Seedling Leaf Rust Resistance Gene Research Articles

Mapping seedling and adult plant leaf rust resistance genes in the durum wheat cultivar Strongfield and other Triticum turgidum (L.) lines.

Durum wheat (T. turgidum L.) is threatened by the appearance of new virulent races of leaf rust, caused by Puccinia triticina, in recent years. This study was conducted to determine the leaf rust resistance in a modern Canadian durum cultivar Strongfield. Six populations derived from crosses of Strongfield with six tetraploid wheat lines, respectively, were tested at seedling plant stage with different P. triticina races. Two of the populations were evaluated for adult plant leaf rust infection in Canada and Mexico. A stepwise regression joint linkage QTL mapping and analysis by MapQTL were performed. Strongfield contributed the majority of QTL detected, contributing seven QTL detected in field tests, and eight QTL conditioning seedling resistance. A 1B QTL, QLr-Spa-1B.1, from Strongfield had a significant effect in both Canadian and Mexican field tests, and corresponded with Lr46/Yr29. The remaining field QTL were found in only the Canadian or the Mexican environment, not both. The QTL from Strongfield on 3A, QLr-Spa-3A, conferred seedling resistance to all races tested and had a significant effect in the field in Canada. This is the first report of the QLr-Spa-3A and Lr46/Yr29 as key components of the genetic resistance in Canadian durum wheat. KASP markers were developed to detect the QLr-Spa-3A for use in marker assisted leaf rust resistance breeding. The susceptible parental lines contributed QTL on 1A, 2B and 5B that were effective in Mexican field tests that may be good targets to integrate into modern durum varieties to improve resistance to new durum virulent races.

Identification of known leaf rust resistance genes in common wheat cultivars from Sichuan province in China

Leaf rust, caused by Puccinia triticina (Pt), is one of the most important wheat diseases of common wheat (Triticum aestivum L.). Using resistant cultivars is the most economical and efficient way to control the disease. A total of 86 wheat cultivars from Sichuan province in China were inoculated with 14 Pt races for postulating seedling leaf rust resistance (Lr) gene(s) in the greenhouse. Meanwhile, these cultivars were also planted in Baoding and Zhoukou trial fields for identifying slow leaf rusting genes during the 2014–2015 and 2015–2016 cropping seasons. Twelve specific markers for ten known Lr genes (Lr1, Lr9, Lr10, Lr19, Lr20, Lr24, Lr26, Lr34, Lr37 and Lr46) were also used for molecular marker detection. Based on the results from the gene postulation and molecular marker detection, nine Lr genes (Lr1, Lr2a, Lr26, Lr3ka, Lr30, Lr36, Lr15, Lr37, and Lr46) were identified in 45 cultivars either singly or in combination. Most frequently identified Lr genes were Lr26 in 25 cultivars and Lr37 in 21 cultivars. Less frequently detected genes were Lr1 and Lr46 each in nine cultivars, Lr3ka in five cultivars, Lr30 in three cultivars and Lr36 in two cultivars. Lr2a and Lr15 were found in Chuannong 16 and Chuanmai 1, respectively. Twenty-nine cultivars were found to have slow rusting resistance during the two growing seasons. The results should be useful for selecting cultivars with combinations of genes for effective resistance to grow and in breeding new cultivars with improved resistance to leaf rust.

Cytogenetic analysis and mapping of leaf rust resistance in Aegilops speltoides Tausch derived bread wheat line Selection2427 carrying putative gametocidal gene(s).

Leaf rust (Puccinia triticina) is a major biotic stress affecting wheat yields worldwide. Host-plant resistance is the best method for controlling leaf rust. Aegilops speltoides is a good source of resistance against wheat rusts. To date, five Lr genes, Lr28, Lr35, Lr36, Lr47, and Lr51, have been transferred from Ae. speltoides to bread wheat. In Selection2427, a bread wheat introgresed line with Ae. speltoides as the donor parent, a dominant gene for leaf rust resistance was mapped to the long arm of chromosome 3B (LrS2427). None of the Lr genes introgressed from Ae. speltoides have been mapped to chromosome 3B. Since none of the designated seedling leaf rust resistance genes have been located on chromosome 3B, LrS2427 seems to be a novel gene. Selection2427 showed a unique property typical of gametocidal genes, that when crossed to other bread wheat cultivars, the F1 showed partial pollen sterility and poor seed setting, whilst Selection2427 showed reasonable male and female fertility. Accidental co-transfer of gametocidal genes with LrS2427 may have occurred in Selection2427. Though LrS2427 did not show any segregation distortion and assorted independently of putative gametocidal gene(s), its utilization will be difficult due to the selfish behavior of gametocidal genes.

