Post-hail associated die-back of Pinus radiata is associated with the opportunistic pine pathogen Sphaeropsis sapinea. This disease has a serious impact on forestry; especially in regions such as South Africa where Pinus spp. are planted as exotics. Current management strategies are based on the selection of disease tolerant Pinus spp. and the planting of these pines in regions less affected by adverse environmental conditions. A possible management strategy for S. sapinea is biocontrol through dsRNA- mediated hypovirulence. Improvement of current management strategies and the implementation of biocontrol, however, require a re-evaluation of the taxonomic status of the pathogen and the assessment of genetic van ability in populations. Chapter One of this thesis reviews knowledge on S. sapinea in order to obtain the ultimate goal of formulating an integrated management strategy for S. sapinea, especially in South Africa and other regions dependent on exotic pines for commercial forestry. Successful management of S. sapinea in exotic forestry plantations has not, as yet, been fully achieved. This is probably due to a lack of knowledge of the current taxonomy of the fungus, the genetic status of S. sapinea populations and the association with dsRNA viral fragments that could be exploited for biocontrol. In Chapter Two, three morpho types (A, B and C) of S. sapinea are reported from diverse geographical areas. This also includes the discovery, for the first time, of a third morpho type of the fungus. Isolates of the C morphotype are characterised based on RAPD analysis and ITS sequences, as well as cultural and conidial characteristics. Isolates of the C morphotype were found to be more similar to those of the A morphotype based on cultural characteristics, texture of their conidial walls and ITS sequences. The three morphotypes are, however, clearly distinguishable based on their RAPD banding profiles and conidial size. Chapter Three represents the characterisation of the C morphotype of S. sapinea based on morphology and pathogenicity. Based on cultural and conidial characteristics, as well as growth rate the A and C morphotypes are very similar. Isolates of the C morphotype were, however, found to be much more virulent than either those of the A or B morphotype. This could have a great impact on the management of S. sapinea die¬back, as well as on quarantine practices. In Chapter Four, I present an assessment of the diversity for S. sapinea populations from various regions. The diversity in the different populations was found to be highly variable, despite the fact that only an asexual state is known for this fungus. The diversity in presumed native populations from Mexico, California and Switzerland was relative low. In the introduced Malagasy population the diversity was very high, probably due to many introductions of the fungus with seed imports. Due to the high variability of the fungus in the Malagasy population, management of S. sapinea from this region will be extremely difficult. In Chapter Five, dsRNA fragments of different sizes were commonly found to occur in isolates of S. sapinea. Two previously described dsRNA viruses (SsRVI and SsRV2) from South Africa were found to also occur in isolates from Madagascar and Colombia. It shows that these dsRNA fragments have been introduced with S. sapinea into new environments. The type and number of dsRNA fragments was found to have some effect on the growth rate and pathogenicity of the fungus. These. alterations in the normal phenotype of the fungus could, however, not be correlated to the presence of dsRNA fragments. The presence of dsRNA fragments does, however, still have the potential to be implemented in future management programmes for S. sapinea. / Dissertation (MSc)--University of Pretoria, 2006. / Microbiology and Plant Pathology / Unrestricted