The ability of eucalypt trees to grow in different soil aspects, under variable climatic and environmental conditions and to grow fast consistently producing straight stems has made them preferred plantation trees in many parts of the world. The world‘s increased demand for pulp wood has been the major contributor to the aggressive extensive development of eucalyptus plantations around the world. The productivity of these trees is however hindered in both native and introduced plantations by pests and pathogens. Chapter 1 of this thesis is a literature review on Teratosphaeria suttonii as the causative agent of Teratosphaeria Leaf Disease on Eucalyptus. The chapter highlights the taxonomic placement of T. suttonii in the genus Teratosphaeria and the name changes of the species as a consequence of molecular taxonomy. The effects of the pathogen on Eucalyptus plantations in different parts of the world as well as the incidence and range of Eucalyptus species that host the pathogen are presented. In addition the review summarizes much of the information published on the distribution, hosts range and evolutionary relationships of T. suttonii with closely related species in the genus Teratosphaeria. In conclusion, it highlights the lack of understanding of the degree of diversity of the pathogen from different parts of the world. In order to characterize and establish the phylogenetic relationships between T. suttonii isolates from different countries, chapter 2 of this thesis considered microscopic examination and DNA sequence comparisons of isolates from its native and introduced localities. The isolates were classified into a single genetically and morphologically diverse group within which representatives of different countries were intermixed in smaller groups. A high level of genetic variation was evident among Australia isolates but despite the diversity there was no overwhelming evidence for sibling species. In Chapter 3 of this thesis eleven microsatellite markers were developed from three T. suttonii isolates obtained from 2 geographic areas. The markers were used in Chapters 4 and 5 to investigate the genetic diversity of T. suttonii in both its native and introduced environments and to establish factors underlying such diversity. The application of the markers in Chapters 4 and 5 provide the first consideration of the diversity of T. suttonii in both its native and introduced localities respectively. Both native and introduced pathogen populations exhibit substantially high genotypic diversities. It is evident from these studies that Australia is the point of origin of T. suttonii and that anthropogenic activities have resulted in repeated introductions of the pathogen from Australia into many countries. Teratosphaeria suttonii is of great significance to the eucalyptus plantation industry across the world. Movement of diseased germplasm and introduction of new genotypes favors pathogen adaptability. In order to reduce the distribution of the pathogen into new areas or any other places where it might have negative effects, strict quarantine procedures should be followed. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Microbiology and Plant Pathology / Unrestricted