Characterization of Leaf Rust Resistance in Hard Red Spring Wheat Cultivars.

A genetic analysis of the landrace‐derived wheat accessions Americano 25e, Americano 26n, and Americano 44d, from Uruguay was conducted to identify the leaf rust resistance genes present in these early wheat cultivars. The three cultivars were crossed with the leaf rust susceptible cultivar ‘Thatcher’ and approximately 80 backcross (BC1) F2 families were derived for each cross. The BC1F2 families and selected BC1F4 lines were tested for seedling and adult plant leaf rust resistance with selected isolates of leaf rust, Puccinia triticina. The segregation and infection type data indicated that Americano 25e had seedling resistance genes Lr3, Lr16, an additional unidentified seedling gene, and one adult plant resistance gene that was neither Lr12 nor Lr13, and did not phenotypically resemble Lr34. Americano 26n was postulated to have genes Lr11, Lr12, Lr13, and Lr14a. Americano 44d appeared to have two possibly unique adult plant leaf rust resistance genes.

Leaf rust, caused by the fungal pathogen Puccinia triticina, is a major threat to wheat production in many wheat-growing regions of the world. The introduction of leaf rust resistance genes into elite wheat germplasm is the preferred method of disease control, being environmentally friendly and crucial to sustained wheat production. Consequently, there is considerable value in identifying and characterizing new sources of leaf rust resistance. While many major, qualitative leaf rust resistance genes have been identified in wheat, a growing number of valuable sources of quantitative resistance have been reported. Here we review the progress made in the genetic identification of quantitative trait loci (QTL) for leaf rust resistance detected primarily in field analyses, i.e., adult plant resistance. Over the past 50 years, leaf rust resistance loci have been assigned to genomic locations through chromosome analyses and genetic mapping in biparental mapping populations, studies that represent 79 different wheat leaf rust resistance donor lines. In addition, seven association mapping studies have identified adult plant and seedling leaf rust resistance marker trait associations in over 4,000 wheat genotypes. Adult plant leaf rust resistance QTL have been found on all 21 chromosomes of hexaploid wheat, with the B genome carrying the greatest number of QTL. The group 2 chromosomes are also particularly rich in leaf rust resistance QTL. The A genome has the lowest number of QTL for leaf rust resistance. Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Leaf rust (Puccinia triticina) and stripe rust (P. striiformis f. tritici) affect wheat production worldwide. Brazilian 'Toropi' wheat has demonstrated durable leaf rust resistance in South America since its release in 1965. It was previously found to have up to two adult plant leaf rust resistance genes. The leaf and stripe rust resistance of Toropi were studied by analyzing a doubled-haploid population made by crossing with susceptible 'Thatcher'. Toropi expressed good resistance to leaf rust in Canada, Brazil, and New Zealand. Based on field and greenhouse testing, the leaf rust resistance of Toropi is conferred by two race-nonspecific complementary adult plant genes and a race-specific adult plant gene. The stripe rust resistance of Toropi analyzed in New Zealand and in Canada is based on up to two resistance genes. Toropi should provide an important contribution to rust resistance because it expressed good leaf rust and stripe rust resistance in different parts of the world.

Leaf rust caused by Puccinia triticina is an important disease of wheat in many regions worldwide. Durable or long-lasting leaf rust resistance has been difficult to achieve because populations of P. triticina are highly variable for virulence to race-specific resistance genes, and respond to selection by resistance genes in released wheat cultivars. The wheat cultivar Toropi, developed and grown in Brazil, was noted to have long-lasting leaf rust resistance that was effective only in adult plants. The objectives of this study were to determine the chromosome location of the leaf rust resistance genes derived from Toropi in two populations of recombinant inbred lines in a partial Thatcher wheat background. In the first population, a single gene with major effects on chromosome 5DS that mapped 2.2 centimorgans distal to IWA6289, strongly reduced leaf rust severity in all 3 years of field plot tests. This gene for adult plant leaf rust resistance was designated as Lr78. In the second population, quantitative trait loci (QTL) with small effects on chromosomes 1BL, 3BS, and 4BS were found. These QTL expressed inconsistently over 4 years of field plot tests. The adult plant leaf rust resistance derived from Toropi involved a complex combination of QTL with large and small effects.

