Browsing by Author "Carstens, Elma"

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    Citrus black spot is absent in the Western Cape, Northern Cape and Free State Provinces
    (Academy of Science of South Africa, 2012) Carstens, Elma; Le Roux, Hendrik F.; Holtzhausen, Michael A.; Van Rooyen, Liezl; Coetzee, Joey; Wentzel, Ria; Laubscher, Wilhelm; Dawood, Zorina; Venter, Elrita; Schutte, Gerhardus C.; Hattingh, Vaughan; Fourie, Paul H.
    The South African citrus industry is strongly focused on exports and South Africa is a signatory member of both the World Trade Organisation Agreement on the application of Sanitary and Phytosanitary Measures and the International Plant Protection Convention. Citrus black spot, caused by Guignardia citricarpa, does not occur in all the South African citrus production areas and, therefore, South Africa has a responsibility to provide those trading partners that have identified G. citricarpa as a regulated pest with reliable information about the distribution of citrus black spot within South Africa. Detection surveys were conducted in citrus production areas in the Western Cape, Northern Cape and Free State Provinces and appropriate diagnostic protocols were used to ensure reliable detection of G. citricarpa. Trees in commercial orchards and home gardens on farms and in towns of 17, 9 and 5 magisterial districts in the Western Cape, Northern Cape and Free State Provinces, respectively, were sampled between 1995 and 2010. Fruit samples were taken during June and July, and leaf samples from November to January. None of the 3060 fruit and leaf samples collected during these surveys tested positive for G. citricarpa. Phyllosticta capitalensis, a non-pathogenic, ubiquitous, endophytic species was, however, detected during these surveys. In compliance with relevant International Standards for Phytosanitary Measures and based on the outcome of these official surveys, these three provinces in South Africa can be recognised as citrus black spot pest free areas.
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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.
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    Quarantine status of selected fungal pathogens on Malus, Prunus and Vitis species
    (Stellenbosch : Stellenbosch University, 2006-04) Carstens, Elma; Crous, P. W.; Fourie, P. W.; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant and Pathology.
    ENGLISH ABSTRACT: Invasions of alien species into non-native environments pose one of the largest, but least addressed international threats to biodiversity, both within natural ecosystems and agricultural settings. It is without exception ranked as the greatest environmental threat of the 21st century. Their introduction and spread have been identified as one of the six major categories of change that could potentially alter the world's biodiversity. The number and variety of species introduced make it clear that it is no exaggeration to state that biological invasions are breaking down biogeographic barriers that created and maintained the major floral and faunal regions of Earth. It is, however, difficult to conceive that a single indicator could measure the impact of an invader on a country due to the difficulty in measuring their environmental as well as their financial impacts. Another contributing factor to this problem is the scarcity of data available on the impact of the thousands of invaders in other countries. For the agricultural sectors, alien invasive species are likened to a two-edged sword: on the one hand they are used in the development of new plant varieties and products and on the other hand they have the potential to threaten agricultural production and as a consequence agricultural economic development. An important challenge for countries lies in the development of improved protocols for the prevention, identification and managing of potential invasive alien species. Plant health has always influenced international trade and the backbone for nondiscriminatory, fair, predictable and transparent international trade is the set of rules that national governments have agreed to follow with the foundation of the World Trade Organisation (WTO) in 1995. With the establishment of the WTO, the "WTO - Agreement on the Application of Sanitary and Phytosanitary measures (WTO-SPS)" also came into force. This agreement lays out the provisions, rights and obligations of countries in setting measures to protect human, animal and plant life and health. It also guides quarantine policy and decision-making, with the objective to prevent the use of quarantine measures by governments as disguised or unjustified trade barriers to protect their agricultural industries from import competition. As a signatory member of the WTO-SPS, South Africa has the right to implement appropriate measures to protect our plant health and environment. To set the appropriate levels for protection, South Africa must either apply to international standards or undertake a scientific based risk analysis process, to justify quarantine measures. These measures must also be the minimum necessary to protect plant health. A key element in negotiations for market access is the provision of a list of regulated pests, as well as a list of all pests associated with the crop within the exporting country, to the trading partners. In this study lists of fungal pathogens associated with Malus spp. and Vitis spp. worldwide, including South Africa, were compiled to enable the National Plant Protection Organisation in South Africa to comply with their responsibilities as a signatory member of the international regulatory bodies and to assist them to safeguard our country against harmful invasive species. It was concluded that to have access to accurate plant health status information, all researchers are summoned to validate new pest records and to submit voucher specimen to our National Collection. Geographical distribution records of pathogens and pests are the basis for phytosanitary decision-making and therefore it is imperative for countries to have access to accurate information regarding the geographical distribution of pathogens within their boundaries. For a pathogen to be classified as an A 1-pest, the pest should be of economic importance to the endangered area and not yet present there or present, but not widely distributed and being officially controlled. Many disputes have arisen during the past years concerning the classification for Monilinia fructicola and Neonectria galligena as A 1- regulated pests for South Africa, due to official records of the presence of these pathogens dating back as early as 1917. The situation was further complicated by recent reports from some European countries that M fructicola was detected on stone fruit imported from South Africa. These issues were resolved in this study by following a molecular approach and guidelines as stipulated by the international regulatory bodies. The absence of M fructicola as well as N galligena from South African stone and pome fruit orchards was confirmed. The regulated status of M fructicola and N galligena in South Africa is therefore scientifically justified.