Characterization of Stem Rust Resistance in ICARDA/CWANA Elite Wheat Germplasm Using Linked Molecular Markers.

Stem rust, caused by Puccinia graminis f. sp. tritici, was one of the major biotic constraints to wheat production in many parts of the world. Until recently, the disease has been under control through the deployment of host resistance. However, in 1999 a race of stem rust with virulence to one of the most widely deployed resistance genes in wheat, Sr31, was identified in Uganda. Using the North American stem rust race analysis system, this race was designated as TTKSK (syn. Ug99). This race, initially virulent on Sr31 has evolved since then to variants TTKST and TTTSK carrying additional virulence to the majority of commonly used stem resistance genes in bread wheat Sr24 and  Sr36 respectively. Success in breeding for durable stem rust resistance will depend on comprehensive knowledge and understanding of genetic variation available in existing wheat germplasm.

To date, about 50 stem rust resistance genes have been identified and catalogued (McIntosh et al., 2008; http://rustopedia.get-traction.com/traction: accessed 14/01/2010). Some of these have been mapped with linked molecular markers identified. Amongst these, some have been found effective against Ug99. Molecular markers linked to effective Sr genes to Ug99 can assist in the identification of lines currently carrying resistance to Ug99 and will be useful in the future development of resistant cultivars. This study reports the characterization of more than 1000 ICARDA wheat germplasm including cultivars and advanced breeding lines from the CWANA region using linked and diagnostic markers in particular for genes effective against race TTKS. These include markers linked to Sr2, Sr24, Sr25, Sr26, Sr36 and Sr39. Further, the various markers including SSR and Est-derived STS reputedly linked to the slow rusting gene, Sr2 were also validated in diverse genetic background. Results from the analysis showed that a high proportion of ICARDA’s elite wheat germplasm possess the durable stem rust resistance gene, Sr2 with more than 50% of the germplasm showing  SSR haplotype associated with Sr2 resistance. Some of the lines possessed two stem rust resistance genes such as Sr2 and Sr25 or Sr2 and Sr24. A high proportion of these lines also showed the diagnostic fragment associated with Lr34/Yr18. The results is assisting the ICARDA Spring bread wheat breeding program to more effectively target the incorporation of  these diverse resistance genes into relevant adapted elite germplasm in addition to  the efficient pyramiding of  diverse sources/genes  in adapted local germnplasm relevant to its NARS partners. With the improved capacity of deploying resistance gene pyramids, the durability of resistance to Ug99 will be greatly enhanced.