Leaf rust, caused by Puccinia triticina, is a common and widespread disease of wheat in the US. On an annual basis, over 50 races of the leaf rust fungus are detected. There are at least 5 major groups of genetically distinct P. triticina isolates in the US based on allelic variation at microsatellite loci. Distinct regional race populations of P. triticina are found in the US, due to the widespread use of race-specific leaf rust resistance (Lr) genes in different market classes of wheat. In the south-eastern States where soft red winter wheats are grown, races with virulence to Lr9, Lr11, and Lr18 are predominant. In the southern Great Plains region where hard red winter wheats are grown, races with virulence to genes Lr9, Lr17, Lr24, and Lr26 are common. In the northern Great Plains region where hard red spring wheats are grown, races with virulence to Lr2a and Lr16 are common. Due to the wide dispersal of P. triticina, some races are found in all regions of the US. Highly effective durable resistance to leaf rust has been difficult to achieve due to the high degree of virulence variation in the P. triticina population and the rapid selection of races with virulence to effective Lr genes in wheat cultivars. Hard red spring wheat cultivars with genes Lr16, Lr23, and Lr34 have been highly resistant for more than 10 years in Minnesota and the Dakotas. Stem rust, caused by P. graminis f. sp. tritici, has not been a common disease of wheat in the US since the last major epidemics in the 1950s. The low levels of stem rust infections in the US can be attributed to the increasing use of highly resistant winter and spring wheat cultivars, which has greatly reduced the overall level of stem rust urediniospores. Eradication of the alternate host, Berberis vulgaris, has reduced the number of races and slowed the emergence of new races. Resistance genes Sr2, Sr6, Sr17, Sr24, Sr31, Sr36, and SrTmp are common in the winter wheats. Genes Sr6, Sr9b, Sr11, and Sr17 are common in the spring wheats. Spring wheat cultivars may also have adult plant stem rust resistance derived from cv. Thatcher. Many of the winter and spring wheats are susceptible to the new stem rust race from East Africa; however, cultivars with resistance to this race can be found in each of the major wheat classes.

Triticum timopheevii (Zhuk.) Zhuk. is noted for its resistance to diseases including leaf and stem rust of wheat. Only one gene (Lr18) for leaf rust resistance has been transferred from T. timopheevii to bread wheat. The objectives of this work were to study the inheritance of leaf rust resistance in five accessions of T. timopheevii and to transfer genes for resistance into durum and bread wheats. A diallel set of crosses was made among five T. timopheevii accessions that gave a fleck infection type with an isolate of leaf rust race CBB. None of the F2 populations of the 10 crosses segregated for resistance, indicating that the five accessions all had at least one gene for resistance in common. Several accessions were crossed and backcrossed twice to durum and to bread wheat. At least three genes for leaf rust resistance were transferred to durum wheat and one to bread wheat. The gene transferred to bread wheat and one of those transferred to durum wheat conditioned good resistance to a set of 10 diverse races of leaf rust. Resistance conditioned by all three genes was dominant in durum wheat but the one gene was recessive in bread wheat. Monosomic analysis of the bread wheat line showed that the gene is on chromosome 1A. It should be useful in breeding for leaf rust resistance in both durum and bread wheat. Key words: Triticum timopheevii, leaf rust resistance, durum wheat, bread wheat

Stem rust [causal organism: Puccinia graminis f. sp. tritici (Pgt)], stripe rust [Puccinia striiformis f. sp. tritici (Pst)], and leaf rust [Puccinia triticina (Pt)] are important fungal diseases of wheat in Central Asia and worldwide. Therefore, identification of seedling and adult plant resistance (APR) genes is of major importance for the national wheat breeding program in many countries. The objectives of this study were to identify genes that confer seedling and APR resistances in widely grown wheat cultivars, landraces and advanced lines from Tajikistan. A total of 41 wheat accessions were inoculated with eleven races of Pgt, twelve races of Pst and nine races of Pt for postulation of Sr (stem rust), Yr (yellow or stripe rust), and Lr Lr (leaf rust) resistance genes at the seedling stage. In addition, all of the accessions were tested in field trials for the response to stem rust and stripe rust. Genes for seedling stem rust resistance (i.e. Sr5, Sr6, Sr11, Sr31, and Sr38), stripe rust resistance (Yr9, Yr17, and Y27), and leaf rust resistance (Lr16 and Lr26) were postulated in the Tajik wheat. The presence of the pleiotropic APR genes Sr2/Yr30/Lr27 (associated with pseudo-black chaff phenotype) and Lr34/Yr18/Sr57 (associated with leaf tip necrosis phenotype), and also Lr37 were assessed in the field and confirmed with linked molecular markers. In most of the wheat accessions, resistance genes could not be postulated because their infection types did not match the avirulence or virulence profile of the Pgt, Pst and Pt races tested. Six, seven, and nine accessions were identified that likely possess new genes for resistance to stem rust, stripe rust, and leaf rust, respectively, which have not been described previously. The research demonstrates the presence of effective seedling resistance and APR genes in widely grown wheat accessions that could facilitate further rust resistance breeding in the national wheat breeding program in Tajikistan.

