Stem Rust Resistance Research Articles

Performance Evaluation of Elite Durum Wheat Genotypes for Yield and Stem Rust Response

Durum wheat is one of the most important crops worldwide and Ethiopia is the major producer in sub-Saharan Africa (SSA). However, there is a huge gap between domestic production and demand in Ethiopia. Hence, getting high yielder with resistant for multiple races of rusts especially for stem rust became challenging. This could be due to limited availability of resistance to multiple stem rust races coupled with the rapid occurrence and spread of virulent races of stem rust. Hence, objective of this study is to identify high yielding and stem rust resistant genotypes and use them as parents in the breeding program and advance to national variety trial to select for wide adaptability. Sixty advanced genotypes were evaluated across three locations during 2020/21 cropping season. The trial was set up using row-column design in two replications. Data on grain yield, yield related traits and stem rust were collected. Stem rust was scored following the modified Cobb’s scale. Under high disease pressure the genotypes showed disease response and severity variations from RMR to S and from 5% to 100%, respectively indicating the presenece of variation for stem rust response. There were also genotypes which showed suceptable reaction and high grain yield and yield related traits. Hence, this may be due to minor gene resistance to stem rust. Generally, about 13 genotypes had good grian yield, thousand kernel weight and stem rust resistance. We can recommend these genotypes to test across country for their wide adaptability and as a parents for crossing.

Stacking effective ASR and APR rust genes for multiple disease resistance in bread wheat cultivars

Rusts and powdery mildews pose serious threats to wheat and have caused substantial yield losses worldwide. Host resistance is the most economical and sustainable approach for managing such diseases. In this study, an effective leaf rust resistance gene Lr45-derived from Secale cereale L. and a linked stem rust and powdery mildew resistance gene Sr36/Pm6-derived from Triticum timopheevii were successfully pyramided. They were validated into well-adapted Indian wheat cultivars that were already carrying the APR stem rust gene Sr2/Lr27/Yr30 through marker-assisted backcross selection (MABC) following two parallel backcrossing schemes. Three efficiently linked microsatellite markers, G372185 (Lr45), Stm773-2(Sr36), and Xgwm533 (Sr2), were used to confirm introgression of these genes. Lines with resistance genes in each background showed improved agronomic traits in comparison to their recurrent parents. These lines could be used in wheat improvement programs as potentially resistant stocks for leaf, stem rusts and powdery mildew to develop new wheat cultivars.

The wheat stem rust resistance gene Sr43 encodes an unusual protein kinase

The wheat stem rust resistance gene Sr43 encodes an unusual protein kinase

The Screening of Pakistani Wheat Landraces to Stem Rust (Puccinia gramminis f. sp. tritici) resistance under field conditions.

The stem rust diseases of wheat have historically been the major biotic stress responsible for destabilizing production in Asia and other parts of the world .Stem rust has continuously been in surge of destroying wheat genome especially after its race Ug99 that surfaced in Uganda in year 1999 that suppressed vital stem rust resistance gene Sr31.It was the pivotal gene that had immunity to wheat against lethal stem rust fungus.In order to trace genetic resources inside the wheat genome; a set of 112 landraces was evaluated for stem rust resistance under field condition at Jhuddo, a hot spot of stem rust in southern Pakistan. The results indicated that 80% landraces were susceptible to intensity of stem rust at Jhuddo,4% moderately suscesptible,11% moderately resistant while 17% shown moderetly resistance moderetly susceptible types of reactions according to modified Cobbs Scale. .

Silicon improves the photosynthetic performance of oat leaves infected with Puccinia graminis f. sp. avenae.

