As the Eucalyptus globulus plantation industry expands and matures in southwestern Australia (WA), the impact of disease within the plantation environment is predicted to increase. This thesis investigated the most abundant canker-causing pathogen associated with branch and stem cankers, Endothiella eucalypti the anamorph of Cryphonectria eucalypti. Endothiella eucalypti was widespread, although at low incidence, throughout the WA plantation estate and was frequently observed sporulating on the bark of healthy hosts in the absence of disease. Regions with a long (approximately 20 years) plantation history, such as Bunbury, had the highest incidence of this pathogen. A high degree of variability in pathogenicity, growth rate and colony morphology was observed between WA isolates of En. eucalypti. In the glasshouse, a significant variation in susceptibility of seven E. globulus provenances to En. eucalypti was observed. Although an interaction between the E. globulus provenance and En. eucalypti isolate was recorded, some provenances were generally more susceptible than others. In two 18-month-old plantations, the susceptibility of three provenances to En. eucalypti was significantly influenced by environmental conditions. Visual assessment of general tree health indicated that less healthy trees had smaller lesions than healthy trees. It is hypothesised that the selection of E. globulus provenances to suit site conditions in the future should decrease the risk of serious disease, especially on marginal sites. Endothiella eucalypti caused disease in intact stems of two-year-old E. globulus under glasshouse conditions. This suggests that En. eucalypti may not require a wound to infect in the field. Vegetative compatibility groupings between WA En. eucalypti isolates indicated a relatively high degree of genotypic diversity within the WA asexual population of En. eucalypti, whereas inter-simple sequence repeats PCR (ISSR-PCR) analysis indicated a lower level of genotypic diversity. Discrepancies between traditional and molecular techniques, such as ISSR-PCR, was attributed to the more specific gene-togene analysis afforded by molecular techniques. ISSR-PCR successfully distinguished variability within the En. eucalypti population and with the teleomorph, isolated in South Africa. It also separated Cryphonectria cubensis isolates from the C. eucalypti isolates. As copper is the micronutrient most limiting growth of E. globulus in WA, its role in the resistance of two E. globulus provenances was examined in a glasshouse trial. Lesion extension or defence responses of E. globulus to En. eucalypti did not differ between Cu-adequate and Cu-deficient plants. It is suggested that constitutive levels of host defence enzymes played a more important role in providing protection for the host against En. eucalypti than the external supply of copper. A reduction in the canopy volume of E. globulus within plantations due to insect herbivory or foliar pathogens, such as Mycosphaerella spp., has been reported to predispose the host to disease caused by non-aggressive canker-causing fungi. Under two separate glasshouse trials, conditions of 100% defoliation and 80% defoliation maintained over six weeks prior to inoculation, were required to significantly increase lesion extension caused by En. eucalypti in E. globulus stems. The ability of defoliated E. globulus to retain a degree of resistance to En. eucalypti was attributed to the rapid replacement of foliage and up-regulation of photosynthesis in remaining leaves. The carbohydrate reserves of the plant were depleted following defoliation and remained depressed regardless of the length of time the trees remained defoliated. In conclusion, the endophytic habit of En. eucalypti poses a threat to highly stressed trees, however it does not appear to be an immediate threat to WA plantation health. Although En. eucalypti has not yet been responsible for a major disease outbreak in WA, the impact of this disease on plantation-grown eucalypts elsewhere in Australia and worldwide serves as an indication of its potential to affect WA plantations.