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Wheat stem rust caused by Puccinia graminis f. sp. tritici is a widespread and recurring threat to wheat production. Emerging P. graminis f. sp. tritici variants are rapidly overcoming major gene resistance deployed in wheat cultivars and new sources of race-nonspecific resistance are urgently needed. The National Small Grains Collection (NSGC) contains thousands of wheat landrace accessions that may harbor unique and broadly effective sources of resistance to emerging P. graminis f. sp. tritici variants. All NSGC available facultative and winter-habit bread wheat landraces were tested in a field nursery in St. Paul, Minnesota, against a bulk collection of six common U.S. P. graminis f. sp. tritici races. Infection response and severity data were collected on 9,192 landrace accessions at the soft-dough stage and resistant accessions were derived from single spikes. Derived accessions were tested in St. Paul a second time to confirm resistance and in a field nursery in Njoro, Kenya against emerging races of P. graminis f. sp. tritici with virulence to many known resistance genes including Sr24, Sr31, Sr38, and SrTmp. Accessions resistant in the St. Paul field were also tested at the seedling stage with up to 13 P. graminis f. sp. tritici races, including TTKSK and TKTTF, and with 19 molecular markers linked with known stem rust resistance genes or genes associated with modern breeding practices. Forty-five accessions were resistant in both U.S. and Kenya field nurseries and lacked alleles linked with known stem rust resistance genes. Accessions with either moderate or strong resistance in the U.S. and Kenya field nurseries and with novel seedling resistance will be prioritized for further study.

AC Taber is a hard red spring wheat cultivar that has had long-lasting resistance to the leaf rust fungus Puccinia triticina. The objective of this study was to determine the chromosome location of the leaf rust resistance genes in AC Taber. The leaf rust-susceptible cultivar Thatcher was crossed with AC Taber to develop an F6 recombinant inbred line (RIL) population. The RILs and parents were evaluated for segregation of leaf rust resistance in five field plot tests and in two seedling tests to race BBBDB of P. triticina. A genetic map of the RIL population was developed using 90,000 single nucleotide polymorphism markers with the Illumina Infinium iSelect 90K wheat bead array. Quantitative trait loci (QTLs) with significant effects for lower leaf rust severity in the field plot tests were found on chromosomes 2BS and 3BS. The same QTLs also had significant effects for lower infection type in seedlings to leaf rust race BBBDB. The gene on 2BS was the adult plant resistance gene Lr13, and the gene on 3BS mapped to the same region as the adult plant resistance gene Lr74 and other QTLs for leaf rust resistance. Kompetitive allele-specific PCR assay markers linked to the 2BS and 3BS regions were developed and should be useful for marker-based selection of these genes.

