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Title:Identifying loci conferring quantitative disease resistance in sorghum and maize against fungal foliar pathogens
Author(s):Lipps, Sarah
Advisor(s):Jamann, Tiffany
Contributor(s):Lipka, Alexander; Kleczewski, Nathan
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:M.S.
Genre:Thesis
Subject(s):quantitative disease resistance
exserohilum turcicum
phyllachora maydis
maize
sorghum
Abstract:There are multiple mechanisms by which hosts defend themselves against pathogens. Broadly disease resistance can be split into qualitative and quantitative resistance. In qualitative resistance the presence of a single resistance gene (R gene) confers complete resistance against the pathogen. R proteins can interact directly as receptors or indirectly as guardees/decoys with pathogen Avr proteins. R genes are generally race specific. Quantitative disease resistance (QDR) is genetically complex with a partial phenotype conferred by multiple small effect genes. In some instances genes and loci associated with QDR confer broad spectrum resistance against multiple of pathogens (Singh et al., 2011a). Many QDR genes are not directly associated with host defense responses but with host morphology and development, signal transduction, and the general defense response, among other mechanisms. The mechanisms underlying qualitative resistance are well understood through many detailed analyses (Spoel and Dong, 2012; Song et al., 2015; Balint-Kurti, 2019; Dievart et al., 2020; Sun et al., 2020). By contrast the mechanisms associated with QDR remains largely elusive. The deployment of resistant germplasm is an economically efficient means to manage disease. However, high selection pressure on pathogens with high evolutionary potential can lead to pathogens overcoming host resistance. Durability of resistance can be maintained through genetic complexity. Genetic complexity is established by stacking both R genes and QDR genes through pyramiding (Mundt, 2018). As complexity increases, pathogens are required to evade multiple forms of detection. Maize is a model organism and has been studied for resistance to a variety of diseases. Northern corn leaf blight and sorghum leaf blight are both caused by the hemibiotrophic fungus Exserohilum turcicum. Both qualitative and quantitative resistance against E. turcicum have been described in maize. In contrast, the genetic architecture of resistance against E. turcicum in sorghum is poorly understood. Maize and sorghum are closely related C4 grass species, and likely share mechanisms of resistance to E. turcicum. Identification of resistance in sorghum against E. turcicum can enhance management strategies and understanding of the pathosystem. By definition, emerging diseases are not as well understood. Phyllachora maydis, the causal agent of tar spot in maize was identified in the United States in 2015. The role of environmental conditions is important in the P. maydis pathosystem. Resistance has been identified in tropical germplasm but not in temperate-adapted germplasm. Accessions developed by the Germplasm Enhancement of Maize (GEM) project contain introgressions from landraces in elite backgrounds. Screening exotic-derived germplasm will aid in the identification of donors for alleles contributing towards resistance to tar spot. Chapter 1 is a literature review of disease resistance in crops, specifically on quantitative and qualitative disease resistance in maize. Chapter 2 is a QTL mapping study that identifies loci for resistance to E. turcicum in two sorghum recombinant inbred line populations. The distribution of resistance loci in sorghum is non-random. The relationship of resistance to E. turcicum between maize and sorghum is examined, and maize genes for resistance colocalize within the sorghum leaf blight resistance loci. Sorghum and maize likely utilize similar mechanisms of resistance against E. turcicum. The main objective of Chapter 3 is to identify sources of resistance to tar spot in exotic-derived germplasm. Environmental conditions have a strong influence on disease severity, but consistent phenotypic responses are observed across environments. Disease symptoms vary between juvenile and adult plants. Accessions with potential for use in breeding for tar spot resistance are described. Chapter 4 summarizes the findings from Chapters 2 and 3 and discusses implications for future research goals of confirming a QTL on chromosome 3 in sorghum associated with resistance against E. turcicum. Chapter 4 also discusses implications of environmental modeling for predicating disease development and further identifying sources of resistance to tar spot in maize.
Issue Date:2021-04-28
Type:Thesis
URI:http://hdl.handle.net/2142/110843
Rights Information:Copyright 2021 Sarah Lipps
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05


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