Seedling Evaluation, Gene Postulation, and Characterization of Bread Wheat Genotypes against Wheat Stem Rust (Puccinia graminis f. sp. tritici) Pathotypes in the Highlands of Bale, Oromiya, Ethiopia

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is one of the most destructive diseases of wheat worldwide. In Ethiopia, the disease is widespread in humid and warm wheat-growing environments, particularly in central and southeastern regions. Understanding resistance genes to Puccinia graminis f. sp. tritici is a fundamental requirement for the effective utilization and deployment of resistant sources in integrated wheat stem rust management. Field and greenhouse experiments were conducted to evaluate slow-rusting resistance and to investigate stem rust resistance spectra in bread wheat genotypes at the seedling stage during the 2021/22 main cropping season. A total of 121 bread wheat genotypes were included in the study. Field experiments were conducted at Sinana and Agarfa, while seedling evaluations were carried out under greenhouse conditions at Debre Zeit Agricultural Research Center. The genotypes were tested against seven dominant stem rust races: TTTTF, TTKTT, TTKTTF, TTRTF, TKTTF, TKKTF, and TTKSK (UG99), which were originally obtained from Ambo Agricultural Research Center. Using 20 near-isogenic lines (NILs), four resistance genes—Sr11, Sr36, Sr30, and Sr24—were postulated in 15 wheat genotypes. Twenty genotypes showed resistance, while 88 genotypes carried unidentified Sr genes. A total of 28 genotypes exhibited intermediate to susceptible reactions at the seedling stage (infection types ranging from 2+ to 4). Among the evaluated genotypes, three varieties—Kingbird, Pavon 76, and Sofumer—exhibited slow-rusting resistance with an area under disease progress curve (AUDPC) value of less than 300. Fifteen genotypes were identified as true slow-rusting types based on combined field and seedling evaluations. These genotypes, carrying Sr11, Sr36, Sr30, and Sr24 genes, along with the identified slow-rusting lines, can be utilized in future breeding programs aimed at developing durable resistance to wheat stem rust. Further studies incorporating additional NILs and molecular markers are recommended to effectively identify both seedling and adult plant resistance (APR) genes in bread wheat genotypes.