Highly predictive SNP markers for efficient selection of the wheat leaf rust resistance gene Lr16

BackgroundLr16 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).ResultsForty-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.ConclusionsLr16 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.

English

Barley nurseries comprising 820 lines with 479 unique pedigrees sourced from the International Centre for Agricultural Research in the Dry Areas (ICARDA) were screened for seedling and adult plant resistance (APR) against Australian isolates of barley leaf rust pathogen Puccinia hordei Otth. Ninety three percent of the lines were postulated to carry the seedling leaf rust resistance gene Rph3 based on their susceptibility in the greenhouse and field against Rph3 virulent pathotype and resistance to Rph3 avirulent pathotypes. The remaining lines showed either presence of uncharacterised seedling resistance (1%) and uncharacterised APR (1%). Five percent of lines were susceptible at both seedling and adult plant growth stages. Of the six lines identified to carry uncharacterised APR, three likely carried Rph20 based on the presence of the Rph20-linked marker bPb-0837. The results suggested that most of the ICARDA germplasm tested is not suitable for leaf rust resistance in Australia due to the presence of virulence for Rph3. Lines carrying uncharacterised seedling resistance and APR are potentially new sources of resistance, and are recommended for genetic analysis. Key words: Hordeum vulgare, Puccinia hordei, adult plant resistance, Rph3.

De Novo Assembled Wheat Transcriptomes Delineate Differentially Expressed Host Genes in Response to Leaf Rust Infection.

Pathogens like Puccinia triticina, the causal organism for leaf rust, extensively damages wheat production. The interaction at molecular level between wheat and the pathogen is complex and less explored. The pathogen induced response was characterized using mock- or pathogen inoculated near-isogenic wheat lines (with or without seedling leaf rust resistance gene Lr28). Four Serial Analysis of Gene Expression libraries were prepared from mock- and pathogen inoculated plants and were subjected to Sequencing by Oligonucleotide Ligation and Detection, which generated a total of 165,767,777 reads, each 35 bases long. The reads were processed and multiple k-mers were attempted for de novo transcript assembly; 22 k-mers showed the best results. Altogether 21,345 contigs were generated and functionally characterized by gene ontology annotation, mining for transcription factors and resistance genes. Expression analysis among the four libraries showed extensive alterations in the transcriptome in response to pathogen infection, reflecting reorganizations in major biological processes and metabolic pathways. Role of auxin in determining pathogenesis in susceptible and resistant lines were imperative. The qPCR expression study of four LRR-RLK (Leucine-rich repeat receptor-like protein kinases) genes showed higher expression at 24 hrs after inoculation with pathogen. In summary, the conceptual model of induced resistance in wheat contributes insights on defense responses and imparts knowledge of Puccinia triticina-induced defense transcripts in wheat plants.

Postulation of seedling leaf rust resistance genes in 84 Chinese winter wheat cultivars

Wheat leaf rust (caused by Puccinia triticina) is one of the most important fungal diseases in China. There are tens of winter wheat cultivars which are approved to be released by the government at a national level and more than 100 wheat cultivars at the provincial level. But there is no information about leaf rust (Lr) genes in these cultivars, which makes it difficult for farmers and breeders to select which cultivars they should plant in their fields and use in their breeding programs. The objective of this paper was to identify the leaf rust resistant genes at seedling stage present in the 84 commercial wheat cultivars from China that have been released in the past few years. A set of 20 near isogenic lines with Thatcher background and 6 lines with known Lr genes were used to test the virulence of 12 races of P. triticina (Pt). By comparing the infection types (ITs) produced on the 84 cultivars by the 12 Pt races with the ITs on the differential sets, the Lr genes were postulated. In addition, 8 molecular markers of Lr genes such as Lr9, Lr10, Lr19, Lr20, Lr21, Lr24, Lr26 and Lr29, which are closely linked to or co-segregated with the Lr gene, were used for further validation of the genes in the 84 Chinese winter wheat cultivars. Twelve Lr genes, including Lr1, Lr3, (Lr3bg), (Lr3ka), Lr11, Lr13, Lr14a, Lr16, Lr26, Lr27, Lr30 and Lr31 were postulated to be present either singly or in combinations in these Chinese wheat cultivars. Lr3 and Lr26 were detected most often in the tested cultivars, with frequencies of 51.2 and 38.1%, respectively. No wheat Lr genes were detected in 16 cultivars, and 4 cultivars may carry unknown Lr genes other than those used in this study. Lr9, Lr20, Lr21, Lr24, Lr25 and Lr29 were not present in any of the 84 tested accessions.

