Sierra Moya, Yeidymar2023-11-282023-11-282023-06https://hdl.handle.net/11299/258610University of Minnesota M.S. thesis. June 2023. Major: Plant Pathology. Advisor: Deborah Samac. 1 computer file (PDF); x, 116 pages.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.enalfalfabacterial stem blightMedicago sativaPseudomonas syringaePseudomonas syringae pv. syringaeThesis or Dissertation