Nearly twenty thousand wheat lines were phenotyped for the presence of leaf tip necrosis (LTN), a phenotypic trait linked to adult plant leaf rust resistance (APR) genes, viz. Lr34, Lr46 and Lr67 having pleiotropic association with multiple disease resistance genes. Thirty-six lines showed varied expression of LTN and moderate level of leaf rust severity at adult plant stage with slow rusting (disease progress at a retarded rate). Seedling resistance test (SRT) revealed susceptible and mixed infection types, a characteristic of adult plant resistance (APR) genes. Further molecular confirmation for the presence of these genes using available microsatellite markers revealed that of the 36 lines, five lines carried Lr46 + alone and five other lines carried Lr67 + alone. Seven lines carried the combination of Lr34 + and Lr67 + while six lines confirmed to carry the combination of Lr46 + and Lr67 +. Remarkably three lines carried all the three APR genes, viz. Lr34 +, Lr46 + and Lr67 +. All these stocks can be a source of APR multiple disease resistance genes. Ten lines were not confirmed to carry any of the genes but still had LTN and SRT results showing an infection type typical of APR genes and these can be the source of identifying newer APR genes. The resistance based on minor APR genes when combined with a few additional minor genes in the background of high yielding cultivars is expected to have high level of race non-specific resistance and to be durable.

The practical development and application of molecular markers in barley breeding has been examined, using barley leaf rust (Puccinia hordei) resistance genes as a model system. Molecular markers can be powerful selection tools to improve plant breeding efficiency. However, to process large breeding populations with confidence, marker systems must be robust and reliable. This requires the development of practical marker systems and their validation to demonstrate the transferability of specific markers. Of particular importance is the ability to process many assays at relatively low cost, requiring rapid and high-throughput DNA extraction and marker assay systems. Initial experiments optimised DNA extraction and polymerase chain reaction (PCR) protocols for use with barley. A modified extraction protocol using a CTAB-based buffer produced adequate yields of good quality DNA from seedling leaf tissue. An alternative protocol with a different buffering system was used in later experiments. More samples could be processed with the latter system, however lower yields of lesser quality DNA were produced. Titration experiments using PCR and random oligonucleotide primers demonstrated that Mg++ concentration and the balance of template and primer DNA were key factors for consistent amplification of random amplified polymorphic DNA (RAPD) products from barley. Validation experiments demonstrated the effectiveness of the protocols and RAPD data were used to resolve issues of genotype identification and demonstrate non-selective marker uses. An F2 population from a cross between Q21861, a barley accession with multiple disease resistances, and Galleon, a rust susceptible cultivar, was screened for seedling resistance to barley leaf rust and stem rust (P. graminis f sp. tritici). F3 progeny lines were also screened for leaf rust reaction. Segregation patterns supported the presence in Q21861 of one dominant and one recessive stem rust resistance gene, likely to be Rpg1 and rpg4, respectively. A single dominant leaf rust resistance gene from Q21861 was designated RphQ. Preliminary RAPD-PCR screening of the parents indicated sufficient polymorphism for marker development. Bulk segregant analysis (BSA) of 8-plant DNA bulks from resistant and susceptible F2 plants produced a 2.7 kb polymorphic marker using the Operon Inc. RAPD primer OPU-02. Analysis of individual DNA samples indicated linkage with RphQ at a distance of 12.0 cM. The marker was designated OPU-022700.A backcross derived breeding population (TG306-5*2/Q21861) made up of three BC1F2 derived families was selected to validate OPU-022700. It was chosen as representative material in which DNA markers might be used for pyramiding leaf rust resistance genes. The population was expected to segregate for RphQ, Rph12 and possibly Rph4. A series of experiments using marker assays and phenotyping with leaf rust pathotypes virulent to Rph12, Rph4 and Rph2 confirmed linkage between OPU-022700 and RphQ in the validation population. However, OPU-022700 could not be reliably assayed in a second laboratory. Linkages between RphQ and the RAPD markers OPAH-03600 and OPI-O42200, identified in North America as linked to leaf rust resistance in Q21861, were also validated. However, OPI-042200 proved to be unreliable when assayed in our laboratory. Candidate simple sequence repeat (SSR) markers in the vicinity of Rph2 were identified from consensus maps and assessed for parental polymorphism. Linkage between RphQ and the markers Bmag387 and Bmac96 was established, placing the Q21861 resistance gene on chromosome 5H and in the vicinity of Rph2. Combined marker and phenotypic data from these experiments with additional phenotypic data from Rph2 virulent leaf rust pathotypes supported a North American conclusion that RphQ was probably allelic to Rph2. Candidate SSR markers for Rph12 (chromosome 5HL) and Rph4 (chromosome IHS) were identified. Two markers on 5HL, GMS61 and GMS27, showed linkage to each other but not Rph12, possibly because of the small size of the single BC1F2 derived BC1F4 family segregating for that gene. However, GMS61 and GMS27 showed common inheritance with Rph12 in the three BC-derived families, indicating potential association. All candidate SSR markers for Rph4 (1HS) were monomorphic, however the gene's presence in the BC-derived population was confirmed by phenotypic assessment. The information generated through the experiments was useful at the practical plant breeding level. The marker data confirmed distorted inheritance of chromosome 5H regions in the validation population, which could have confused interpretation of Q2186rs resistance genetics. The data were also used to determine the chromosomal order of 10 loci and construct graphical genotypes of chromosome 5H in the backcross derived breeding lines. This enabled decisive interpretation of the inheritance of 5H segments in the population. This conclusion was supported by use of the data to accurately predict rust resistance patterns in a set of BC1F7 lines.

