Resistance in U.S. Wheat to Recent Eastern African Isolates of Puccinia graminis f. sp. tritici with Virulence to Resistance Gene Sr31.

The stem rust resistance gene Sr31 derived from rye has been used as an important source of stem rust resistance in many wheat cultivars worldwide. Isolates of Puccinia graminis f. sp. tritici with virulence to Sr31 were identified from Uganda in 1999. Stem rust susceptibility in wheat lines with Sr31 was observed in Kenya in 2003 and 2004. An isolate collected from Uganda in 1999 and an isolate collected from Kenya in 2004, identified to be race TTKS, were used in the rust evaluations. Selected cultivars and advanced breeding lines (450 in total) of wheat from the United States were tested against these two stem rust isolates. Resistance to race TTKS was detected in major classes of wheat with the following frequencies: 16% of hard red spring wheat, 48% of hard red winter wheat, and 27% of soft winter wheat. The genes that conferred resistance in the spring wheat have not been conclusively identified. Resistance in cultivar Ivan was likely due to Sr24. Resistance in hard red winter wheat was postulated to be primarily due to Sr24, and resistance in soft winter wheat was postulated to be primarily due to Sr36. The 1AL.1RS translocation present in many U.S. winter wheat cultivars and breeding lines appears to carry an effective resistance gene different from Sr31. The presence of resistance to race TTKS in the adapted germ plasm presents an opportunity to improve stem rust resistance in wheat.