Identification, Validation of a SSR Marker and Marker Assisted Selection for the Goat Grass Derived Seedling Resistance Gene Lr28 in Wheat

The goat grass (Aegilops speltoides) derived seedling leaf rust resistance gene Lr28 is effective in providing resistance against infection to leaf rust including its most virulent strain, 77-5 (121R63-1) of the pathogen. A polymorphic SSR marker specific to Lr28 was identified by employing bulk segregant analysis on an F2 population derived from the cross between PBW343-Lr28, a leaf rust resistant near isogenic line of the most cultivated variety PBW343 and CSP44-Lr48, the Australian cultivar Condor derived CSP44 line carrying the APR gene Lr48. The marker amplified a polymorphic fragment which was particular to the presence of the seedling resistance gene and it was mapped at a distance of 2.9 cM from the Lr28 resistance locus on chromosome 4AL. It was also validated on a set of 42 NILs which carried other potent leaf rust resistance genes of diverse origin. Such a polymorphic codominant SSR marker will be useful in wheat breeding programmes to differentiate plants homozygous at the Lr28 locus from those that are heterozygous.

The genetic relationship between barley leaf rust resistance genes located on chromosome 2HS

Four seedling leaf rust resistance genes (Rph14, Rph15, Rph16 and Rph17) described in barley are located on chromosome 2HS. A fifth, present in barley accession HOR 1063, was reported to map to the same region as Rph16. A comprehensive analysis of the allelic relationship between Rph14, Rph15, Rph17 and RphHOR1063 was undertaken. Two additional genotypes (HOR 15560 and Zhu 4) that were initially thought to carry Rph16 were also included. While no segregation was observed in F2 populations derived from crosses between PI 584760 (carrying Rph14), PI 355447 (carrying Rph15), Zhu 4 and HOR 1063, the lines did exhibit differential reactions across 10 Australian Puccinia hordei pathotypes. This suggests that the Rph14 locus may be complex. An evaluation of F3 populations confirmed that resistance genes in Zhu 4 and in HOR 1063 were allelic to both Rph14 and Rph15, however A susceptible line was found among F3 progeny of the cross PI 584760/PI 355447, suggesting that Rph14 and Rph15 may be independent. Rph17 and the gene present in HOR 15560 were shown to be independent of each other and of Rph14 and Rph15. The response of HOR 15560 across an array of P. hordei pathotypes revealed that it possesses the same specificity as Rph1. Because the resistance in Zhu 4 and HOR 1063 were shown to be allelic to Rph14, they should be designated Rph14.am and Rph14.an, respectively.

Virulence of Puccinia triticina in Turkey and leaf rust resistance in Turkish wheat cultivars

Leaf rust caused by Puccinia triticina is a common disease on wheat in the coastal regions of Turkey. Collections of P. triticina from infected wheat leaves were obtained from the main wheat production zones of Turkey in 2009 and 2010. A total of 104 single uredinial isolates were tested for virulence on 20 lines of Thatcher wheat that differ for single leaf rust resistance genes. Forty-four different virulence phenotypes were identified over both years. Four phenotypes were found in both years. Phenotype FHPTQ found in 2009, with virulence to genes Lr2c, Lr3, Lr16, Lr26, Lr3ka, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr18, Lr3bg, and Lr14b, was the most common phenotype at 15.4 % of the total isolates. Forty-three winter and spring wheat cultivars from Turkey were tested as seedlings with 13 different P. triticina virulence phenotypes from Canada, the US and Turkey. The infection types on the cultivars were compared with infection types on the Thatcher near isogenic lines to postulate the presence of seedling leaf rust resistance genes in the cultivars. Resistance genes Lr1, Lr3a, Lr10, Lr14a, Lr17a, Lr20, Lr23, and Lr26 were postulated to be present in the Turkish wheat cultivars. DNA of the wheat cultivars was tested with PCR markers to determine the presence of the adult plant resistance genes Lr34 and Lr37. Marker data indicated the presence of Lr34 in 20 cultivars and Lr37 in three cultivars. Field plot evaluations of the wheat cultivars indicated that no single Lr gene conditioned highly effective leaf rust resistance. Resistant cultivars varied for combinations of seedling and adult plant resistance genes.