Diseases caused by Puccinia species can have devastating effects on wheat and other crops, causing severe economic losses by reducing grain yield and quality. Generally, three strategies are used to control leaf and stem rust wheat diseases: (i) incorporating genetic resistance into new cultivars (ii) the use of fungicides (iii) crop management. The first approach is the most environmentally and economically efficient. Combined with traditional selection techniques, marker-assisted selection (MAS) has become a valuable tool in selection of individuals carrying genes that control traits of interest, such as disease resistance. The objective of this study is to postulate leaf and stem rust resistance genes in 50 mostly Croatian wheat cultivars. Most of the cultivars were screened for the first time for rust diseases. The studies detected six leaf rust resistance genes (Lr2a, Lr3, Lr10, Lr14a, Lr17 and Lr26) and four stem leaf resistance genes (Sr8a, Sr31, Sr36 and Sr38). Nine wheat cultivars had one or more unidentified Lr genes that were not included in the set of 24 isolines, and six wheat cultivars had one or more unidentifed Sr genes. Knowledge on the identity of the seedling rust resistance genes in released cultivars and germplasm is essential for the incorporation of effective resistance genes into breeding programs to maintain a diversity of resistance genes in commonly grown cultivars.

Collections of Puccinia triticina were obtained from rust-infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, Gulf Coast, California, Pacific Northwest, and Atlantic Coast States in order to determine the virulence of the wheat leaf rust fungus in 2001. Single uredinial isolates (477 in total) were derived from the wheat leaf rust collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17, Lr30, LrB, Lr10, Lr14a, and Lr18. The isolates also were tested for virulence on adult plants with leaf rust resistance genes Lr12, Lr13, Lr22a, Lr22b, Lr34, Lr35, and Lr37. In the United States in 2001, 44 virulence phenotypes of P. triticina were found. Virulence phenotype MBDS, which is virulent to resistance gene Lr17, was the most common phenotype in the United States. MBDS was found in the Southeast, Great Plains, and Ohio Valley regions. Virulence phenotype THBJ, which is virulent to Lr16 and Lr26, was the second most common phenotype, and occurred almost exclusively in the north-central Great Plains region. Phenotype MCDS, which is virulent to Lr17 and Lr26, was the third most common phenotype and was found primarily in the Southeast, Ohio Valley, and Great Plains regions. The Southeast and Ohio Valley regions differed from the Great Plains region for predominant virulence phenotypes, which indicate that populations of P. triticina in those areas are not closely connected. The northern and southern areas of the Great Plains region differed for phenotypes with virulence to Lr16; however, the two areas had other phenotypes in common. Virulence to the adult plant resistance genes Lr35 and Lr37 was detected for the first time in North America in the MBDS, MCJS, and MCDS phenotypes.

