Doctoral Degrees (Plant Sciences)

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Browsing Doctoral Degrees (Plant Sciences) by Advisor "Boshoff, W. H. P."

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    Pathogen variation and genetic control of Puccinia triticina in Zimbabwe
    (University of the Free State, 2022) Chiuraise, Nyashadzashe; Boshoff, W. H. P.; Visser, B.; Maré, A.
    Genetic resistance is the most cost-effective approach to manage wheat leaf rust caused by Puccinia triticina Eriks. (Pt). However, the continuous emergence of more virulent races can deplete monogenic sources of resistance. The aim of this study was to determine the distribution, race and genetic diversity of Pt isolates in Zimbabwe and to characterise the sources of resistance in selected wheat accessions. In total, 104 single pustule isolates of Pt were established from infected wheat samples that were collected from the main wheat production regions of Zimbabwe during surveys from 2019 to 2021. Results from phenotyping a set of 46 differential and additional wheat lines revealed Pt race MCDS as dominant in Zimbabwe. Genotyping of 48 Pt isolates with 19 microsatellite markers, followed by DARwin and STRUCTURE analyses, confirmed a high genetic similarity between the Zimbabwean isolates and representative isolates of the South African Pt races MCDS, MCPS and MFPS. However, five isolates (19_1_2019, 24_3_2019, 5_1_2020, 20_1_2020, 23_2_2020) with genetic similarity to South African races SDDN and SCDS were detected. The detection of the five genetically distinct Pt isolates among the Zimbabwean isolates indicates genetic variation that could have arisen from foreign introductions. The infection type (IT) data from screening the 39 differential lines and 72 Zimbabwean wheat accessions with nine Pt races were not informative in postulating the presence of any all-stage resistance genes (ASR). Forty-nine Zimbabwean varieties showed low (resistant) seedling ITs to all nine Pt races tested in the greenhouse and at least 53 varieties were strongly resistant with immune responses to races CFPS+Lr20 and MFPS in the field. From these, 25 wheat lines with ASR to all Pt race isolates were crossed with an MCDS susceptible variety. Twenty-three varieties displayed an F2 segregation ratio of 3:1, indicating the inheritance of a single dominant leaf rust (Lr) resistance gene. Molecular markers detected Lr19 in 20 of these varieties. Five adult plant resistance genes (APR) namely Lr27, Lr34, Lr37, Lr46 and Lr68 were detected in the Zimbabwean germplasm, with Lr46 being the most common and Lr34 the least common. A multi-environmental trial (MET) conducted over two seasons in Zimbabwe identified wheat varieties SC001, SC002, SC004, SC027 and SC W9101 as widely adapted with stable yields, acceptable leaf rust resistance while meeting the quality traits required in the wheat value chain. Overall, the outcomes of this study make a valuable contribution to shaping longer term strategies to control wheat leaf rust in Zimbabwe.
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    Priming effect of leaf rust and salicylic acid in Russian wheat aphid resistance
    (University of the Free State, 2022) Bilal, Huzaifa; Mohase, L.; Boshoff, W. H. P.
    Wheat (Triticum aestivum L.) is one of the primary sources of carbohydrates for humans and livestock (Karakas et al., 2021). It is an essential cereal for the human diet and contributes to global food security. Almost 50% of calories for human consumption come from grains; out of this, about a quarter comes from wheat (González-Esteban, 2017). Wheat grain is a rich source of carbohydrates, dietary fibre, vitamins (B-vitamins) and phytochemicals (Shewry and Hey, 2015). In addition to this, it has 13-17% bran, 2-3% germ and 80-85% mealy endosperm (Šramková et al., 2009). Wheat is a significant source of globulin, albumin, and amphiphilic protein content (Dubreil et al., 1998). Furthermore, wheat provides lipids and essential minerals like calcium, copper, iron, phosphorus, potassium, manganese, magnesium, and zinc (Rachon et al., 2015). The current global wheat production is 642 million tons, and the future (2050) demand is about 840 million tons. This demand may be attained on limited resources (water, land) if new agronomic, physiological and genetic research strategies and practices are introduced (Sharma et al., 2015). Domestication of wheat occurred 10,000 years ago, and wheat spread worldwide as a major cereal crop. Its diverse adaptability to different environments makes it easy to domesticate. Genetic miscellany (ploidy level) of wheat and its progenitors reward novel diversity quickly in different climatic zones (Dubcovsky and Dvorak, 2007). Commercial wheat cultivation started in South Africa in the early 1910s in Cape Town, with seeds introduced earlier by the Dutch traders (Nhemachena and Kirsten, 2017), and has become the second most crucial grain crop cultivated in South Africa after maize (Anonymous, 2021; Bester, 2014). Both tetraploid and hexaploid wheat cultivars are produced in approximately 90% of the available agro-climatic regions of South Africa (Lantican et al., 2005). The dominant wheat-producing areas are the Western Cape (winter rainfall, mainly dryland), Free State (summer rainfall, both dryland and irrigated), Northern Cape (irrigated) and North West (mainly irrigated) provinces. Even though cultivation occurs in winter and summer rainfall regions, between 1983 and 2008, wheat was cultivated predominantly under dryland conditions where annual production averaged 1.5 to 3 million tonnes (2-2.5 tons/ha) (Nhemachena and Kirsten, 2017). However, about 30% of harvested wheat is produced under irrigation, where the yield potential varies between 6 to 12 tons/ha, with higher winter temperatures being the main limitation in the lower-yielding areas (Anonymous, 2021). The major companies or institutions supplying improved wheat cultivars in South Africa are Sensako (now part of Syngenta), Pannar Seed (Corteva AgrisciencesTM) and the Agricultural Research Council-Small Grains (ARC-SG) (Nhemachena and Kirsten, 2017). The wheat varieties are constantly improved for high yield and tolerance or resistance to prevailing drought, salinity, heat, pests and diseases. In South Africa, the wheat industry contributes about USD 40 billion to the gross value of agricultural production (Jankielsohn, 2016) and 28 000 jobs (Bester, 2014). Some pathogens (causing diseases like rust and powdery mildew) and pests similar to the Russian wheat aphid (RWA) significantly reduce yield and flour quality (Kazi et al., 2013). Russian wheat aphid infestations significantly challenge successful wheat production (Njom et al., 2017) because they reduce wheat yield and deteriorate flour quality (Girma et al., 1993). The emergence of RWA biotypes with increased virulence threatens wheat production and reduces the desired targets to meet the South African demand for high-quality wheat grain.