BackgroundThe recently identified Puccinia graminis f. sp. tritici (Pgt) race TTKSK (Ug99) poses a severe threat to global wheat production because of its broad virulence on several widely deployed resistance genes. Additional virulences have been detected in the Ug99 group of races, and the spread of this race group has been documented across wheat growing regions in Africa, the Middle East (Yemen), and West Asia (Iran). Other broadly virulent Pgt races, such as TRTTF and TKTTF, present further difficulties in maintaining abundant genetic resistance for their effective use in wheat breeding against this destructive fungal disease of wheat. In an effort to identify loci conferring resistance to these races, a genome-wide association study was carried out on a panel of 250 spring wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT), six wheat breeding programs in the United States and three wheat breeding programs in Canada.ResultsThe lines included in this study were grouped into two major clusters, based on the results of principal component analysis using 23,976 SNP markers. Upon screening for adult plant resistance (APR) to Ug99 during 2013 and 2014 in artificial stem rust screening nurseries at Njoro, Kenya and at Debre Zeit, Ethiopia, several wheat lines were found to exhibit APR. The lines were also screened for resistance at the seedling stage against races TTKSK, TRTTF, and TKTTF at USDA-ARS Cereal Disease Laboratory in St. Paul, Minnesota; and only 9 of the 250 lines displayed seedling resistance to all the races. Using a mixed linear model, 27 SNP markers associated with APR against Ug99 were detected, including markers linked with the known APR gene Sr2. Using the same model, 23, 86, and 111 SNP markers associated with seedling resistance against races TTKSK, TRTTF, and TKTTF were identified, respectively. These included markers linked to the genes Sr8a and Sr11 providing seedling resistance to races TRTTF and TKTTF, respectively. We also identified putatively novel Sr resistance genes on chromosomes 3B, 4D, 5A, 5B, 6A, 7A, and 7B.ConclusionOur results demonstrate that the North American wheat breeding lines have several resistance loci that provide APR and seedling resistance to highly virulent Pgt races. Using the resistant lines and the SNP markers identified in this study, marker-assisted resistance breeding can assist in development of varieties with elevated levels of resistance to virulent stem rust races including TTKSK.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0628-9) contains supplementary material, which is available to authorized users.

Key message Dwarf bunt-resistant bread wheat accessions and SNP markers associated with DB resistance identified in this study are valuable resources for characterization and deployment of DB resistance in bread wheat. Abstract Dwarf bunt (DB), caused by Tilletia controversa J.G. Kühn, can significantly reduce grain yield and quality on autumn-sown wheat in regions with prolonged snow cover. DB can be managed with the use of resistant cultivars. The objectives of the present study were to characterize DB resistance in a large set of bread wheat accessions from the National Small Grains Collection and use a genome-wide association study approach to identify genetic loci associated with DB resistance. A total of 292 accessions were selected using historical DB resistance data recorded across many trials and years in the Germplasm Resources Information Network (GRIN) and re-tested for DB resistance in replicated field nurseries in Logan, UT, in 2017, 2018, and 2019. Ninety-eight accessions were resistant with DB normalized incidence ≤ 10%, and twenty-eight of these were highly resistant with DB normalized incidence ≤ 1% in both GRIN and the field nurseries. Based on the presence of marker haplotypes of the four published dwarf bunt QTL on 6DS, 6DL, 7AL, and 7DS, highly resistant accessions identified in this study may provide novel resistance and should be further evaluated. This study validated one previously identified QTL on 6DS and identified an additional locus on 6DS. These loci explained 9-15% of the observed phenotypic variation. The resistant accessions and molecular markers identified in the present study may provide valuable resources for characterization and deployment of DB resistance in bread wheat.

Accessions of common wheat (Triticum aestivum L. subsp. aestivum) from the USDA-ARS National Small Grains Collection (NSGC) are a resource for wheat scientists worldwide. The

A new gene for Ug99 resistance from wheat landrace PI 374670 was detected on the long arm of chromosome 7A. Wheat landrace PI 374670 has seedling and field resistance to stem rust caused by Puccinia graminis f. sp tritici Eriks. & E. Henn (Pgt) race TTKSK. To elucidate the inheritance of resistance, 216 BC1F2 families, 192 double haploid (DH) lines, and 185 recombinant inbred lines (RILs) were developed by crossing PI 374670 and the susceptible line LMPG-6. The parents and progeny were evaluated for seedling resistance to Pgt races TTKSK, MCCFC, and TPMKC. The DH lines were tested in field stem rust nurseries in Kenya and Ethiopia. The DH lines were genotyped with the 90K wheat iSelect SNP genotyping platform. Goodness-of-fit tests indicated that a single dominant gene in PI 374670 conditioned seedling resistance to the three Pgt races. The seedling resistance locus mapped to the long arm of chromosome 7A and this result was verified in the RIL population screened with the flanking SNP markers using KASP assays. In the same region, a major QTL for field resistance was detected in a 7.7 cM interval and explained 34-54 and 29-36% of the variation in Kenya and Ethiopia, respectively. Results from tests with specific Pgt races and the csIH81 marker showed that the resistance was not due to Sr22. Thus, a new stem rust resistance gene or allele, either closely linked or allelic to Sr15, is responsible for the seedling and field resistance of PI 374670 to Ug99.

