Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Author "Bishop, Robyn"

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    Characterisation and pathogenicity of soilborne fungi from Western Cape olive orchards
    (Stellenbosch : Stellenbosch University, 2022-03) Bishop, Robyn; Spies, Chris, F. J.; Mostert, Lizel; Halleen, Francois; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Soilborne diseases are of concern to the olive industry worldwide, causing significant financial losses. The most devastating of these diseases is Verticillium wilt caused by Verticillium dahliae, followed by Phytophthora root rot. Species of Cylindrocarpon-like fungi, Fusarium, Macrophomina, Pythium and Rhizoctonia have also been associated with these diseases on olive, resulting in symptoms of dieback, defoliation, wilting, root rot, stunted growth, and tree death. In 2019, a survey targeting Cylindrocarpon-like fungi, Phytophthora and Verticillium revealed a high incidence and wide distribution of Cylindrocarpon-like fungi (mainly the Dactylonectria macrodidyma species complex) and Pythium irregulare in Western Cape olive orchards. However, Phytophthora (three species) was recovered from only 8% of samples, and no Verticillium was found. The survey also uncovered other potential pathogenic genera such as Fusarium, Diaporthe, Macrophomina and Rhizoctonia, but these were not identified to species level. Consequently, the aims of this study were to identify the most frequently recovered fungi, refine the species identities of isolates previously identified as the Da. macrodidyma species complex, and evaluate the pathogenicity of widely distributed species and species suspected to be pathogens. Unidentified isolates form the 2019 survey were divided into morphological groups. Groups containing isolates recovered from five or more of the thirteen sampling sites were selected for identification to species level using PCR, sequencing and phylogenetic analyses. Phylogenetic analyses revealed four species of Diaporthe, fourteen species of Fusarium, one species of Macrophomina, eight species of Neocosmospora, and nine anastomosis groups (AGs) of Rhizoctonia. Morphological descriptions of two new Diaporthe species were compiled. Isolates previously identified as the Da. macrodidyma species complex were delineated into Da. macrodidyma, Da. novozelandica, Da. sp., Da. torresensis, and Da. pauciseptata. The most widely distributed species included Da. sp., Da. macrodidyma, Diaporthe sp RB-2019a., F. fabacearum, F. nirenbergiae, M. phaseolina, and Rhizoctonia AG-G. Species of Fusarium, Macrophomina, Neocosmospora, Phytophthora, Pythium and Rhizoctonia have been reported as olive pathogens in other countries, however, none of these reports identified phylogenetic species within Fusarium and Neocosmospora, and none reported AGs of Rhizoctonia. Species identified in this study that have been reported on olives previously include Da. ecuadoriensis, Da. macrodidyma, Da novozelandica, Da. sp., Da. torresensis, Da. valentina, Di. ambigua, F. oxysporum, M. phaseolina, N. solani, Pleurostoma richardsiae, and Rhizoctonia spp. In addition to the two new Diaporthe species, Cadophora constrictospora, Da. pauciseptata, F. clavus, F. udum, nine species from the Fusarium oxysporum species complex, seven species of Neocosmospora, as well as nine different AGs of Rhizoctonia are reported on olive for the first time worldwide. The pathogenicity of selected species was evaluated on three-month-old ‘Mission’ cuttings under wet, moderate and dry irrigation regimes in glasshouse trials. Twenty–nine isolates representing Da. macrodidyma, Da. novozelandica, Da. sp., Da. pauciseptata, Da. torresensis, Di. ambigua, Diaporthe. sp., F. fabacearum, F. nirenbergiae, M. phaseolina, N. solani, Ph. multivora, Ph. pseudocryptogea, Py. irregulare, Py. oligandrum, Rhizoctonia AG- G, and V. dahliae were included. Species of Da. macrodidyma, F. nirenbergiae, Ph. Multivora, Ph. pseudocryptogea, Rhizoctonia AG-G, and V. dahliae were found to be pathogenic towards olive trees. The different watering regimes had no consistent effect on pathogenicity. This study was the first to evaluate the pathogenicity of soilborne fungi on olive trees in South Africa and to report the pathogenicity of three species for the first time on olive, including F. nirenbergiae, Ph. pseudocryptogea, and Rhizoctonia AG-G. Future research should aim to address the management of these diseases on olive.