Stem rust, caused by Puccinia graminis f. sp. avenae (Pga) is a key disease affecting oat production worldwide. Silicon (Si) plays an essential role in enhancing plant resistance against pathogens. However, the scientific evidence of Si-mediated stem rust resistance of oat from the photosynthetic perspective has not been reported. The specific objective of this research was to investigate the effects of Si application on disease inhibition, photosynthetic gas exchange parameters, light response parameters, photosynthetic pigments and chlorophyll fluorescence parameters under Pga infection. Our results illustrated that Si application significantly reduced rust severity while the other parameters like net photosynthetic rate (P n), stomatal conductance (Gs), intercellular CO2 concentration (C i) and transpiration rate (T r) were significantly increased. Si application increased maximum photosynthetic rate (P nmax) and light saturation point (LSP), while reduced the dark respiration rate (Rd) and light compensation point (LCP). The results also indicated that Si application significantly increased the activities of maximum fluorescence (F m), variable fluorescence (F v), maximum quantum yield of photosystem II (F v/F m), photochemical quenching (qP), photosynthetic performance index (PI ABS), actual PSII quantum yield (ΦPSII), electron transfer rate (ETR), the absorbed light energy per unit reaction center (ABS/RC) and the dissipated energy per unit reaction center (DIo/RC), whereas it decreased the minimal fluorescence (F o), non-photochemical quenching (NPQ), the absorbed light energy used for electron transfer per unit reaction center (ETo/RC) and the absorbed light energy used for reduction of QA per unit reaction center (TRo/RC). The contents of chlorophyll a, b and carotenoids were also increased due to the change in the activity of parameters due to Si application as mentioned above. In conclusion, the results of the current study suggests that Si imparts tolerance to the stem rust possibly by the underlying mechanisms of improving gas exchange performance, and efficiency of the photochemical compounds in oat leaves.

Studying Stem Rust and Leaf Rust Resistances of Self-Fertile Rye Breeding Populations.

Stem rust (SR) and leaf rust (LR) are currently the two most important rust diseases of cultivated rye in Central Europe and resistant cultivars promise to prevent yield losses caused by those pathogens. To secure long-lasting resistance, ideally pyramided monogenic resistances and race-nonspecific resistances are applied. To find respective genes, we screened six breeding populations and one testcross population for resistance to artificially inoculated SR and naturally occurring LR in multi-environmental field trials. Five populations were genotyped with a 10K SNP marker chip and one with DArTseqTM. In total, ten SR-QTLs were found that caused a reduction of 5-17 percentage points in stem coverage with urediniospores. Four QTLs thereof were mapped to positions of already known SR QTLs. An additional gene at the distal end of chromosome 2R, Pgs3.1, that caused a reduction of 40 percentage points SR infection, was validated. One SR-QTL on chromosome 3R, QTL-SR4, was found in three populations linked with the same marker. Further QTLs at similar positions, but from different populations, were also found on chromosomes 1R, 4R, and 6R. For SR, additionally seedling tests were used to separate between adult-plant and all-stage resistances and a statistical method accounting for the ordinal-scaled seedling test data was used to map seedling resistances. However, only Pgs3.1 could be detected based on seedling test data, even though genetic variance was observed in another population, too. For LR, in three of the populations, two new large-effect loci (Pr7 and Pr8) on chromosomes 1R and 2R were mapped that caused 34 and 21 percentage points reduction in leaf area covered with urediniospores and one new QTL on chromosome 1R causing 9 percentage points reduction.

QTL mapping of seedling and field resistance to stem rust in DAKIYE/Reichenbachii durum wheat population.