Developing oat cultivars with partial resistance to crown rust would be beneficial and cost-effective for disease management. Two recombinant inbred-line populations were generated by crossing the susceptible cultivar Provena with two partially resistant sources, CDC Boyer and breeding line 94197A1-9-2-2-2-5. A third mapping population was generated by crossing the partially resistant sources to validate the quantitative trait locus (QTL) results. The three populations were evaluated for crown rust severity in the field at Louisiana State University (LSU) in 2009 and 2010 and at the Cereal Disease Laboratory (CDL) in St. Paul, MN, in 2009, 2010, and 2011. An iSelect platform assay containing 5,744 oat single nucleotide polymorphisms was used to genotype the populations. From the 2009 CDL test, linkage analyses revealed two QTLs for partial resistance in the Provena/CDC Boyer population on chromosome 19A. One of the 19A QTLs was also detected in the 2009 LSU test. Another QTL was detected on chromosome 12D in the CDL 2009 test. In the Provena/94197A1-9-2-2-2-5 population, only one QTL was detected, on chromosome 13A, in the CDL 2011 test. The 13A QTL from the Provena/94197A1-9-2-2-2-5 population was validated in the CDC Boyer/94197A1-9-2-2-2-5 population in the CDL 2010 and 2011 tests. Comparative analysis of the significant marker sequences with the rice genome database revealed 15 candidate genes for disease resistance on chromosomes 4 and 6 of rice. These genes could be potential targets for cloning from the two resistant parents.

Key message Two major dwarf bunt resistance QTLs were mapped to a known Bt9 locus and a novel locus. The associated KASP markers were developed and validated in other two populations. Abstract Dwarf bunt (DB), caused by Tilletia controversa J.G. Kühn, and common bunt (CB), caused by T. caries and T. foetida, are two destructive diseases that reduce grain yield and quality in wheat. Breeding for bunt-resistant cultivars is important in many wheat production areas, especially where organic wheat is grown. However, few molecular markers have been used in selection of bunt resistance. In the present study, a doubled haploid (DH) population derived from the buntresistant line 'IDO835' and the susceptible cultivar 'Moreland' was evaluated for DB resistance in a field nursery in Logan, Utah, for four growing seasons. The population was genotyped with the Illumina 90 K SNP iSelect marker platform. Two major QTLs were consistently identified on chromosomes 6DL (Q.DB.ui-6DL) and 7AL (Q.DB.ui-7AL), explaining up to 53% and 38% of the phenotypic variation, respectively. Comparative study suggested that Q.DB.ui-6DL was located in the same region as the CB resistance gene Bt9, and Q.DB.ui-7AL was located at a novel locus for bunt resistance. Based on Chinese Spring reference sequence and annotations (IWGSC RefSeq v1.1), both resistance QTLs were mapped to disease resistance gene-rich (NBS-LRR and kinase genes) regions. To validate the identified QTL and design user-friendly markers for MAS, five SNPs were converted to Kompetitive Allele-Specific PCR (KASP) markers and used to genotype two validation panels, including a DH population and a diverse winter wheat population from USDA-ARS National Small Grain Collection, as well as a Bt gene investigation panel, consisting of 15 bunt differential lines and 11 resistant lines.Communicated by Beat Keller.Blair Goates: Retired.

Quantitative trait loci (QTL) for barley stripe rust resistance were mapped in recombinant inbred lines (RIL) from a 'Lenetah' × 'Grannelose Zweizeilige' (GZ) cross. GZ is known for a major seedling resistance QTL on chromosome 4H but linked markers suitable for marker-assisted selection have not been developed. This study identified the 4H QTL (log of the likelihood [LOD] = 15.94 at 97.19 centimorgans [cM]), and additional QTL on chromosomes 4H and 6H (LOD = 5.39 at 72.7 cM and 4.24 at 34.46 cM, respectively). A QTL on chromosome 7H (LOD = 2.04 at 81.07 cM) was suggested. All resistance alleles were derived from GZ. Evaluations of adult plant response in Corvallis, OR in 2013 and 2015 provided evidence of QTL at the same positions. However, the minor QTL on 4H was not statistically significant in either location/year, while the 7H QTL was significant in both. The single-nucleotide polymorphism markers flanking the resistance QTL were validated in RIL from a '95SR316A' × GZ cross for their ability to predict seedling resistance. In 95SR316A × GZ, 91 to 92% of RIL with GZ alleles at the major 4H QTL and at least one other were resistant to moderate in reaction. In these populations, at least two QTL were required to transfer the barley stripe rust resistance from GZ.

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.