Schizolobium parahybum is native to Ecuador, South America, where plantations of this tree is being established. Development of these plantations has been unsuccessful due to a serious die-back disease. A survey on the role fungal pathogens might play in disease development were conducted by isolating possible pathogenic fungi from streaks in the xylem and from machete wounds from dying trees. The primary aim of this study was to identify the isolated fungi and to determine their pathogenicity to S. parahybum through a series of greenhouse trials. Ceratocystis fimbriata, C. moniliformis, Thielaviopsis basicola, Graphium penicillioides, Ophiostoma quercus and a Pesotum species were identified as possible pathogens of S. parahybum. Inoculation trials conducted with these fungi revealed that C. fimbriata and C. moniliformis were able to cause significant lesions on young S. parahybum trees under greenhouse conditions. These fungi were, however, not consistently isolated from diseased trees and the lesions in greenhouse trials were not consistently produced. Graphium penicillioides, O. quercus and the Pesotum species were unable to cause any notable lesions. Thielaviopsis basicola caused lesions that differed significantly from the control inoculations of S. parahybum. These results were intriguing, as T. basicola is not a known tree pathogen, but is predominantly known to cause disease of agricultural crops such as groundnuts and chicory. The ability of T. basicola to cause lesions on S. parahybum initiated the second part of the thesis that dealt with a population diversity study of this pathogen. Comparison of T. basicola isolates from Ecuador and other parts of the world with isolates from groundnut and chicory in South Africa revealed that the Ecuador isolates did not originate from S. parahybum but from the carrots that was used to isolate them. Although T. basicola from carrots were able to cause lesions on S. parahybum, we did not investigate this phenomenon further as this fell beyond the scope of this thesis. Seven polymorphic primers were developed for T. basicola using the ISSR-PCR technique. These primers were used to determine the population diversity of T. basicola from groundnuts and chicory populations in South Africa. Results showed a low diversity for both populations and suggest that T. basicola was introduced to South Africa. The markers were also used to compare isolates from the groundnut and chicory populations with T. basicola isolates from other hosts and geographical regions. This indicated that T. basicola may be native to Europe from where it possibly spread to other countries.