Stem rust caused by the fungus Puccinia graminis f.sp. tritici Eriks. & E. Henn. (Pgt) threatens the global production of both durum wheat (Triticum turgidum L. ssp. durum (Desf.) Husnot) and common wheat (Triticum aestivum L.). The objective of this study was to evaluate a durum wheat recombinant inbred line (RIL) population from a cross between a susceptible parent 'DAKIYE' and a resistant parent 'Reichenbachii' developed by the International Center for the Improvement of Maize and Wheat (CIMMYT) 1) for seedling response to races JRCQC and TTRTF and 2) for field response to a bulk of the current Pgt races prevalent in Ethiopia and Kenya and 3) to map loci associated with seedling and field resistances in this population. A total of 224 RILs along with their parents were evaluated at the seedling stage in the Ethiopian Institute for Agricultural Research greenhouse at Debre Zeit, Ethiopia and in the EIAR and KALRO fields in Ethiopia and Kenya, for two seasons from 2019 to 2020. The lines were genotyped using the genotyping-by-sequencing approach. A total of 843 single nucleotide polymorphism markers for 175 lines were used for quantitative trait locus (QTL) analyses. Composite interval mapping (CIM) identified three QTL on chromosomes 3B, 4B and 7B contributed by the resistant parent. The QTL on chromosome 3B was identified at all growth stages and it explained 11.8%, 6.5%, 6.4% and 15.3% of the phenotypic variation for responses to races JRCQC, TTRTF and in the field trials ETMS19 and KNMS19, respectively. The power to identify additional QTL in this population was limited by the number of high-quality markers, since several markers with segregation distortion were eliminated. A cytological study is needed to understand the presence of chromosomal rearrangements. Future evaluations of additional durum lines and RIL families identification of durable adult plant resistance sources is crucial for breeding stem rust resistance in durum wheat in the future.

QTL Mapping of Stem Rust Resistance in Populations of Durum Wheat.

Stem rinfectionust, caused by the fungus Puccinia graminis f. sp. tritici (Pgt), is one of the most devastating fungal diseases of durum and common wheat worldwide. The identification of sources of resistance and the validation of QTLs identified through genome-wide association studies is of paramount importance for reducing the losses caused by this disease to wheat grain yield and quality. Four segregating populations whose parents showed contrasting reactions to some Pgt races were assessed in the present study, and 14 QTLs were identified on chromosomes 3A, 4A, 6A, and 6B, with some regions in common between different segregating populations. Several QTLs were mapped to chromosomal regions coincident with previously mapped stem rust resistance loci; however, their reaction to different Pgt races suggest that novel genes or alleles could be present on chromosomes 3A and 6B. Putative candidate genes with a disease-related functional annotation have been identified in the QTL regions based on information available from the reference genome of durum cv. ‘Svevo’.

English

Production of wheat in Ethiopia is under danger from wheat stem rust, which is brought on by the disease Puccinia graminis f.sp. tritici and has resulted in a yield loss of 100%. An investigation was conducted in a hotspot area (Debere Zeit, Ethiopia) to evaluate the status of Ethiopian durum wheat cultivars for their level of stem rust resistance in the field. During the main seasons of 2017, 2020, and 2021, 34 durum wheat varieties with three susceptibility checks were examined. Terminal Rust Severity (TRS), Coefficient of Infection (CI) and Area under Disease Progress Curve (AUDPC) were used to measure stem rust field resistance. Analysis of all disease parameters was carried out using R studio. Due to their low levels of AUDPC, ACI, and TRS values combined with MR infection type, two commercial varieties (Oda and Tob-66) may have a significant resistance gene(s) and seven varieties (Boohie, Kilinto, Ginchi, Robe, Toltu, Lelisso, and Bekelcha) showed a moderate level of field resistance with MR to MRMS responses and may have adult plant resistance mediated by minor resistance genes. Stem rust was found to affect 73.5% of the Ethiopian durum cultivars. Therefore, this study suggests that the national breeding effort should focus on local durum cultivars rather than the ones that were imported. Key words: Field resistance, durum wheat, stem rust, Puccinia graminis f.sp tritici.

Geographical Correlation and Genetic Diversity of Newly Emerged Races within the Ug99 Lineage of Stem Rust Pathogen, Puccinia graminis f. sp. tritici, in Different Wheat-Producing Areas.