In the last 20 years, severe wheat stem rust outbreaks have been recorded in Africa, Europe, and Central Asia. This previously well controlled disease, caused by the fungus Puccinia graminis f. sp. tritici, has reemerged as a major threat to wheat cultivation. The stem rust (Sr) resistance gene Sr22 encodes a nucleotide-binding and leucine-rich repeat receptor which confers resistance to the highly virulent African stem rust isolate Ug99. Here, we show that the Sr22 gene is conserved among grasses in the Triticeae and Poeae lineages. Triticeae species contain syntenic loci with single-copy orthologs of Sr22 on chromosome 7, except Hordeum vulgare, which has experienced major expansions and rearrangements at the locus. We also describe 14 Sr22 sequence variants obtained from both Triticum boeoticum and the domesticated form of this species, T. monococcum, which have been postulated to encode both functional and nonfunctional Sr22 alleles. The nucleotide sequence analysis of these alleles identified historical sequence exchange resulting from recombination or gene conversion, including breakpoints within codons, which expanded the coding potential at these positions by introduction of nonsynonymous substitutions. Three Sr22 alleles were transformed into wheat cultivar Fielder and two postulated resistant alleles from Schomburgk (hexaploid wheat introgressed with T. boeoticum segment carrying Sr22) and T. monococcum accession PI190945, respectively, conferred resistance to P. graminis f. sp. tritici race TTKSK, thereby unequivocally confirming Sr22 effectiveness against Ug99. The third allele from accession PI573523, previously believed to confer susceptibility, was confirmed as nonfunctional against Australian P. graminis f. sp. tritici race 98-1,2,3,5,6. [Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, is a re-emerging disease exemplified by recent epidemics caused by new virulent races. Understanding the sources and origins of genetic variations in the pathogen populations globally can facilitate the development of better strategies in disease management. We analyzed 68 wheat stem rust samples collected between 2013 and 2015 from Georgia where stem rust incidences are frequent and the alternate host, common barberry, is present. A total of 116 single-pustule isolates were derived and evaluated on stem rust differential lines to determine the virulence phenotypes and 23 races were identified, many of which were detected for the first time. Unique virulence combinations including, Sr22+Sr24 and Sr13b+Sr35+Sr37 were detected. These virulence combinations pose new challenges to breeding programs because many of these genes are used in breeding for resistance to the Ug99 race group. Sixty-one isolates were genotyped using a custom single-nucleotide polymorphism chip and 17 genotypes were identified. The 2013 isolates contained 11 multilocus genotypes compared with isolates of 2014 and 2015, with five and three genotypes, respectively. The higher levels of virulence and genotypic diversity observed in the 2013 samples strongly indicated that sexual recombination occurs in the Georgian P. graminis f. sp. tritici population, and that the Caucasus region of Eurasia may be an important source of new races. [Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

In Ethiopia, breeding rust resistant wheat cultivars is a priority for wheat production. A stem rust epidemic during 2013 to 2014 on previously resistant cultivar Digalu highlighted the need to determine the effectiveness of wheat lines to multiple races of Puccinia graminis f. sp. tritici in Ethiopia. During 2014 and 2015, we evaluated a total of 97 bread wheat and 14 durum wheat genotypes against four P. graminis f. sp. tritici races at the seedling stage and in single-race field nurseries. Resistance genes were postulated using molecular marker assays. Bread wheat lines were resistant to race JRCQC, the race most virulent to durum wheat. Lines with stem rust resistance gene Sr24 possessed the most effective resistance to the four races. Only three lines with adult plant resistance possessed resistance effective to the four races comparable with cultivars with Sr24. Although responses of the wheat lines across races were positively correlated, wheat lines were identified that possessed adult plant resistance to race TTKSK but were relatively susceptible to race TKTTF. This study demonstrated the importance of testing wheat lines for response to multiple races of the stem rust pathogen to determine if lines possessed non-race-specific resistance. [Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Dasypyrum villosum is a wild relative of common wheat (Triticum aestivum L.) with resistance to Puccinia graminis f. tritici, the causal agent of stem rust, including the highly virulent race TTKSK (Ug99). In order to transfer resistance, T. durum–D. villosum amphiploids were initially developed and used as a bridge to create wheat–D. villosum introgression lines. Conserved ortholog set (COS) markers were used to identify D. villosum chromosome introgression lines, which were then subjected to seedling P. graminis f. tritici resistance screening with race TTKSK. A COS marker-verified line carrying chromosome 2V with TTKSK resistance was further characterized by combined genomic in situ and fluorescent in situ analyses to confirm a monosomic substitution line MS2V(2D) (20″ + 1′ 2V[2D]). This is the first report of stem rust resistance on 2V, which was temporarily designated as SrTA10276-2V. To facilitate the use of this gene in wheat improvement, a complete set of previously developed wheat–D. villosum disomic addition lines was subjected to genotyping-by-sequencing analysis to develop D. villosum chromosome-specific markers. On average, 350 markers per chromosome were identified. These markers can be used to develop diagnostic markers for D. villosum-derived genes of interest in wheat improvement.

