Molecular Mapping of an Adult Plant Resistance Gene for Leaf Rust in Soft Red Winter Wheat Cultivar AGS 2000.

Leaf rust, caused by Puccinia triticina, is a major disease in southeastern US soft red winter wheat growing regions. In Georgia the gene combination of Lr37/Yr17/Sr38 from Aegilops ventricosa from the 2NS (2AS) translocation has been used to prevent leaf rust. With new emerging virulent races of leaf rust, these resistance genes have lost their effectiveness. AGS 2000 is one of the check cultivars in Uniform Southern Soft Red Winter Wheat Nursery. Its resistance to leaf rust in adult plant stage is a good complement to Lr37, but its genetic basis remains unknown. AGS CL7, a CLEARFIELD herbicide resistance cultivar, inherited most its agronomic traits from AGS 2000, but has lost its leaf rust resistance during the introduction of herbicide resistance genes. However, while both cultivars are susceptible to leaf rust in seedling, at adult plant stage, AGS CL7 remains susceptible whereas AGS 2000 is completely resistant. In this study, a mapping population of 121 F_2:3 lines from an AGS CL7 and AGS 2000 cross was developed to map the Leaf rust resistance gene(s) in AGS 2000. Results indicate that a region on 7BL chromosome was identified to be associated with this resistance. The gene, we identified and temporarily designated LrA2K, was close to Lr14 and Lr68. Both Lr14a and Lr14b were susceptible in the same field as AGS 2000, which indicate that they are different from Lr14 and LrA2K. Additionally, AGS 2000 did not have leaf tip necrosis, a trait associated with Lr68, a slow rusting gene. The marker data showed that LrA2K was about 5.0 cM proximally from Lr68. LrA2K is probably a new gene. The two closely linked markers Xbarc92 and Xbarc182 to LrA2K can initially be used in marker-assisted selection for leaf rust resistance.