Stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), is one of the most damaging diseases in common wheat (Triticum aestivum L.). With the objective of identifying and tagging new genes for resistance to stripe rust, F1, F2 and F3 populations from the cross Zhou 8425B/Chinese Spring were inoculated with Chinese PST isolate CYR32 in the greenhouse. A total of 790 SSR primers were used to test the parents and resistant and susceptible bulks. The resulting seven polymorphic markers on chromosome 7BL were used for genotyping F2 and F3 populations. Results indicated that Zhou 8425B carries a single dominant resistance gene, temporarily designated YrZH84, closely linked to SSR markers Xcfa2040-7B and Xbarc32-7B with genetic distances of 1.4 and 4.8 cM, respectively. In a seedling test with 25 PST isolates, the reaction patterns of YrZH84 were different from those of lines carrying Yr2 and Yr6. It was concluded that YrZH84 is probably a new stripe rust resistance gene.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most widespread and destructive wheat diseases worldwide. Growing resistant cultivars is the preferred control of the disease. The spring wheat cultivar 'Alpowa' has both race-specific, all-stage resistance and non-race-specific, high-temperature adult-plant (HTAP) resistances to stripe rust. To identify genes for the stripe rust resistances, Alpowa was crossed with 'Avocet Susceptible' (AVS). Seedlings of the parents, and F(1), F(2) and F(3) progeny were tested with races PST-1 and PST-21 of P. striiformis f. sp. tritici under controlled greenhouse conditions. Alpowa has a single partially dominant gene, designated as YrAlp, conferring all-stage resistance. Resistance gene analog polymorphism (RGAP) and simple sequence repeat (SSR) techniques were used to identify molecular markers linked to YrAlp. A linkage group of five RGAP markers and two SSR markers was constructed for YrAlp using 136 F(3) lines. Amplification of a set of nulli-tetrasomic Chinese Spring lines with RGAP markers Xwgp47 and Xwgp48 and the two SSR markers indicated that YrAlp is located on the short arm of chromosome 1B. To map quantitative trait loci (QTLs) for the non-race-specific HTAP resistance, the parents and 136 F(3) lines were tested at two sites near Pullman and one site near Mount Vernon, Washington, under naturally infected conditions. A major HTAP QTL was consistently detected across environments and was located on chromosome 7BL. Because of its chromosomal location and the non-race-specific nature of the HTAP resistance, this gene is different from previously described genes for adult-plant resistance, and is therefore designated Yr39. The gene contributed to 64.2% of the total variation of relative area under disease progress curve (AUDPC) data and 59.1% of the total variation of infection type data recorded at the heading-flowering stages. Two RGAP markers, Xwgp36 and Xwgp45 with the highest R (2) values were closely linked to Yr39, should be useful for incorporation of the non-race-specific resistance gene into new cultivars and for combining Yr39 with other genes for durable and high-level resistance.

The locus Lr34/Yr18/Pm38 confers partial and durable resistance against the devastating fungal pathogens leaf rust, stripe rust, and powdery mildew. In previous studies, this broad-spectrum resistance was shown to be controlled by a single gene which encodes a putative ATP-binding cassette transporter. Alleles of resistant and susceptible cultivars differed by only three sequence polymorphisms and the same resistance haplotype was found in the three independent breeding lineages of Lr34/Yr18/Pm38. Hence, we used these conserved sequence polymorphisms as templates to develop diagnostic molecular markers that will assist selection for durable multi-pathogen resistance in breeding programs. Five allele-specific markers (cssfr1-cssfr5) were developed based on a 3 bp deletion in exon 11 of the Lr34-gene, and one marker (cssfr6) was derived from a single nucleotide polymorphism in exon 12. Validation of reference genotypes, well characterized for the presence or absence of the Lr34/Yr18/Pm38 resistance locus, demonstrated perfect diagnostic values for the newly developed markers. By testing the new markers on a larger set of wheat cultivars, a third Lr34 haplotype, not described so far, was discovered in some European winter wheat and spelt material. Some cultivars with uncertain Lr34 status were re-assessed using the newly derived markers. Unambiguous identification of the Lr34 gene aided by the new markers has revealed that some wheat cultivars incorrectly postulated as having Lr34 may possess as yet uncharacterised loci for adult plant leaf and stripe rust resistance.

We demonstrate that Lr67/Yr46 has pleiotropic effect on stem rust and powdery mildew resistance and is associated with leaf tip necrosis. Genes are designated as Sr55, Pm46 and Ltn3, respectively. Wheat (Triticum aestivum) accession RL6077, known to carry the pleiotropic slow rusting leaf and yellow rust resistance genes Lr67/Yr46 in Thatcher background, displayed significantly lower stem rust (P. graminis tritici; Pgt) and powdery mildew (Blumeria graminis tritici; Bgt) severities in Kenya and in Norway, respectively, compared to its recurrent parent Thatcher. We investigated the resistance of RL6077 to stem rust and powdery mildew using Avocet × RL6077 F6 recombinant inbred lines (RILs) derived from two photoperiod-insensitive F3 families segregating for Lr67/Yr46. Greenhouse seedling tests were conducted with Mexican Pgt race RTR. Field evaluations were conducted under artificially initiated stem rust epidemics with Pgt races RTR and TTKST (Ug99 + Sr24) at Ciudad Obregon (Mexico) and Njoro (Kenya) during 2010-2011; and under natural powdery mildew epiphytotic in Norway at Ås and Hamar during 2011 and 2012. In Mexico, a mean reduction of 41 % on stem rust severity was obtained for RILs carrying Lr67/Yr46, compared to RILs that lacked the gene, whereas in Kenya the difference was smaller (16 %) but significant. In Norway, leaf tip necrosis was associated with Lr67/Yr46 and RILs carrying Lr67/Yr46 showed a 20 % reduction in mean powdery mildew severity at both sites across the 2 years of evaluation. Our study demonstrates that Lr67/Yr46 confers partial resistance to stem rust and powdery mildew and is associated with leaf tip necrosis. The corresponding pleiotropic, or tightly linked, genes, designated as Sr55, Pm46, and Ltn3, can be utilized to provide broad-spectrum durable disease resistance in wheat.

