Crop wild relatives (CWRs) such as wild soybean (Glycine soja) are critical andunderutilized sources of genetic diversity with the potential to improve stress resilienceof crops in modern agriculture. Wild soybean, the wild progenitor of cultivated soybean(Glycine max), is a reservoir of genetic and phenotypic variation that was previously lostthrough domestication. The genetic diversity of wild soybean provides an opportunity toidentify novel mechanisms of resistance and stress tolerance that may contribute to thedevelopment of more durable crops. This dissertation investigates G. soja as a source of resistance to the soybean cystnematode (SCN; Heterodera glycines), the most damaging pathogen to soybean yields thatis increasing in virulence worldwide. Wild soybean is used as a model to elucidate howresistance is shaped by genetic architecture, pathogen diversity, and environmental stress.First, quantitative trait loci (QTL) mapping in a recombinant inbred population identifiedtwo new resistance-associated loci, located in chromosomes19 (QTL-19) and 13 (QTL-13).Additionally, epistatic interactions involving these QTLs indicate that resistance is alsoaffected by interactions among loci. Interestingly, QTL-19 represents a genomic regiondistinct from those commonly identified in cultivated soybean.Second, a diverse panel of wild soybean accessions was screened against two SCNtypes (HG 0 and HG 1.2.5.7) to evaluate resistance breadth. Resistance levels varied amongdifferent accessions and across pathogen types, indicating strong host-pathogen specificity.Excitingly, accessions S54 and S55 exhibited broad-spectrum resistance. Root architecturaltraits, particularly root length as a function of perimeter (RLP), were associated with SCNreproduction in a population-dependent manner. These findings suggest that phenotypicresponses to SCN infection are affected both by host accession and pathogen virulenceivprofile. Finally, the stability of SCN resistance was evaluated under combined stress con-ditions, SCN infection with water deficit. While no overall treatment effect was detected,accession-specific responses revealed variable shifts in SCN reproduction and plant per-formance. Integration of SCN reproduction and growth performance identified accessionswith potentially stable resistance across environmental conditions. Multivariate modeling ofphysiological and spectral traits demonstrated that SCN reproductive success is influencedby multiple, modest-effect associations, including plant health, photosynthetic efficiency,and resource allocation, rather than a single dominant driver.In summary, these findings demonstrate that SCN resistance in G. soja is a mul-tidimensional, context-dependent trait governed by interacting genetic, physiological, andenvironmental factors. This work highlights the value of wild soybean as a crop wild rela-tive with an important role in future soybean improvement. It establishes a framework forintegrating diverse resistance sources, phenotypic traits, and environmental conditions intosoybean breeding strategies aimed at improving resistance durability.