Wheat stem rust caused by Puccinia graminis f. sp. tritici is one of the most destructive wheat diseases worldwide. Identifying stem rust races in general, Ug99 lineage particularly, and determining resistance genes are critical goals for disease assessment. Thirty wheat varieties and monogenic lines with major stem rust resistance genes (Sr) were examined here over the course of three succeeding seasons from 2020 to 2022. Fourteen stem rust races have been identified in ten African countries, as well as Central and West Asia and North Africa (CWANA) and ten European countries. The Ug99 group (Clade I) included four races (TTKSK, TTKST, TTKTK, and TTKTT) and was reported in five African countries (Egypt, Kenya, Rwanda, Tanzania, and Uganda) and Iran, but none of the European countries. On the other hand, none of the races in Clade III-B (TTRTF) and Clade IV-B (TKTTF and TTTTF) were found in Egypt. Furthermore, Egyptian races were clustered separately from races identified from other countries, and six races were found only in Egypt, including PKSTC, RKTTH, TKTTC, TTTSK, TCKTC, and TKTTH. Races from Kenya, Tanzania, Uganda, Rwanda, and Iran were all closely associated with one another, according to correlation analysis. However, most races identified from other investigated regions, including Eritrea, Spain, Ethiopia, Morocco, Italy, Poland, Kenya, Tanzania, and Uganda, were adversely linked with Egyptian races. The diagnostic 350 bp long PCR fragment linked with virulence to Sr31, Clement (Sr31), and Brigardier (Sr31) was used to identify the TTKSK (Ug99) race. The identification of the regional associations and genetic diversity of newly emerged races within the Ug99 lineage of P. graminis tritici in Africa, Asia, and Europe is one of the key goals of this study. It will help plant breeders to develop new resistant lines against the virulent races, especially TTKSK (Ug99) and TTTSK. This helps in ensuring global food security in the context of climate change.

Diversity of Ethiopian Durum Wheat Landraces for Resistance to Stem Rust Seedling Resistance Genes

Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is one of the most important diseases of wheat worldwide. Breeding for resistance to diseases is the most important approach for mitigation of yield losses. This study was conducted to estimate the diversity of all stage stem rust resistance (ASR) genes on the 142 durum wheat landrace accessions at seedling stage. The study was conducted in greenhouse at Ambo Plant Protection Research Center on the 142 durum wheat landrace accessions using 20 differential lines, one susceptible line (McNair), and eight Pgt races. The result depicted the presence of Sr7b, Sr8a, Sr9b, Sr10, Sr11, Sr13, Sr17, Sr30, Sr31, Sr36, and SrTmp in the Ethiopian durum wheat accessions. Among the 142 durum wheat accessions, 83 accessions were identified for possessing single ASR genes, and four accessions including the universal susceptible line (McNair) did not have effective resistance genes to the pathogen races tested in this study. The remaining 55 accessions had either a combination of two resistance genes, unknown number and kind of genes, or unidentified genes displaying resistance across all the pathogen races. This study demonstrated the prevalence of significant genetic diversity for stem rust ASR genes in the Ethiopian durum wheat landraces.

Registration of six synthetic hexaploid wheat germplasm as sources of resistance to tan spot, stem rust, and stripe rust and adult plant resistance to leaf rust

AbstractAegilops tauschii is the donor of D‐genome of bread wheat (Triticum aestivum L.) as well as several agronomically desirable genes important for wheat improvement. The genetic transfers from Ae. tauschii to bread wheat are achieved either through direct crosses or indirectly through production of synthetic hexaploid wheat (SHW). Synthetic hexaploid wheat is produced by crossing tetraploid wheat (Triticum turgidum L.) with Ae. tauschii, followed by chromosome doubling of the F1 hybrid by colchicine. Six Ae. tauschii accessions based on their seedling resistance to tan spot, stem rust, stripe rust and adult plant resistance to leaf rust and two tetraploid wheat genotypes (tetraPrelude and tetraThatcher) were used for the production of SHWs. The present report describes the production and evaluation of six new SHWs, TA4161‐L1 (Reg. no. GP‐1071, PI 700779), TA4161‐L2 (Reg. no. GP‐1072, PI 700780), TA4161‐L3 (Reg. no. GP‐1073, PI 700781), TA4161‐L4 (Reg. no. GP‐1074, PI 700782), TA4161‐L5 (Reg. no. GP‐1075, PI 700783), and TA4161‐L6 (Reg. no. GP‐1076, PI 700784), for seedling resistance to tan spot, stem rest, stripe rust, and adult plant resistance to leaf rust. The SHWs reported here with higher levels of resistance to race 1 of Pyrenospora tritici‐repentis would be an excellent addition to the current collection of tan spot–resistant germplasm. The resistance to the three rusts was not expressed in the SHWs due to the presence of rust‐suppressor factors in tetraploid parents. However, the SHWs will be good source material for new resistance genes to the three rusts in derivative lines upon breeding with nonsuppressive wheat genotypes as well as for the mapping of rust‐suppressive factors.