Hexaploid bread wheat (Triticum aestivum L. em. Thell) is one of the world's most important crop plants and displays a very low level of intraspecific polymorphism. We report the development of highly polymorphic microsatellite markers using procedures optimized for the large wheat genome. The isolation of microsatellite-containing clones from hypomethylated regions of the wheat genome increased the proportion of useful markers almost twofold. The majority (80%) of primer sets developed are genome-specific and detect only a single locus in one of the three genomes of bread wheat (A, B, or D). Only 20% of the markers detect more than one locus. A total of 279 loci amplified by 230 primer sets were placed onto a genetic framework map composed of RFLPs previously mapped in the reference population of the International Triticeae Mapping Initiative (ITMI) Opata 85 x W7984. Sixty-five microsatellites were mapped at a LOD >2.5, and 214 microsatellites were assigned to the most likely intervals. Ninety-three loci were mapped to the A genome, 115 to the B genome, and 71 to the D genome. The markers are randomly distributed along the linkage map, with clustering in several centromeric regions.

Identification of resistance genes is important for developing leaf rust resistant wheat (Triticum aestivum) cultivars. A total of 102 Chinese winter wheat cultivars and advanced lines were inoculated with 24 pathotypes of Puccinia triticina for postulation of leaf rust resistance genes effective at the seedling stage. These genotypes were also planted in the field for characterization of slow rusting responses to leaf rust in the 2006-07 and 2007-08 cropping seasons. Fourteen leaf rust resistance genes-Lr1, Lr2a, Lr3bg, Lr3ka, Lr14a, Lr16, Lr17a, Lr18, Lr20, Lr23, Lr24, Lr26, Lr34, and LrZH84-either singly or in combinations, were postulated in 65 genotypes, whereas known resistance genes were not identified in the other 37 accessions. Resistance gene Lr26 was present in 44 accessions. Genes Lr14a and Lr34 were each detected in seven entries. Lr1 and Lr3ka were each found in six cultivars, and five lines possessed Lr16. Lr17a and Lr18 were each identified in four lines. Three cultivars were postulated to possess Lr3bg. Genes Lr20, Lr24, and LrZH84 were each present in two cultivars. Each of the genes Lr2a and Lr23 may exist in one line. Fourteen genotypes showed slow leaf rusting resistance in two cropping seasons.

Background: Lr16 is a widely deployed leaf rust resistance gene in wheat (Triticum aestivum L.) that is highly effective against the North American Puccinia triticina population when pyramided with the gene Lr34. Lr16 is a seedling leaf rust resistance gene conditioning an incompatible interaction with a distinct necrotic ring surrounding the uredinium. Lr16 was previously mapped to the telomeric region of the short arm of wheat chromosome 2B. The goals of this study were to develop numerous single nucleotide polymorphism (SNP) markers for the Lr16 region and identify diagnostic gene-specific SNP marker assays for marker-assisted selection (MAS).Results: Forty-three SNP markers were developed and mapped on chromosome 2BS tightly linked with the resistance gene Lr16 across four mapping populations representing a total of 1528 gametes. Kompetitive Allele Specific PCR (KASP) assays were designed for all identified SNPs. Resistance gene analogs (RGAs) linked with the Lr16 locus were identified and RGA-based SNP markers were developed. The diagnostic potential of the SNPs co-segregating with Lr16 was evaluated in a diverse set of 133 cultivars and breeding lines. Six SNP markers were consistent with the Lr16 phenotype and are accurately predictive of Lr16 for all wheat lines/cultivars in the panel.Conclusions: Lr16 was mapped relative to SNP markers in four populations. Six SNP markers exhibited high quality clustering in the KASP assay and are suitable for MAS of Lr16 in wheat breeding programs.

A gene conferring seedling resistance to Puccinia triticina was mapped to chromosome 2BS in the wheat Morocco. The gene was shown to be distinct and was therefore designated Lr73. The wheat genotype Morocco, widely susceptible to isolates of Puccinia triticina, was resistant to an Australian isolate of this pathogen collected in 2004. Genetic studies established that the resistance in Morocco was also present the Australian wheat genotypes Avocet, Halberd, Harrier, Tincurrin and a selection of cultivar Warigal lacking the resistance gene Lr20. Genetic studies based on a cross with Halberd showed that the gene is dominant and located on chromosome 2BS (XwPt8760-4 cM-Lr73-1.4 cM-XwPt8235). The gene was genetically independent of the Lr13, Lr16 and Lr23 loci, also located on chromosome 2BS, indicating that it is distinct. The locus designation Lr73 was therefore assigned. On the basis of multi-pathotype tests, it is likely Lr73 is also present in the Australian wheat cultivars Clearfield STL, Federation (with Lr10), Gatcher (with Lr10 and Lr27+Lr31), Marombi (with Lr1 and Lr37), Pugsley (with Lr1 and Lr37), Spear (with Lr1), Stiletto and Tarsa (with Lr1). Gene Lr73 is unlikely to be of value in resistance breeding. However, recognising Lr73 is important to avoid its inadvertent selection in breeding programmes. Furthermore, the apparent rarity of avirulence for genes like Lr73, sometimes referred to as "fossil" resistance genes, makes them of interest in terms of the evolution of disease resistance in host plants and of virulence in the respective rust pathogens.