QTL mapping of resistance to Ug99 and other stem rust pathogen races in bread wheat

Most wheat cultivars planted worldwide are susceptible to the stem rust Ug99 race group. To prepare for the potential spread of these races into South America, we aimed to identify genomic regions responsible for resistance to Ug99 race group in germplasm adapted to South America. Two RIL populations from a cross between a stem rust susceptible parent “Baguette 13” and resistant local parents “INIA Tero” and “BR23//CEP19/PF85490” were developed. Phenotypical evaluation was completed at the seedling stage in Uruguay and under field conditions in Uruguay and Kenya. Both RIL populations were genotyped using the GBS approach. Besides Sr24, three other resistance loci in “INIA Tero” were detected on chromosomes 2B, 6A, and 7B. All four QTL were effective to local races, whereas only the QTL on chromosome 2B was effective against the Ug99 race group. Besides Sr31, “BR23//CEP19/PF85490” also carries two other stem rust resistance loci on chromosomes 2B and 6A. All three explained the resistance in Uruguay, while only the QTL on 2B was effective to Ug99 in Kenya. The physical location suggested that the QTL identified on chromosome 2B in both populations may correspond to Sr28, which was confirmed using specific molecular markers. Further studies are needed to determine the relationship between QTL for resistance to local races identified on chromosomes 6A and 7B and previously reported resistance genes and QTL. The results of this study are highly relevant for breeding wheat cultivars with diverse and durable resistance to stem rust.