Characterisation of stem rust resistance in the South African winter wheat cultivar PAN 3161

Characterisation of stem rust resistance in the South African winter wheat cultivar PAN 3161

Characterization of wheat monogenic lines with known Sr genes and wheat cultivars for resistance to three new races of Puccinia graminis f. sp. tritici in China

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a potentially devastating fungal disease of wheat worldwide. The present study was to evaluate the resistance of 42 wheat monogenic lines with known stem rust resistance (Sr) genes and 69 wheat cultivars to three new Pgt races (34C0MRGQM, 34C3MKGQM, and 34C6MTGSM) identified from aeciospores at the seedling and adult-plant stages. The phenotyping results revealed that monogenic lines harboring resistance genes Sr9e, Sr17, Sr21, Sr22, Sr26, Sr30, Sr31, Sr33, Sr35, Sr36, Sr37, Sr38, Sr47, SrTmp, and SrTt3 were effectively resistant to all three Pgt races at the seedling and adult-plant stages. In contrast, monogenic lines containing Sr5, Sr6, Sr7b, Sr9a, Sr9d, Sr9f, Sr9g, Sr9b, Sr16, Sr24, Sr28, and Sr39 were highly susceptible to these races at both seedling and adult-plant stages. The other lines with Sr8a, Sr10, Sr11, Sr13, Sr14, Sr15, Sr18, Sr20, Sr19, Sr23, Sr25, Sr27, Sr29, Sr32, and Sr34, displayed variable levels of resistance to one or two of the tested races. Seedling infection types (ITs) and adult-plant infection responses (IRs) indicated that 41 (59.4%) of the wheat cultivars showed high resistance to all the three races. Molecular marker analysis showed that four wheat culitvars likely carried Sr2, 20 wheat culitvars likely carried Sr31, 9 wheat culitvars likely carried Sr38, and none of the cultivars carried Sr24, Sr25, and Sr26. Our results provide a scientific basis for rational utilization of the tested Sr genes and wheat cultivars against these novel Pgt races.

Molecular and Cytogenetic Identification of Stem Rust Resistant Wheat-Thinopyrum intermedium Introgression Lines.

Thinopyrum intermedium (JJJsJsStSt, 2n = 6x = 42), a wild relative of common wheat, possesses many desirable agronomic genes for wheat improvement. The production of wheat-Thinopyrum intermedium introgression lines is a key step for transferring these beneficial genes into wheat. In this study, we characterized three wheat-Thinopyrum intermedium introgression lines TA3681, TA5566, and TA5567 using non-denaturing fluorescence in situ hybridization, genomic in situ hybridization, PCR-based landmark unique gene, and intron targeting markers. Our results showed that TA3681 is a wheat-Thinopyrum intermedium 1Stdisomic addition line, TA5566 is a wheat-Thinopyrum intermedium non-Robertsonian translocation line carrying two pairs of 3A-7Js translocation chromosomes, and that TA5567 is a wheat-Thinopyrum intermedium non-Robertsonian translocation line carrying a pair of 3A-7Js translocation chromosomes. We developed 13, 36, and 15 Thinopyrum intermedium chromosome-specific markers for detecting the introgressed Thinopyrum chromosomes in TA3681, TA5566, and TA5567, respectively. Stem rust assessment revealed that TA3681 exhibited ahigh level of seedling resistance to Chinese-prevalent Puccinia graminis f. sp. tritici pathotypes, and both TA5566 and TA5567 were highly resistant to Australian P. graminis f. sp. tritici pathotypes, indicating that Thinopyrum intermedium chromosomes 1St and 7Js might carry new stem rust resistance genes. Therefore, the new identified introgression lines may be useful for improving wheat stem rust resistance.

