Genome-Wide Association Mapping of Susceptibility to Stagonospora nodorum Blotch in Emmer Wheat

Stagonospora nodorum blotch (SNB), caused by Parastagonospora nodorum, is an important foliar disease of wheat. P. nodorum produces necrotrophic effectors (NEs) that, when recognized by corresponding host sensitivity genes, result in host driven programmed cell death which ultimately leads to disease susceptibility. Emmer wheat (Triticum dicoccum) is a good source of resistance genes for various wheat diseases, but may contain yet unidentified P. nodorum NE sensitivity genes.  The objectives of this study were to evaluate 180 emmer wheat accessions, which were recently genotyped with the wheat 9K single nucleotide polymorphism (SNP) array, for reaction to SNB and identify novel susceptibility genes through association mapping.  The 180 emmer wheat accessions were evaluated for their reaction to the P. nodorum isolate Sn4 at the seedling stage, and 31 (17%) of the accessions showed moderate and high susceptibility.  For identifying the genomic regions associated with susceptibility, 4,134 polymorphic SNPs and 178 of the accessions were used for association analysis. The results revealed that 23 SNP markers on chromosomes 2A, 3A, 5A, 6A, 7A, 2B, 3B, 4B, 5B, 6B, and 7B were significantly associated with SNB susceptibility.  The susceptibility loci on chromosomes 6A, 4B, and 5B may be due to previously identified susceptibility genes including Tsn1, Snn3-B1, Snn5, and Snn6, but the others may represent novel susceptibility genes that can be targeted for further characterization of the wheat-P. nodorum pathosystem. Although the NE triggered susceptibility is predominant in the SNB system, it is also possible that active resistance is contributing quantitatively to this trait.