The number of Ug99-effective wheat stem rust resistance genes has increased from 22 to 45 in the past 13 years. Despite this research progress, most United States spring wheat cultivars and advanced breeding lines remain susceptible to foreign virulent races of the stem rust pathogen, Puccinia graminis f. sp. tritici. Multiple stem rust resistance genes need to be utilized in gene combinations to avoid rapid loss of effectiveness of resistance genes. In order to address the needs for (1) increased stem rust resistance in United States spring wheat and (2) use of resistance gene combinations, linkage blocks of multiple Ug99-effective stem rust resistance genes were backcrossed into U.S. spring wheat cultivars. A linkage block including Sr9h and Sr28 on chromosome arm 2BL was backcrossed to the BC4 generation in the cultivars ‘Linkert’, ‘Bolles’, ‘Advance’ and ‘Forefront’. A linkage block on chromosome arm 7AL with Sr15 and Sr22 was backcrossed to the BC4 generation in Linkert, ‘Lang-MN’, and ‘Prevail’. The backcrossing was facilitated by molecular marker assays. Fixed BC4F3 lines were validated for the presence of stem rust resistance genes using multi-race seedling tests. Multi-location yield trials are planned for fixed BC4F5 lines with and without Sr9h-Sr28 in 2020 to quantify any linkage drag. The fixed lines with linkage blocks of Ug99-effective stem rust resistance genes have been used as parents in crossing blocks of spring wheat breeding programs. Deployment of Ug99-effective stem rust resistance genes in U.S. spring wheat could protect U.S. wheat production from a stem rust epidemic if a member of the Ug99 race group were to arrive in the United States.
