Plantations of Eucalyptus spp. are expanding world-wide to serve growing global requirements for timber and pulp products. Together with this expansion, there has been a concomitant increase in diseases affecting these trees. Most of these are caused by fungi but there are a growing number of diseases caused by bacterial pathogens. Very little is known about them and the focus of this study was to consider species in the genus Pantoea and their association with diseases on Eucalyptus. Pantoea spp. are known pathogens of agricultural crops in South Africa and elsewhere in the world. They are also ubiquitous occurring in diverse ecological niches. Despite their prevalence, little is known about their association with plants, particularly where they occur as pathogens. The first chapter of this thesis presents an overview of the important aspects concerning the identification and classification of bacterial pathogens. Different techniques used for bacterial identification and classification were considered. These techniques are classified into different levels, based on their complexity and level of data resolution. As techniques have developed and been refined, our understanding of how organisms are related to each other has increased. An overview of the taxonomic history of the genus Pantoea was used to illustrate this point. An interaction between two Pantoea spp. and Colletogloeopsis zuluense, a serious fungal pathogen of Eucalyptus has been reported in the past. In the second chapter of this thesis, I considered the view that pathogenicity of C. zuluense is enhanced when infection occurs in conjunction with the two Pantoea spp. The identities of the two Pantoea spp. were confirmed as Pantoea ananatis and Pantoea stewartii subsp. indologenes. Both greenhouse and field inoculation trials with the two Pantoea spp. and C. zuluense failed to confirm that there is an increase in pathogenicity of C. zuluense when these bacteria are present. Studies in chapter three of this thesis, considered the identity of bacteria associated with diseased Eucalyptus leaf material from Uganda, Thailand and Uruguay. Symptoms observed in these countries were very similar and they were also similar to those of bacterial blight observed in South Africa. The majority of isolates obtained from Thailand were identified as Pantoea dispersa, based on both phenotypic and DNA-based data. This is the first report of Pantoea dispersa associated with disease on Eucalyptus. Uganda isolates were identified as Pantoea vagens prov.nom., a new species in the genus Pantoea. The majority of Uruguay isolates were identified as either Pantoea ananatis or Pantoea vagens prov.nom. The remaining isolates from Uruguay were found to belong to Pantoea eucalyptii prov.nom., a proposed new Pantoea sp., as well as Pantoea agglomerans. Pathogenicity results showed that the majority of isolates from all three countries were moderately pathogenic, eliciting moderate reactions in both tobacco and susceptible Eucalyptus grandis x nitens hybrid clones. Overall, results of studies presented in this thesis showed that Pantoea spp. can exist in complex interactions with both plants and fungi. These interactions are, however, not necessary for bacterial survival. We believe that the majority of Pantoea spp. are opportunistic pathogens based on their ability to selectively enter into interactions as well as occur epiphytically on plants. Variability in pathogenicity, both observed in this study and previously reported, further supports this view. Additional studies are needed to determine the conditions conducive to disease development in order to fully understand the threat these pathogens pose to global forestry. / Dissertation (MSc (Microbiology))--University of Pretoria, 2011. / Microbiology and Plant Pathology / unrestricted