Combinations of effective leaf rust resistance genes that condition resistance to many leaf rust races are key to the development of wheat cultivars with long-lasting resistance to leaf rust caused by Puccinia triticina. A population of F6 recombinant inbred lines (RILs) derived from the cross ‘Thatcher*3/Americano 25e’ was evaluated for segregation of adult plant leaf rust resistance in three field plot tests and in a greenhouse test. A genetic map was constructed with 243 diversity array technology markers. Significant effects for reduction in leaf rust severity were found on chromosomes 1BL and 5BL. The sequence-tagged site marker csLV46, closely linked to Lr46, mapped to the logarithm of odds peak on chromosome 1BL, indicating that the RILs were likely segregating for this adult plant resistance gene. Lines with csLV46 had an average leaf rust severity of 35 % in the four tests. RILs with the resistance on chromosome 5BL designated as QLr.cdl-5BL had an average leaf rust severity of 47 %. RILs with both csLV46 and QLr.cdl-5BL had an average leaf rust severity of 22 %, close to the Thatcher*2/Americano 25e parental line. Lines that lacked both csLV46 and QLr-5BL had an average severity of 56 %. The QLr.cdl-5BL region enhanced the leaf resistance conditioned by Lr46 in an additive manner.

The novel, leaf rust seedling resistance gene, Lr81, was identified in a Croatian breeding line and mapped to a genomic region of less than 100Kb on chromosome 2AS. Leaf rust, caused by Puccinia triticina, is the most common and widespread rust disease in wheat. Races of Puccinia triticina evolve rapidly in the southern Great Plains of the USA, and leaf rust resistance genes often lose effectiveness shortly after deployment in wheat production. PI 470121, a wheat breeding line developed by the University of Zagreb in Croatia, showed high resistance to Puccinia triticina races collected from Oklahoma, suggesting that PI 470121 could be a leaf rust resistance source for the southern Great Plains of the USA. Genetic analysis based on an F2 population and F2:3 families derived from the cross PI 470121 × Stardust indicated that PI 470121 carries a dominant seedling resistance gene, designated as Lr81. Linkage mapping delimited Lr81 to a genomic region of 96,148bp flanked by newly developed KASP markers Xstars-KASP320 and Xstars-KASP323 on the short arm of chromosome 2A, spanning 67,030,206-67,132,354bp in the Chinese Spring reference assembly (IWGSC RefSeq v1.0). Deletion bin mapping assigned Lr81 to the terminal bin 2AS-0.78-1.00. Allelism tests indicated that Lr81 is a distinctive leaf rust resistance locus with the physical order Lr65-Lr17-Lr81. Marker-assisted selection based on a set of markers closely linked to leaf rust resistance genes in PI 470121 and Stardust enabled identification of a recombinant inbred line RIL92 carrying Lr81 only. Lr81 is a valuable leaf rust resistance source that can be rapidly introgressed into locally adapted cultivars using KASP markers Xstars-KASP320 and Xstars-KASP323.

Wild relatives of wheat are useful sources of alien resistance genes for wheat breeding. The objective of this review is to document research on the evaluation, transfer, and molecular analysis of alien resistance to wheat leaf rust especially in Aegilops tauschii, the diploid D-genome donor of common wheat. Nine named resistance genes (Lr1, Lr2, Lr15, Lr21, Lr22, Lr32, Lr34, Lr39, and Lr42) occur in the D genome. Twelve new leaf rust resistance genes have been documented in Ae. tauschii. The south-west Caspian Sea region is the centre of genetic diversity for seedling resistance. Adult-plant resistance is widespread in all geographic regions and should be exploited more in the future. Lr1 and Lr21 have been cloned and are typical NBS-LRR genes. The recent documentation of cryptic introgressions of Lr57/Yr40 from Ae. geniculata and Lr58 from Ae. triuncialis offers exciting possibilities for transferring alien genes without linkage drag. Both Lr21 and Lr34 presumably arose during or following the origin of common wheat ~8000 years ago. Leaf rust resistance genes often are located towards the physical ends of wheat chromosomes. These regions are known to be high in recombination, and this may explain their rapid rate of evolution.

Leaf rust is one of the most destructive diseases of wheat causing huge economic losses throughout the world. Several leaf rust resistance genes have been identified and genetically mapped, some of which have also been cloned including the adult plant resistance gene Lr34. Selection of wheat genotypes carrying Lr34 can be accomplished with the help of expression of a morphological marker leaf tip necrosis (LTN) which is linked with Lr34. However, recording of LTN under field conditions is time and labor intensive thereby slowing down the process of selection of desirable genotypes. Use of molecular markers which are tightly linked to Lr34 can be more efficient in precise identification of lines carrying this gene in a short span of time. We selected a total of seven such markers that were earlier reported to be linked with Lr34. The objective of the study was to assess the suitability of these markers in distinguishing Indian bread wheat genotypes carrying Lr34 from those which lacked Lr34. The haplotype analysis suggested that one STS marker (csLV34) and two gene specific markers (cssfr2 and cssfr5) could distinguish wheat genotypes carrying Lr34 from those lacking it. Use of these markers in breeding program can accelerate the speed of selection of desirable genotypes.