This study investigates insect-fungal-plant interactions on tree-nut crops (pistachio, walnut and pecan) in South Africa. The main aim was to determine whether insects occurring in tree-nut crop orchards are assocaited with fungal phytopathogens and whether they have the capability to harbour and dissiminate them. Isolations from Atelocera raptoria (powdery stink bug) on pistachio showed that the largest proportion of fungi associated with these insects are pathogenic, some serious pathogens of the crop. Isolations from Coenomorpha nervosa (grey-brown stink bug) and Empoasca citricola (green citrus leafhopper) on walnut showed that the insects were associated with fungi but the presence of pathogens was minimal. The isolation of fungi associated with insects occurring in pecan orchards showed that the most abundant fungal species from Empoasca sp. (leafhoppers) and the Sciobius cf. granosus (citrus snouted weevil) were pathogenic. Pathogens were also isolated from Panafrolepta dahlmani, however, a large proportion of the fungi isolated from these specimens were non-pathogenic. Fungi from the grey-brown stink bug, collected through a pecan nut production season and comparing it to other ways of dispersal (air, soil, leaf & nut samples), showed that the greatest diversity of fungi was found to be associated with the stink bug specimens. This indicates that stink bugs, most likely insects in general, play an important role in the dynamics of fungal populations within pecan orchards. As a result the isolation of fungi from insects in tree-nut crop orchards may give an indication of the mycoflora in the environment. Overall, the two most abundant fungal species isolated from stink bugs and other niches were Alternaria tenuissima and Cladosporium cladosporioides. The incidence of these two species on stink bugs followed a similar trend to the incidence of these two species in the air and soil. However, the incidence of these two species on the stink bugs differed from their incidence in the leaves and nuts. This indicates that the fungi occurring on the stink bugs are from their environment and that the presence of fungal species such as A. tenuissima and C. cladosporioides in the leaves and nuts cannot necessarily be attributed to vectoring by stink bugs. Neofusicoccum parvum was only isolated from the nuts and from the stink bugs. It was completely absent from all the other samples taken throughout the whole season. This suggests an association between the stink bugs and N. parvum, where the stink bugs act as disseminators spreading this fungus between developing nuts as they feed. Pathogenicity trials indicated that A. tenuissima and N. parvum are pathogenic on pecan nuts and leaves, while C. cladosporioides cannot be considered a pathogen of this crop. It was also found that wounding of pecan nuts increases disease incidence on nuts inoculated with A. tenuissima and is a necessity for both A. tenuissima and N. parvum to be able to infect, colonise and cause symptoms on pecan leaves. Wounding does not increase the rate of disease development, but it reduces the time required by N. parvum to infect and colonise pecan nuts. In contrast, wounding does not provide the same benefit to A. tenuissima. Overall, the study has shown that insects occurring in tree-nut orchards have the capability to harbour and disseminate fungal phytopathogens. These insects feed on various parts of the trees causing damage and, in addition, provide suitable entry points into the trees through which pathogens gain entry to infect, colonise and cause disease.