Browsing by Subject "Medicago sativa"

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    Development of genetic mapping and DNA markers for tolerance to bacterial stem blight caused by Pseudomonas syringae pv. syringae in alfalfa
    (2023-06) Sierra Moya, Yeidymar
    Alfalfa, Medicago sativa, is a legume with an important role in agriculture, livestock nutrition, and human health. It has high nutritive content, providing vitamins, minerals, protein, and antioxidants not only for livestock but also for humans. As a perennial forage crop, it is cultivated worldwide and is considered the third most valuable field crop in the USA. In addition, it is important for sustainable agriculture due to its role in biological nitrogen fixation and the improvement of soil fertility. Unfortunately, even though it is an excellent forage crop, it is constantly facing biotic and abiotic challenges.Pseudomonas syringae is a gram-negative bacterial pathogen capable of affecting a wide range of plants around the world. Pseudomonas syringae pv. syringae is the causal agent of bacterial stem blight (BSB) of alfalfa and losses can reach 50% of the forage production in some cultivars. Yield losses are due to both frost injury and disease. The bacterium promotes frost through ice nucleation then enters plants through frost injury sites and causes water-soaking, chlorosis, and necrosis 7 to 10 days after the frost injury. Currently, multiple strains have been isolated from Australia, Europe, western Iran, and the USA. Although there is a better understanding of the epidemiology of P. syringae infecting model plants, there is a gap in knowledge on the interactions between legumes and this bacterium. For this reason, it is crucial to find genes that provide tolerance, resistance, or susceptibility to this infection to develop efficient genotypic selection tools such as marker-assisted selection for the development of germplasm resistant to BSB in alfalfa. Chapter 1 includes an extensive literature review of alfalfa and P. syringae, as well as the grounding base for our study. Chapter 2 contains the development of standardized methods for the detection and quantification of Pseudomonas syringae pv. syringae causing BSB of alfalfa, as well as a scoring system to evaluate disease severity. Adapting molecular techniques for the detection, identification, and quantification of this pathogen is key understand the epidemiology of the disease. By understanding the disease progression and bacterial load after infection we can provide insights into the plant response against the pathogen. Results from this research showed that alfalfa leaves scored as resistant with few to no symptoms at 9 days post-inoculation had a similar bacterial load as those with severe symptoms, indicating a tolerance response to the foliar phase of BSB. Chapter 3 contains a genome-wide identification study of genes for tolerance to BSB. The scoring guide of host responses and quantification methods from Chapter 2 were employed to classify plants into different levels of response and used to conduct quantitative trait loci (QTL) mapping for genes involved in BSB disease phenotypes. This is the first study to identify DNA markers associated with tolerance to this pathogen, to increase understanding of defense mechanisms, and advance progress in the development of improved alfalfa cultivars. These results will provide a better insight into the genes involved in BSB resistance and facilitate the mapping of other disease-tolerance/resistance QTL for the development of commercial varieties. The investigations of these chapters have significant implications for the understanding of disease resistance mechanisms for BSB in alfalfa.
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    The host range of Fusarium virguliforme on rotational crops and common plant species and its survival and growth on crop residue.
    (2010-03) Kolander, Tammy Mae
    Sudden death syndrome (SDS), caused by Fusarium virguliforme (Fv), is an important soybean (Glycine max) disease. Crop rotation is not an effective management strategy, suggesting that the pathogen may survive long periods in the soil or may infect or be sustained on weeds or crops other than soybean. Minimal research has been conducted to understand Fv survival or its host range and ability to grow on different plants. The objectives of this thesis research were to determine the host range of Fv, the best methods for extracting Fv DNA from crop residue and macrocondia, and to determine how long Fv DNA can be detected on crop residue after burial in a field. Fifteen plant species were inoculated in a greenhouse to determine the host range of Fv. In at least one experiment, soybean, alfalfa (Medicago sativa), pinto bean and navy bean (Phaseolus vulgaris), white clover (Trifolium repens), red clover (T. pretense), pea (Pisum sativum), and Canadian milk vetch (Astragalus canadensis) developed foliar and/or root symptoms. In at least one experiment, corn (Zea mays), wheat (Triticum aestivum), ryegrass (Lolium perenne), pigweed (Amaranthus retroflexus), sugar beet (Beta vulgaris), lambsquarters (Chenopodium album), and canola (Brassica napus) appeared to be asymptomatic hosts for Fv. Thus, multiple plant species may be negatively affected by Fv and/or promote its survival and growth. Three commercial DNA extraction kits were compared to determine which would yield the greatest purity and quantity of Fv DNA from crop residue and macroconidia. The FastDNA® kit was generally most effective for extracting Fv DNA from crop residue and the MO BIO PowerSoil™ kit was superior for extracting Fv DNA from macroconidia. This knowledge was used to determine if Fv DNA can be detected on soybean, corn, alfalfa and wheat residue over time after placement in three crop fields and whether different inoculation methods influence the duration of detection. Soybean and corn tissue that was infected while growing retained detectable amounts of Fv DNA for at least 8 months after burial in the field. Dead tissues inoculated with Fv macroconidia typically did not retain quantifiable amounts of Fv DNA after burial. Results from this study suggest that the inoculation method is important for survival and detection of Fv DNA, and that Fv DNA remains detectable on crop residue from fall into the following summer.