Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Carstens, Elma"

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    Population structure, sex and spatial distribution of phyllosticta citricarpa, the citrus black spot pathogen
    (Stellenbosch : Stellenbosch University, 2018-03) Carstens, Elma; McLeod, Adele; Linde, C. C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Citrus Black Spot (CBS), caused by Phyllosticta citricarpa, is a fungal disease that influences citrus industries worldwide. All commercial Citrus spp. are susceptible to the disease. The pathogen was first described 117 years ago from Australia; subsequently, from summer rainfall citrus production regions in China, Africa, and South America; and, recently, the United States. Limited information is available on the pathogen’s population structure, mode of reproduction, and introduction pathways at a global scale and at a regional (provincial) scale in South Africa. This is also true for the effect of distance (spatial), season (temporal) and Citrus spp. on population structure at the orchard scale. The aforementioned aspects were investigated in the current study. Since limited co-dominant markers are available for P. citricarpa population genetic analyses, one of the first aims of the study was to develop new simple-sequence repeat (SSR) markers. The population structure of P. citricarpa was investigated at a global scale in 12 populations from South Africa, the United States, Australia, China, and Brazil. Seven published and eight newly developed polymorphic SSR markers were used for genotyping populations. The Chinese and Australian populations had the highest genetic diversities, whereas populations from Brazil, the United States, and South Africa exhibited characteristics of founder populations. Based on population differentiation and clustering analyses, the Chinese populations were distinct from the other populations. High connectivity was found, and possibly linked introduction pathways, between South Africa, Australia and Brazil. With the exception of the clonal United States populations that only contained one mating type, the other populations contained both mating types in a ratio that did not deviate significantly from 1:1. Although most populations exhibited sexual reproduction, linkage disequilibrium analyses indicated that asexual reproduction is also important. The effects of distance (spatial) and season (temporal) on the population structure of P. citricarpa were investigated over two seasons, in two lemon orchards in South Africa; one in the Mpumalanga province and the other in the North West province. Spatial analyses indicated that subpopulations separated by a short distance (within 200 m) were typically not significantly genetically differentiated, but that those separated by longer distances were sometimes significantly differentiated. Temporal analyses in the North West orchard showed that seasonal populations were not significantly genetically differentiated. In contrast, seasonal populations from the Mpumalanga orchard were significantly differentiated, most likely due to higher rainfall and disease pressure, and the spatial scale of sampling. Based on linkage disequilibrium analyses, sexual and asexual reproduction occurred in both orchards. In each orchard, two dominant multilocus genotypes (MLGs) were identified in most of the subpopulations, as well as in the seasonal populations. Pycnidiospores are therefore important in the development of CBS at the temporal and spatial scales in South African lemon orchards. Population genetic studies on a regional (provincial) scale in South Africa showed that ten P. citricarpa populations, representing five provinces (North West, Mpumalanga, Limpopo, KwaZulu-Natal and Eastern Cape), were not significantly genetically differentiated. Based on gene and genotypic diversities and private allele richness, the KwaZulu-Natal or the Limpopo provinces are likely the provinces where the pathogen was first introduced. There might have been at least two separate introductions of the pathogen into the country. The Eastern Cape province was confirmed as being the province where the latest introduction occurred in South Africa. Despite lemon trees having overlapping fruit crops, potentially providing increased opportunities for clonal reproduction, Citrus spp. (lemons vs. oranges) did not have an effect on population structure; not all lemon populations were significantly genetically differentiated from all orange populations. The current study has revealed novel information on the population structure of P. citricarpa at global and regional (South Africa) scales, which have implications for the epidemiology and management of the disease. The finding that pycnidiospores, in addition to ascospores, are also important in the epidemiology of the disease in South Africa, contradicts previous reports that pycnidiospores are of minor significance. Future studies should reinvestigate the role of these spore types in the epidemiology of CBS in South Africa using conventional orchard inoculation and leaf removal studies, combined with a population genetic data analyses. The role that distance and season have on the population structure should also be considered in orchard trial designs. Ascospore spore trap data should be generated that involve the differentiation of P. citricarpa from P. capitalensis.