Characterization of Sr9h, a wheat stem rust resistance allele effective to Ug99

Wheat stem rust resistance gene SrWeb is an allele at the Sr9 locus that confers resistance to Ug99. Race TTKSK (Ug99) of Puccinia graminis f. sp. tritici, the causal fungus of stem rust, threatens global wheat production because of its broad virulence to current wheat cultivars. A recently identified Ug99 resistance gene from cultivar Webster, temporarily designated as SrWeb, mapped near the stem rust resistance gene locus Sr9. We determined that SrWeb is also present in Ug99 resistant cultivar Gabo 56 by comparative mapping and an allelism test. Analysis of resistance in a population segregating for both Sr9e and SrWeb demonstrated that SrWeb is an allele at the Sr9 locus, which subsequently was designated as Sr9h. Webster and Gabo 56 were susceptible to the Ug99-related race TTKSF+ from South Africa. Race TTKSF+ possesses unique virulence to uncharacterized Ug99 resistance in cultivar Matlabas. This result validated that resistance to Ug99 in Webster and Gabo 56 is conferred by the same gene: Sr9h. The emergence of pathogen virulence to several resistance genes that are effective to the original Ug99 race TTKSK, including Sr9h, suggests that resistance genes should be used in combinations in order to increase resistance durability.