English: 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.
Afrikaans: Hierdie studie ondersoek insek-swam-plant interaksies op neut gewasse (pistachio, okkerneut en pekanneute) in Suid-Afrika. Die hoofdoel was om te bepaal of insekte wat in neutboorde voorkom geassosieer is met swam fitopatogene en of hulle oor die vermoë beskik om vir hierdie 'n hawe te bied en hulle te versprei. Isolasie vanaf Atelocera raptoria (wollerige stinkbesie) op pistachio het getoon dat die grootste gedeelte van swamme wat op hierdie insekte voorkom patogene is, insluitend ernstige patogene van die gewas. Isolasie vanaf Coenomorpha nervosa (grys-bruin stinkbesie ) en Empoasca citricola (groen sitrus bladspringer) individue wat versamel is op okkerneute het getoon dat die insekte geassosieer is met swamme, maar die teenwoordigheid van patogene was minimaal. Die isolasie van swamme vanaf insekte wat in pekanneutboorde versamel is het getoon dat die mees prominente swamspesies wat voorkom op Empoasca sp. (bladspringers) en Sciobius cf. granosus (sitrus kalander) patogenies is. Patogene is ook geïsoleer vanaf die Panafrolepta dahlmani individue. 'n Groot gedeelte van hierdie swamme was egter nie-patogenies nie. Die isolasie van swamme vanaf die grys-bruin stinkbesie wat versamel is deur die loop van 'n pekanneut produksie seisoen, en wat vergelyk is met ander maniere van verspreiding (lug, grond , blaar en neut monsters), toon dat die grootste verskeidenheid van swamme geassosieer word met die stinkbesies. Dit dui daarop dat stinkbesies, waarskynlik insekte in die algemeen, 'n belangrike rol speel in die dinamika van swampopulasies binne pekanneutboorde. Die dui ook aan dat die isolasie van swamme vanaf insekte, wat in neutboorde voorkom, moontlik gebruik kan word om 'n goeie aanduiding van die omgewing se mikoflora te kry. Die twee mees prominente swam spesies wat algeheel die meeste geïsoleer is vanaf stinkbesies en vanaf ander nisse was Alternaria tenuissima en Cladosporium cladosporioides. Die teenwoordigheid van hierdie twee spesies op die stinkbesies het 'n soortgelyke patroon gevolg as die teenwoordigheid in die lug en grond. In teenstelling het die teenwoordigheid van hierdie twee spesies op die stinkbesies verskil van hul teenwoordigheid in die blare en neute. Dit kan daarop dui dat die swamme wat op die stinkbesies voorkom verkry word uit die omgewing en dat die teenwoordigheid van swam spesies soos A. tenuissima en C. cladospioroides in die blare en neute nie, in geheel, toegeskryf kan word aan verspreiding deur stinkbesies nie. Neofusicocum parvum is net geïsoleer vanaf die neute en die stinkbesies. Dit was afwesig in al die ander monsters wat geneem was tydens die hele seisoen. Dit dui 'n assosiasie aan tussen die stinkbesies en N. parvum, waar die stinkbesies optree as verspreiders wat hierdie swam versprei tussen ontwikkelende neute tydens voeding. Patogenisiteitsproewe het gevind dat A. tenuissima en N. parvum patogenies is op pekanneute en pekanneut blare, terwyl C. cladosporioides nie as 'n patogeen van hierdie gewas beskou kan word nie. Dit is ook bevind dat wonding van pekanneute die voorkoms van siektes verhoog op neute wat geïnokuleer is met A. tenuissima, en dat dit 'n noodsaaklikheid is vir beide A. tenuissima en N. parvum om in staat te wees om pekanneutblare te infekteer, te koloniseer en simptome te kan veroorsaak. Wonding versnel nie die tempo van siekte ontwikkeling nie, maar dit verkort die tyd wat benodig word deur N. parvum om pekanneute te infekteer en te koloniseer. In teenstelling verskaf wonding nie dieselfde voordeel aan A. tenuissima nie. In die geheel het die studie bewys dat insekte wat in neutboorde voorkom die vermoë het om 'n hawe te bied vir fitopatogene en hulle te kan versprei. Hierdie insekte voed op verskillende dele van die bome, wat skade veroorsaak en daarbenewens geskikte toegangs punte verskaf waardeur patogene toegang verkry om bome te infekteer, te koloniseer en siekte te veroorsaak.