Wheat landrace CItr 15026 previously showed adult plant resistance (APR) to the Ug99 stem rust race group in Kenya and seedling resistance to Puccinia graminis f. sp. tritici races QFCSC, TTTTF, and TRTTF. CItr 15026 was crossed to susceptible accessions LMPG-6 and Red Bobs, and 180 double haploid (DH) lines and 140 recombinant inbred lines (RIL), respectively, were developed. The 90K wheat iSelect single-nucleotide polymorphism platform was used to genotype the parents and populations. Parents and 180 DH lines were evaluated in the field in Kenya for three seasons. A major quantitative trait locus (QTL) for APR was consistently detected on chromosome arm 6AS. This QTL was further detected in the RIL population screened in Kenya for one season. Parents, F1, and the two populations were tested as seedlings against races TRTTF and TTTTF. In addition, the DH population was tested against race QFCSC. Goodness-of-fit tests indicated that the TRTTF resistance in CItr 15026 was controlled by two complementary genes whereas the TTTTF and QFCSC resistance was conditioned by one dominant gene. The TRTTF resistance loci mapped to chromosome arms 6AS and 6DS, whereas the TTTTF and QFCSC resistance locus mapped to the same region on 6DS as the TRTTF resistance. The APR identified in CItr 15026 should be useful in developing cultivars with durable stem rust resistance.

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.

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, can cause severe yield losses on susceptible wheat varieties and cultivars. Although stem rust can be controlled by the use of genetic resistance, population dynamics of P. graminis f. sp. tritici can frequently lead to defeat of wheat stem rust resistance genes. P. graminis f. sp. tritici race TKTTF caused a severe epidemic in Ethiopia on Ug99-resistant ‘Digalu’ in 2013 and 2014. The gene Sr11 confers resistance to race TKTTF and is present in ‘Gabo 56’. We identified seven single-nucleotide polymorphism (SNP) markers linked to Sr11 from a cross between Gabo 56 and ‘Chinese Spring’ exploiting a 90K Infinium iSelect Custom beadchip. Five SNP markers were validated on a ‘Berkut’/‘Scalavatis’ population that segregated for Sr11, using KBioscience competitive allele-specific polymerase chain reaction (KASP) assays. Two of the SNP markers, KASP_6BL_IWB10724 and KASP_6BL_IWB72471, were predictive of Sr11 among wheat genetic stocks, cultivars, and breeding lines from North America, Ethiopia, and Pakistan. These markers can be utilized to select for Sr11 in wheat breeding and to detect the presence of Sr11 in uncharacterized germplasm.

© Her Majesty the Queen in Right of Canada, as represented by the Minister of Agriculture and Agri-Food Canada, 2015. Race TTKSK (or Ug99) of Puccinia graminis f. sp. tritici, the causal agent of wheat stem rust, is a serious threat to wheat production worldwide. Diploid wheat, Triticum monococcum (genome Am), has been utilized previously for the introgression of stem rust resistance genes Sr21, Sr22, and Sr35. Multipathotype seedling tests of biparental populations demonstrated that T. monococcum accession PI 306540 collected in Romania contains a recessive resistance gene effective to all P. graminis f. sp. tritici races screened, including race TTKSK. We will refer to this gene as SrTm4, which is the fourth stem rust resistance gene characterized from T. monococcum. Using two mapping populations derived from crosses of PI 272557 × PI 306540 and G3116 × PI 306540, we mapped SrTm4 on chromosome arm 2AmL within a 2.1 cM interval flanked by sequence-tagged markers BQ461276 and DR732348, which corresponds to a 240-kb region in Brachypodium chromosome 5. The eight microsatellite and nine sequence-tagged markers linked to SrTm4 will facilitate the introgression and accelerate the deployment of SrTm4-mediated Ug99 resistance in wheat breeding programs.