Estimation results of the spring bread wheat varieties on disease resistance in the Kazan Research Center

In the Republic of Tatarstan, due to favorable soil-climatic and market conditions, spring wheat production is developed, but wheat grain yields are limited by a number of unfavorable factors, one of which is a spread of fungal diseases. At the same time, the pressure of pathogenic fungi makes it possible to carry out fruitful breeding work in Tatarstan on resistance to diseases, which is on of the main activity of the Kazan Research Center of the RAS. The purpose of the current study was to estimate ten new varieties for resistance to major fungal diseases. The study on resistance to leafy diseases, such as powdery mildew, stem rust, brown leaf rust, dark brown leaf blotch was carried out at a natural infectious background, to kernel smut on an artificial infectious background inoculating seeds with spores of kernel smut. The study was carried out between the years of 2017 and 2021. The analysis for the possible presence of stem rust resistance genes was performed by a PCR analysis. As a result of the study, there was found out that the varieties ‘Sto let TASSR’, ‘Balkysh’, ‘Khazine’, ‘Chistopolskaya’, ‘Bulyak’ had a complex resistance to all studied diseases. The variety ‘Sitara’ had a strong field resistance to powdery mildew and kernel smut. The variety ‘Barakat’ had a strong field resistance to powdery mildew and leaf rust. The variety ‘Nadira’ was moderately resistant to powdery mildew and dark brown leaf spot. The varieties ‘Ekada 265’ and ‘Ekada 282’ were highly resistant to local stem rust populations. The molecular genetic estimation of the stem rust resistant varieties of spring bread wheat ‘Chistopolskaya’, ‘Sto let TASSR’, ‘Balkysh’, ‘Ekada 282’ for the identification of effective Sr-genes has shown that their resistance is apparently regulated by the Sr31 gene.

Haplotype variants of Sr46 in Aegilops tauschii, the diploid D genome progenitor of wheat.

Stem rust resistance genes, SrRL5271 and Sr672.1 as well as SrCPI110651, from Aegilops tauschii, the diploid D genome progenitor of wheat, are sequence variants of Sr46 differing by 1-2 nucleotides leading to non-synonymous amino acid substitutions. The Aegilops tauschii (wheat D-genome progenitor) accessions RL 5271 and CPI110672 were identified as resistant to multiple races (including the Ug99) of the wheat stem rust pathogen Puccinia graminis f. sp. tritici (Pgt). This study was conducted to identify the stem rust resistance (Sr) gene(s) in both accessions. Genetic analysis of the resistance in RL 5271 identified a single dominant allele (SrRL5271) controlling resistance, whereas resistance segregated at two loci (SR672.1 and SR672.2) for a cross of CPI110672. Bulked segregant analysis placed SrRL5271 and Sr672.1 in a region on chromosome arm 2DS that encodes Sr46. Molecular marker screening, mapping and genomic sequence analysis demonstrated SrRL5271 and Sr672.1 are alleles of Sr46. The amino acid sequence of SrRL5271 and Sr672.1 is identical but differs from Sr46 (hereafter referred to as Sr46_h1 by following the gene nomenclature in wheat) by a single amino acid (N763K) and is thus designated Sr46_h2. Screening of a panel of Ae. tauschii accessions identified an additional allelic variant that differed from Sr46_h2 by a different amino acid (A648V) and was designated Sr46_h3. By contrast, the protein encoded by the susceptible allele of Ae. tauschii accession AL8/78 differed from these resistance proteins by 54 amino acid substitutions (94% nucleotide sequence gene identity). Cloning and complementation tests of the three resistance haplotypes confirmed their resistance to Pgt race 98-1,2,3,5,6 and partial resistance to Pgt race TTRTF in bread wheat. The three Sr46 haplotypes, with no virulent races detected yet, represent a valuable source for improving stem resistance in wheat.

Establishment of a set of wheat-rye addition lines with resistance to stem rust.

