includes important tree pathogens. Four genera occur in Africa, namely Aurifilum, Celoporthe, Chrysoporthe and Holocryphia and are known tree pathogens able to infect native trees, non-native ornamentals and forestry trees in the Myrtales. In Chapter One of this thesis, the taxonomic history, pathology, possible origins, host range, distribution, symptoms, impact and possible control of Cryphonectriaceae in Africa was considered. This review aimed to provide a background for the study of specimens resembling Cryphonectriaceae collected from southern Africa during surveys throughout 2005-2009. Furthermore, it provided a background for the study of the population diversity of Chr. austroafricana in southern Africa and its possible movement within the country and between known hosts. Although great progress has been made in studying the Cryphonectriaceae it is clear that our knowledge of these pathogens in Africa remain limited, despite evidence that the centers of origin of some of them are most likely in Africa. Chapter Two reported, for the first time, Chr. austroafricana from Namibia as well as areas in South Africa, outside of its previously known range. Holocryphia eucalypti was reported from Swaziland and the host and distribution range of Celoporthe species are now known to include S. legatti in the Soutpansberg area in South Africa and S. cordatum in Zambia. Latruncellus aurora gen. sp. nov. was discovered on Galpinia transvaalica (Lythraceae, Myrtales) in Swaziland, representing the first report of a member of the Cryphonectriaceae infecting Lythraceae in Africa. In Chapter Three of this thesis the diversity of Chr. austroafricana was assessed based on Vegetative Compatibility Groupings (VCGs). A high diversity was observed for Chr. austroafricana in southern Africa, with very limited movement of VCGs between different locations and hosts. This high VCG diversity is indicative of a native pathogen, confirming previous hypotheses in this regard. This study provides a VCG tester database that will allow assignment of newly discovered isolates to a specific VCG grouping and allowing more rapid identification of new introductions. This technique is especially valuable in that it is cheap and easy to apply, allowing for all laboratories in Africa to be able to use it. Two previously unknown Celoporthe species were described in Chapter Four. These are Cel. fontana, described from S. cordatum in Zambia, and Cel. woodiana from T. granulosa in South Africa. This chapter relied strongly on the phylogenetic species concept (PSC) to recognize these new species. The PSC is especially helpful in a genus such as Celoporthe where limited specimens are available for morphological comparisons and where morphological differences are difficult to identify. This thesis greatly expanded our knowledge on the diversity, distribution and host range of the Cryphonectriaceae in southern Africa and described a novel genus and three novel species. Based on pathogenicity tests all three new species have the potential to cause disease on Eucalyptus, a tree of importance in forestry in various southern African countries, indicating that in the future these fungi could emerge as important pathogens of Eucalyptus species in Africa and other continents.