Genetics and mapping of seedling resistance to Ug99 stem rust in winter wheat cultivar Triumph 64 and differentiation of SrTmp, SrCad, and Sr42

Key Message: Resistance to Ug99 stem rust in Triumph 64 was conferred bySrTmpon chromosome arm 6DS and was mapped to the same position asSrCadandSr42, however, the three genes show functional differences. Abstract: Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is an important disease of wheat that can be controlled by effective stem rust resistance (Sr) genes. The emergence of virulent Pgt races in Africa, namely Ug99 and its variants, has stimulated the search for new Sr genes and genetic characterization of known sources of resistance. Triumph 64 is a winter wheat cultivar that carries gene SrTmp, which confers resistance to Ug99. The goals of this study were to genetically map SrTmp and examine its relationship with other Sr genes occupying a similar chromosome location. A doubled haploid (DH) population from the cross LMPG-6S/Triumph 64 was inoculated with Ug99 at the seedling stage. A single gene conditioning resistance to Ug99 segregated in the population. Genetic mapping with SSR markers placed SrTmp on chromosome arm 6DS in a region similar to SrCad and Sr42. SNP markers developed for SrCad were used to further map SrTmp and were also added to a genetic map of Sr42 using a DH population (LMPG-6S/Norin 40). Three SNP markers that co-segregated with SrTmp also co-segregated with SrCad and Sr42. The SNP markers showed no difference in the map locations of SrTmp, SrCad, and Sr42. Multi-race testing with DH lines from the Triumph 64 and Norin 40 populations and a recombinant inbred-line population from the cross LMPG-6S/AC Cadillac showed that SrTmp, SrCad, and Sr42 confer different spectra of resistance. Markers closely linked to SrTmp are suitable for marker-assisted breeding and germplasm development.