RESUMEN
Wheat stem rust caused by Puccinia graminis f. sp. tritici is a widespread and recurring threat to wheat production. Emerging P. graminis f. sp. tritici variants are rapidly overcoming major gene resistance deployed in wheat cultivars and new sources of race-nonspecific resistance are urgently needed. The National Small Grains Collection (NSGC) contains thousands of wheat landrace accessions that may harbor unique and broadly effective sources of resistance to emerging P. graminis f. sp. tritici variants. All NSGC available facultative and winter-habit bread wheat landraces were tested in a field nursery in St. Paul, Minnesota, against a bulk collection of six common U.S. P. graminis f. sp. tritici races. Infection response and severity data were collected on 9,192 landrace accessions at the soft-dough stage and resistant accessions were derived from single spikes. Derived accessions were tested in St. Paul a second time to confirm resistance and in a field nursery in Njoro, Kenya against emerging races of P. graminis f. sp. tritici with virulence to many known resistance genes including Sr24, Sr31, Sr38, and SrTmp. Accessions resistant in the St. Paul field were also tested at the seedling stage with up to 13 P. graminis f. sp. tritici races, including TTKSK and TKTTF, and with 19 molecular markers linked with known stem rust resistance genes or genes associated with modern breeding practices. Forty-five accessions were resistant in both U.S. and Kenya field nurseries and lacked alleles linked with known stem rust resistance genes. Accessions with either moderate or strong resistance in the U.S. and Kenya field nurseries and with novel seedling resistance will be prioritized for further study.