Complete new wheat-rye disomic, telosomic addition lines and various chromosomal aberrations were developed and characterized by molecular cytogenetic method as novel chromosome engineering materials. A new stem rust resistance (Ug99) gene was located on 3RL. Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a devastating fungal disease worldwide. A recently emerged great threat to global wheat production is Pgt strain Ug99 and its derivatives, which have overcome most of the commonly used resistance genes. Rye (Secale cereale L.), closely related to wheat (Triticum aestivum L.), is a significant and valuable resource of resistance genes for wheat germplasm improvement. It is of great importance and urgency to identify new resistance gene sources of rye and transfer them into wheat. In this study, two complete sets of wheat-rye addition lines were established through wide hybridization, chromosome doubling and backcrossing. A wheat-rye 3RL telosomic addition line was identified with high resistance to stem rust strain Ug99. PCR-based markers specific for the rye chromosome were developed. Furthermore, abundant chromosomal aberrations such as minichromosomes, ring chromosomes as well as centromere reduction and expansion were identified in the progeny of wheat-rye addition lines by multicolor GISH and FISH. The line carrying a novel resistance gene to stem rust can be utilized as a bridge material for wheat disease resistance breeding. The chromosomal and centromeric variation within the wheat-rye hybrids can further contribute to genetic diversity of their offspring.

Wheat germplasm development by gene pyramiding for resistance to race TTKSK of stem rust

To develop wheat germplasm with resistance to race TTKSK of stem rust, five stem rust resistance genes were used to produce pyramids with up to 4 Sr genes of various combinations by marker-assisted selection (MAS) and doubled haploid (DH) technology. Thirteen desired pyramids with 2 to 4 of the Sr genes were recovered, and wheat cultivar Hoffman was improved by stacking Sr35 and Sr36. TTKSK resistance test showed that pyramided lines with three or four genes plus improved Hoffman with Sr35+Sr36 were all immune to TTKSK. This type of germplasm will be a useful tool for the production of cultivars with long-term sustainability.

Genetic Diversity and Population Structure Reveal Cryptic Genetic Variation and Long Distance Migration of Puccinia graminis f. sp. tritici in the Indian Subcontinent.

Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is a devastating disease of wheat worldwide since time immemorial. Several wheat stem rust outbreaks have been reported worldwide including India. Approximately 7 mha wheat area in central and peninsular India is highly vulnerable to stem rust epidemics. In this study, a repository of 29 single genotype uredospore pathotypes, representing five geographical regions, was characterized by investigating their virulence phenotype and simple sequence repeat (SSR) genotypes using 37 reproducible polymorphic SSR markers, 32 of which had ≥ 0.50 polymorphic information content (PIC) value. Virulence phenotypes were used to evaluate the virulence frequency (VF) and construct a hypothetical evolutionary hierarchy of these pathotypes. We projected seven lineages to explain the evolutionary pattern of the Pgt population. The VF of these pathotypes ranged between 0% and 100%. The virulence-based neighbor-joining (NJ) cluster analysis grouped Pgt pathotypes into five virulence groups. Likewise, five molecular groups were categorized using molecular genotypes. The molecular grouping was supported by principal coordinate analysis (PCoA), which revealed 25% of the cumulative variance contributed by the first two axes. Analysis of molecular variance (AMOVA) revealed 8 and 92% of the variation among and within the populations, respectively. The Mantel test confirmed a positive but weak correlation (R2 = 0.15) between virulence phenotypes and SSR genotypes. The highest and lowest values of different genetic diversity parameters (Na, Ne, I, He, uHe, and %P) revealed maximum and minimum variability in the Pgt population from Maharashtra and Uttar Pradesh, respectively. The population structure analysis clustered 29 Pgt pathotypes into two subpopulations and an admixture. Our results demonstrated that there was significant genetic diversity among Pgt pathotypes resulting from their long-distance dispersal ability complemented by gene flow. These findings provide insights into the virulence patterns, genetic variations, and possible evolution of Pgt pathotypes, which would support strategic stem rust resistance breeding.