North American durum lines, selected for resistance to TTKSK (Ug99) and related races of Puccinia graminis f. sp. tritici in Kenya, became susceptible in Debre Zeit, Ethiopia, suggesting the presence of stem rust races that were virulent to the TTKSK-effective genes in durum. The objective of this study was to characterize races of P. graminis f. sp. tritici present in the Debre Zeit, Ethiopia stem rust nursery. Three races of P. graminis f. sp. tritici were identified from 34 isolates: JRCQC, TRTTF, and TTKSK. Both races JRCQC and TRTTF possess virulence on stem rust resistance genes Sr13 and Sr9e, which may explain why many TTKSK-resistant durum lines tested in Kenya became susceptible in Debre Zeit. The Sr9e-Sr13 virulence combination is of particular concern because these two genes constitute major components of stem rust resistance in North American durum cultivars. In addition to Sr9e and Sr13 virulence, race TRTTF is virulent to at least three stem rust resistance genes that are effective to race TTKSK, including Sr36, SrTmp, and resistance conferred by the 1AL.1RS rye translocation. Race TRTTF is the first known race with virulence to the stem rust resistance carried by the 1AL.1RS translocation, which represents one of the few effective genes against TTKSK in winter wheat cultivars in the United States. Durum entries exhibiting resistant to moderately susceptible infection response at the Debre Zeit nursery in 2009 were evaluated for reaction to races JRCQC, TRTTF, and TTKSK at the seedling stage. In all, 47 entries were resistant to the three races evaluated at the seedling stage, whereas 26 entries exhibited a susceptible reaction. These results suggest the presence of both major and adult plant resistance genes, which would be useful in durum-wheat-breeding programs. A thorough survey of virulence in the population of P. graminis f. sp. tritici in Ethiopia will allow characterization of the geographic distribution of the races identified in the Debre Zeit field nursery.

Wheat stem rust (Puccinia graminis f. sp. tritici) race TTKSK (Ug99), with virulence to the majority of the world's wheat (Triticum aestivum) cultivars, has spread from Uganda throughout eastern Africa, Yemen, and Iran. The identification and spread of variants of race TTKSK with virulence to additional stem rust resistance genes has reminded breeders and pathologists of the danger of deploying major resistance genes alone. In order to protect wheat from this rapidly spreading and adapting pathogen, multiple resistance genes are needed, preferably from improved germplasm. Preliminary screening of over 700 spring wheat breeding lines and cultivars developed at least 20 years ago identified 88 accessions with field resistance to Ug99. We included these resistant accessions in the stem rust screening nursery in Njoro, Kenya for two additional seasons. The accessions were also screened with a bulk of North American isolates of P. graminis f. sp. tritici in the field in St. Paul, MN. In order to further characterize the resistance in these accessions, we obtained seedling phenotypes for 10 races of P. graminis f. sp. tritici, including two races from the race TTKSK complex. This phenotyping led to the identification of accessions with either adult-plant or all-stage resistance to race TTKSK, and often North American races of P. graminis f. sp. tritici as well. These Ug99 resistant accessions can be obtained by breeders and introgressed into current breeding germplasm.

Cheatham, M. R., Rouse, M. N., Esker, P. D., Ignacio, S., Pradel, W., Raymundo, R., Sparks, A. H., Forbes, G. A., Gordon, T. R., and Garrett, K. A. 2009. Beyond yield: Plant disease in the context of ecosystem services. Phytopathology 99:1228-1236. The ecosystem services concept provides a means to define successful disease management more broadly, beyond short-term crop yield evaluations. Plant disease can affect ecosystem services directly, such as through removal of plants providing services, or indirectly through the effects of disease management activities, including pesticide applications, tillage, and other methods of plant removal. Increased plant biodiversity may reduce disease risk if susceptible host tissue becomes less common, or may increase risk if additional plant species are important in completing pathogen life cycles. Arthropod and microbial biodiversity may play similar roles. Distant ecosystems may provide a disservice as the setting for the evolution of pathogens that later invade a focal ecosystem, where plants have not evolved defenses. Conversely, distant ecosystems may provide a service as sources of genetic resources of great value to agriculture, including disease resistance genes. Good policies are needed to support conservation and optimal use of genetic resources, protect ecosystems from exotic pathogens, and limit the homogeneity of agricultural systems. Research is needed to provide policy makers, farmers, and consumers with the information required for evaluating trade-offs in the pursuit of the full range of ecosystem services desired from managed and native ecosystems.