Association mapping for resistance to tan spot induced by Pyrenophora tritici-repentis race 1 in CIMMYTs historical bread wheat set

Tan spot, a major foliar disease of wheat, is caused by an ascomycete Pyrenophora tritici - repentis ( PTR ). Association mapping on germplasm collection is a novel approach for the discovery and validation of major genes/QTLs for a trait. Linkage disequilibrium, the genetic basis of association mapping, can be an important tool to identify genomic regions associated with tan spot resistance. Association analysis utilizing the population structure and additive genetic covariance between relatives was conducted for tan spot resistance on a set of historical bread wheat germplasm comprising of 170 lines developed at CIMMYT, Mexico with the genotypic data generated with 1644 molecular markers. Tan spot resistance data was obtained by screening the germplasm with PTR race 1 isolate Ptr-1. Two weeks old seedlings were inoculated and rated 8 days later on a lesion type disease scale. Three experiments were conducted in the greenhouse with each experiment as arranged in completely randomized design with two replicates. Over half of the entries were resistant to tan spot induced by PTR race 1. The genome-wide scan revealed significant marker-phenotype associations on the short arm of chromosomes 1A, 1B, and 6B and long arm of chromosomes 4A, 6A 1 , 6A 2 , 2B, 3B, 5B, and 7B that play an important role in conferring resistance to tan spot. Although some genomic regions contributing to tan spot resistance have been previously identified; novel regions on long arm of chromosomes 6A 1 , 6A 2 , and 7B, were identified in this study. Findings of this study reveal that a high proportion of CIMMYT wheat germplasm is resistant to tan spot caused by PTR race 1 and genetic base of CIMMYT germplasm resistance involves race specific and non-race specific resistance genes.