Cochliobolus sativus (Ito & Kurib.) Drechsl. ex Dast. [anamorph, Bipolaris sorokiniana (Sacc. in Sorok.) Shoem.] is an important fungal pathogen, which causes common root rot (CRR), spot blotch (SB) and black point/kernel blight in barley in North America. Use of genetic resistance has been effective against SB presumably due to presence of durable resistance in North America. However, recently emerged virulence groups have overcome durable resistance in barley. Additionally, the genetics of resistance to CRR is poorly understood. Therefore, the objectives of current studies are multifaceted. To identify the sources of resistance to CRR, 824 contemporary barley lines from the USDA-CSREES Barley Coordinated Agricultural Project (CAP) were evaluated for resistance to CRR under natural inoculum pressure in the field during 2006 - 2008. Additionally, resistance to CRR (n = 384 lines) and SB (n = 386 lines) was also evaluated in the greenhouse. The results indicate that only 0.9% of breeding lines showed resistance to CRR in field experiments during 2006-2008. None of the genotypes showed resistance to virulent isolates in greenhouse experiments. Hordeum jubatum sp. jubatum (Accession # CGN 13044) showed 12% CRR severity against highly virulent isolates. Therefore, this accession can be used as a potential source of resistance to CRR in the future. In the SB experiment, only 0.5% of the 386 genotypes showed resistant responses to isolate 4008. The barley line NDB112 with durable resistance to SB also showed a highly susceptible reaction to this isolate. To map QTL for CRR and SB resistance, association mapping was employed using the CAP06 population (n = 384), CAP07 population (n = 384) for CRR resistance and the CAP06 population (n = 384) for SB resistance. In all association analyses, 3072 single nucleotide polymorphism (SNP) markers were used. The results suggest five QTL resistance to CRR, CRR-3H-28-51, CRR-SH- 180-195, CRR-6H-30-64, CRR-6H-91-97, and CRR-lH-50-86 were detected in chromosomes 3H, 5H, 6H, and 7H. Two QTL resistance to spot blotch, Rcs-1H- 84.6 and Rcs-2H-106-122 were identified in chromosomes 1 Hand 2H, respectively. These QTL didn't coincide with any of the QTL reported earlier and confer resistance to virulence group 7.7.3.6. To investigate the virulence spectrum of C. sativus isolates collected from North Dakota (ND), 12 barley genotypes were inoculated with 12 virulent C. sativus isolates on both root and leaf. The results suggest that different virulence groups for CRR and SB diseases exist in the pathogen population. To understand the population structure of C. sativus populations collected from Australia and the USA (ND), 289 single-spore isolates were analyzed for amplified fragment length polymorphism (AFLP) using three AFLP primer combinations. Moderate to high gene diversity (H = 0.27 - 0.35) and high genotypic diversity (GD= 1) within C. sativus populations indicate occurrence of genetic recombination other than sexual in C. sativus populations. The moderate to high population differentiation (Gst = 0.196), moderate multilocus linkage disequilibrium (rd = 0.046 - 0.118), and low gene flow (Nm= 2.0) suggest the occurrence of different populations of C. sativus in the field.