Mapping Resistance to the Ug99 Race Group of the Stem Rust Pathogen in a Spring Wheat Landrace

Wheat landrace PI 374670 has seedling and field resistance to stem rust caused by Puccinia graminis f. sp tritici Eriks. & E. Henn (Pgt) race TTKSK. To elucidate the inheritance of resistance, 216 BC₁F₂ families, 192 double haploid (DH) lines, and 185 recombinant inbred lines (RILs) were developed by crossing PI 374670 and the susceptible line LMPG-6. The parents and progeny were evaluated for seedling resistance to Pgt races TTKSK, MCCFC, and TPMKC. The DH lines were tested in field stem rust nurseries in Kenya and Ethiopia. The DH lines were genotyped with the 90K wheat iSelect SNP genotyping platform. Goodness-of-fit tests indicated that a single dominant gene in PI 374670 conditioned seedling resistance to the three Pgt races. The seedling resistance locus mapped to the long arm of chromosome 7A and this result was verified with the RIL population screened with the flanking SNP markers using KASP assays. In the same region, a major QTL for adult plant resistance was detected in a 7.7 cM interval and explained 34-54% and 29-36% of the variation in Kenya and Ethiopia, respectively. Only two genes for resistance to Pgt, Sr15 and Sr22, have been mapped to chromosome 7AL and of these only Sr22 is reported to be effective against the Ug99 race group.  Results from tests with specific Pgt races and the csIH81 marker showed that the resistance from PI 374670 was not due to Sr22. Results from the KASP assay of PI 374670 and ‘Norka’, an accession with Sr15, showed that the four markers linked to the resistance locus were all monomorphic between the two accessions. Thus, a new stem rust resistance gene or allele, either closely linked or allelic to Sr15, is responsible for the seedling and field resistance of PI 374670 to Ug99.