Stem rust resistance in North American spring wheat to Ug99 and other virulent pathotypes of Puccinia graminis f. sp. tritici

KEY MESSAGE: North American spring wheat shows limited seedling resistance to a diverse set of exotic Pgt races but harbors valuable field resistance, including a strong locus for field resistance on chromosome 7AL. The emergence and spread of Ug99 and other races of Puccinia graminis f. sp. tritici (Pgt) with virulence to widely deployed resistance genes threaten global wheat production. This study evaluated North American spring wheat germplasm for seedling responses to four Pgt races (TTKSK, TTKSF+, TKTTF, and TRTTF) and for field response across five environments in Ethiopia and Kenya. Seedling evaluation revealed significant race-specific variation, with 82% of the lines exhibiting resistance to race TKTTF but very limited resistance to the other races. Gene postulation based on resistance patterns suggested a high frequency of Sr7a (~ 92%) in this panel, with only a few lines (~ 5%) carrying Sr8a, Sr11, and/or Sr31. Field evaluations in Ethiopia and Kenya demonstrated quantitative variation for field response, with several lines showing consistent field resistance. Association mapping identified 37 significant associations for seedling resistance, representing 17 unique genomic regions. For field response, 15 significant associations were detected, including a region on chromosome arm 7AL that was consistently detected across environments. Further analysis indicated that this locus confers adult-plant resistance (APR) and might be independent of known stem rust resistance genes located in the region. Hence, KASP assay was developed to enable marker-assisted selection and further investigation of this locus. This study highlights limited seedling resistance but identifies promising sources of field resistance against exotic Pgt races and provides valuable insights into the genetics underlying both seedling and field responses in North American germplasm.