Inheritance and Mapping of Novel Stem Rust Resistance of Wheat Landrace, PI 410966.

Stem rust (caused by Puccinia graminis f. sp tritici, abbreviated Pgt) of wheat (Triticum aestivum L.), is one of the most destructive cereal diseases globally.  Concerns have grown since 1999 with the discovery in Uganda of a new virulence race of Pgt, known as Ug99 (also TTKSK), which is capable of attacking a majority of current wheat cultivars worldwide. The spring wheat landrace, PI 410966, was identified as resistant to race TTKSK in international stem rust nurseries carried out by the USDA-ARS in 2005 and 2006.  The objectives of this experiment are to characterize the resistance and to determine the chromosomal location of the stem rust resistance gene(s) in PI 410966. Two mapping populations were developed from crosses between PI 410966 and two susceptible spring wheat lines: LMPG-6 and OK3040.  Since PI 410966 serves as the only source of resistance for these two populations, the phenotypic data from both populations can be combined and analyzed as one large population. Tests for resistance were carried out on the F_2:3 families, one test with inoculations of Pgt race TPMKC (or 15B) at Purdue University and one test with TTKSK at the USDA-ARS Cereal Disease Laboratory, St. Paul, MN.  F_2:3 segregation ratios of 1:2:1 (resistant:segregating:susceptible) in both F_2:3 tests indicated that resistance of PI 410966 is conditioned by one dominant gene. Genomic mapping of the resistance gene is in progress.  Several wheat simple sequence repeat (SSR) markers have been identified as polymorphic between the resistant and susceptible parents, and additional polymorphic markers are being identified.  The USDA conducted a second inoculation test on F_6:7 families using Ug99, and the results were used to select families for Bulk Segregant Analysis (BAS).  The polymorphic markers will be screened on the F_7:8 lines to map the stem rust resistance gene of PI 410966.  By identifying markers that are closely linked or co-segregating with the stem rust resistance, wheat breeders can use marker-based selection to efficiently integrate the resistance into their new and improved cultivars.