Fusarium circinatum is the causal agent of the disease known as pitch canker of pine. The fungus causes resinous cankers on stems and branches of mature trees, dieback of female flowers and cones, as well as root rot and pre- and post emergence damping off of seedlings. Little is known regarding the epidemiology and biology of F. circinatum in South African pine seedling nurseries, where the fungus has been causing major economic losses since its introduction into the country in the early 1990s. The objectives of this study were, therefore, to study the infection biology and epidemiology of F. circinatum on pine seedlings, the organism’s saprophytic biology. I also considered approaches to rapidly diagnose plants infected by F. circinatum and its relatedness to other species. Much research has been done on the pitch canker disease and the causal agent F. circinatum. Chapter 1 of this thesis aimed to summarize the available knowledge on the pitch canker fungus and its biology, ecology and epidemiology. Trials to screen for resistance of Pinus spp. to the pitch canker fungus have been conducted by many research groups and also by industries that rely on Pinus spp. for pulp and wood production. In Chapter 2, parameters for such trials, including optimal wounding methods, spore concentrations, plant physiological considerations and time elapsed between wounding and inoculation, were investigated. Temperature and ambient humidity are considered important factors in plant disease epidemiology. The effect of these factors on pitch canker epidemics has not yet been studied. In Chapter 3, a survey of disease incidence in pine nurseries from different geographic areas in South Africa with different climates is presented. This was done by correlating disease incidence data from the nurseries with temperature and humidity measurements. The saprophytic biology of a plant pathogen is of great importance in its epidemiology. The extent of any plant pathogen’s saprophytic survival determines the initial inoculum levels at the onset of an epidemic. In Chapter 4, I investigated the saprophytic growth and survival of F. circinatum in various substrates, temperatures and moisture regimes. Fusarium circinatum is morphologically similar to fungi referred to as Fusarium subglutinans sensu lato. Distinguishing F. circinatum from other species in this group has in the past required pathogenicity tests and sexual crosses, which are labour intensive and time consuming. In Chapter 5, a molecular diagnostic technique, based on real-time PCR, with which identification of Fusarium spp. commonly occurring in South African nurseries is possible, was developed. Fusarium circinatum and other fungi referred to as F. subglutinans sensu lato are members of the Gibberella fujikuroi species complex. Molecular taxonomic studies have shown that F. subflutinans sensu lato is a polyphyletic taxon. The objective of the study presented in Chapter 6 was to resolve the taxon phylogenetically with the use of multiple loci. The studies in the individual chapters of this thesis present individual aspects of the biology, ecology, epidemiology and molecular ecology of Fusarium circinatum. Each chapter represents an independent entity and consequently repetition between chapters has been unavoidable.