UC Davis

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (synonym: Zymoseptoria tritici; asexual stage: Septoria tritici), is an important disease of wheat worldwide. Management of the disease usually is by host resistance or fungicides. However, M. graminicola has developed insensitivity to most commonly applied fungicides so there is a continuing need for well-characterized sources of host resistance to accelerate the development of improved wheat cultivars. Gene Stb3 has been a useful source of major resistance, but its mapping location has not been well characterized. Based on linkage to a single marker, a previous study assigned Stb3 to a location on the short arm of chromosome 6D. However, the results from the present study show that this reported location is incorrect. Instead, linkage analysis revealed that Stb3 is located on the short arm of wheat chromosome 7A, completely linked to microsatellite (SSR) locus Xwmc83 and flanked by loci Xcfa2028 (12.4 cM distal) and Xbarc222 (2.1 cM proximal). Linkage between Stb3 and Xwmc83 was validated in BC₁F₃ progeny of other crosses, and analyses of the flanking markers with deletion stocks showed that the gene is located on 7AS between fraction lengths 0.73 and 0.83. This revised location of Stb3 is different from those for other STB resistance genes previously mapped in hexaploid wheat but is approximately 20 cM proximal to an STB resistance gene mapped on the short arm of chromosome 7A^m in Triticum monococcum. The markers described in this study are useful for accelerating the deployment of Stb3 in wheat breeding programs.