The spring wheat belt of Western Siberia and Northern Kazakhstan covers more than 15 million ha. While moisture stress is the main factor limiting production, rusts also represent a major challenge, especially in years with higher rainfall. Stem rust was not considered economically important until 2015 when a local epidemic occurred in the Omsk region of Russia and neighboring areas of Kazakhstan and affected more than 1 million ha. It occurred again in 2016 though the spread, severity and losses were less. This study used 16 pathotypes and 17 molecular markers to characterize a set of 146 spring wheat varieties and breeding lines identified as stem rust resistant in Kenya and the Kazakhstan–Siberia region for the presence of major genes. The genetic basis of resistance in the material was limited to Sr25, Sr31, Sr36, Sr6Ai, Sr6Ai#2, and some unknown major genes. Genes Sr25 and Sr6Ai#2 also provided high levels of resistance to leaf rust through linkages with Lr19 and Lr6Ai#2. Adult plant resistance to stem rust was observed in 26 genotypes (16.5 %), including eight possessing Sr57 gene. The high risk of stem rust—as indicated by the 2015 Siberian epidemic—means that there is an urgent need to diversify the genetic bases of resistance and to promote resistant varieties with farmers.
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Stem rust (Puccinia graminis f. sp. tritici, Pgt) races belonging to the Ug99 (TTKSK) race group pose a serious threat to global wheat (Triticum aestivum L.) production. To improve Pgt host resistance, the Ug99-effective resistance gene SrTA10187 previously identified in Aegilops tauschii Coss. was introgressed into wheat, and mapped to the short arm of wheat chromosome 6D. In this study, high-resolution mapping of SrTA10187 was done using a population of 1,060 plants. Pgt resistance was screened using race QFCSC. PCR-based SNP and STS markers were developed from genotyping-by-sequencing tags and SNP sequences available in online databases. SrTA10187 segregated as expected in a 3:1 ratio of resistant to susceptible individuals in three out of six BC3F2 families, and was fine-mapped to a 1.1 cM region on wheat chromosome 6DS. Marker context sequence was aligned to the reference Ae. tauschii genome to identify the physical region encompassing SrTA10187. Due to the size of the corresponding region, candidate disease resistance genes could not be identified with confidence. Comparisons with the Ae. tauschii genetic map developed by Luo et al. (PNAS 110(19):7940–7945, 2013) enabled identification of a discrete genetic locus and a BAC minimum tiling path of the region spanning SrTA10187. Annotation of pooled BAC library sequences led to the identification of candidate genes in the region of interest—including a single NB-ARC-LRR gene. The shorter genetic interval and flanking KASP™ and STS markers developed in this study will facilitate marker-assisted selection, gene pyramiding, and positional cloning of SrTA10187.
Wheat landrace PI 177906 has seedling resistance to stem rust caused by Puccinia graminis f. sp. tritici races TTKSK, TTKST, and BCCBC and field resistance to the Ug99 race group. Parents, 140 recombinant inbred lines, and 138 double haploid (DH) lines were evaluated for seedling resistance to races TTKSK and BCCBC. Parents and the DH population were evaluated for field resistance to Ug99 in Kenya. The 90K wheat single nucleotide polymorphism (SNP) genotyping platform was used to genotype the parents and populations. Goodness-of-fit tests indicated that two dominant genes in PI 177906 conditioned seedling resistance to TTKSK. Two major loci for seedling resistance were consistently mapped to the chromosome arms 2BL and 6DS. The BCCBC resistance was mapped to the same location on 2BL as the TTKSK resistance. Using field data from the three seasons, two major QTL were consistently detected at the same regions on 2BL and 6DS. Based on the mapping result, race specificity, and the infection type observed in PI 177906, the TTKSK resistance on 2BL is likely due to Sr28. One SNP marker (KASP_IWB1208) was found to be predictive for the presence of the TTKSK resistance locus on 2BL and Sr28.