Mapping of two loci conferring resistance to wheat stem rust pathogen races TTKSK (Ug99) and TRTTF in the elite hard red spring wheat line SD4279

Since its identification in the late 1990s, the stem rust pathogen (Puccinia graminis. f. sp. tritici (Pgt)) strain Ug99 (race TTKSK) has represented a worldwide wheat production threat due to its ability to overcome most of the resistance genes present in commercial cultivars. In order to address this challenge, resistance genes in wheat cultivars as well as in wild relatives have been identified. However, stem rust resistance breeding is facing a new challenge with the recent discovery in Ethiopia of a new race of Pgt (TRTTF) capable of defeating Sr13, SrTmp, and Sr1R^Amigo genes that conferred resistance to the Ug99 race group. As part of an ongoing screening process at USDA-ARS Cereal Disease Laboratory, SD4279, an elite line from the hard red spring wheat breeding program at South Dakota State University, was found to be resistant to both races TTKSK and TRTTF. The objectives posed in this research were (1) to characterize the genetics of resistance to stem rust in SD4279 and (2) to identify molecular markers linked to race TTKSK (Ug99) and TRTTF resistance in SD4279. A mapping population composed of 92 F_2:3 families was evaluated for resistance to TTKSK and TRTTF. A single-gene conferring resistance to TTKSK, likely Sr9h, was mapped on chromosome arm 2BL. Also, a single gene was located on chromosome arm 6AS conferring resistance to TRTTF. Based on the type of reaction and map location, we postulate that the 6AS resistance gene is Sr8a which has not been mapped previously using DNA markers.