QTL mapping of multiple foliar disease and root-lesion nematode resistances in wheat

A genetic linkage map, based on a cross between the synthetic hexaploid CPI133872 and the bread wheat cultivar Janz, was established using 111 F1-derived doubled haploid lines. The population was phenotyped in multiple years and/or locations for seven disease resistance traits, namely, Septoria tritici blotch (Mycosphaeralla graminicola), yellow leaf spot also known as tan spot (Pyrenophora tritici-repentis), stripe rust (Puccinia striiformis f. sp. tritici), leaf rust (Puccinia triticina), stem rust (Puccinia graminis f. sp. tritici) and two species of root-lesion nematode (Pratylenchyus thornei and P. neglectus). The DH population was also scored for coleoptile colour and the presence of the seedling leaf rust resistance gene Lr24. Implementation of a multiple-QTL model identified a tightly linked cluster of foliar disease resistance QTL in chromosome 3DL. Major QTL each for resistance to Septoria tritici blotch and yellow leaf spot were contributed by the synthetic hexaploid parent CPI133872 and linked in repulsion with the coincident Lr24/Sr24 locus carried by parent Janz. This is the first report of linked QTL for Septoria tritici blotch and yellow leaf spot contributed by the same parent. Additional QTL for yellow leaf spot were detected in 5AS and 5BL. Consistent QTL for stripe rust resistance were identified in chromosomes 1BL, 4BL and 7DS, with the QTL in 7DS corresponding to the Yr18/Lr34 region. Three major QTL for P. thornei resistance (2BS, 6DS, 6DL) and two for P. neglectus resistance (2BS, 6DS) were detected. The recombinants combining resistance to Septoria tritici blotch, yellow leaf spot, rust diseases and root-lesion nematodes from parents CPI133872 and Janz constitute valuable germplasm for the transfer of multiple disease resistance into new wheat cultivars.

Inheritance of Leaf Rust Resistance in the CIMMYT Wheat Weebill 1

The CIMMYT spring wheat Weebill 1 has near‐immune levels of adult plant resistance (APR) to leaf rust caused by Puccinia triticina Eriks. To determine the genetic basis of resistance in seedlings and adult plants Weebill 1 was crossed with two susceptible parents Jupateco 73S and ‘Thatcher’. Advanced F4:5, F4:6, and F5:7 recombinant inbred lines (RIL) populations from Jupateco 73S/Weebill 1 and BC1F2 populations from Thatcher/Weebill 1 were derived. The F5:7 RILs and BC1F2 families were tested as seedlings with five races of P. triticina The F1 and F4:5, F4:6, and F5:7 RILs of Jupateco 73S/Weebill 1 were evaluated in plots in Mexico and St. Paul. The Jupateco 73S/Weebill 1 RILs and Thatcher*2/Weebill 1 F2 families segregated for Lr14b, an unknown seedling leaf rust resistance gene derived from Weebill 1 in both crosses, and Lr17a derived from Jupateco 73S in the RIL population. Lr17a was mapped to the distal end of chromosome 2AS and was linked to microsatellite maker Xgwm636 at a distance of 4.0 cM. The seedling genes did not condition effective resistance in the field plots with races used in the field trials. The segregation ratio in the RILs and F2 families indicated the presence of three to four APR genes in Weebill 1. One of the APR genes was linked to the seedling gene Lr14b on chromosome 7BL and reduced leaf rust by 47% in St. Paul and 31% in Mexico. Based on the haplotypes of DNA markers, we do not believe that any of the APR genes is Lr34

Transfer of leaf rust and stripe rust resistance from Aegilops umbellulata Zhuk. to bread wheat (Triticum aestivum L.)

Aegilops umbellulata acc. 3732, an excellent source of resistance to major wheat diseases, was used for transferring leaf rust and stripe rust resistance to cultivated wheat. An amphiploid between Ae. umbellulata acc. 3732 and Triticum durum cv. WH890 was crossed with cv. Chinese Spring Ph I to induce homoeologous pairing between Ae. umbellulata and wheat chromosomes. The F1 was crossed to the susceptible Triticum aestivum cv. ‘WL711’ and leaf rust and stripe rust resistant plants were selected among the backcross progenies. Homozygous lines were selected and screened against six Puccinia triticina and four Puccinia striiformis f. sp. tritici pathotypes at the seedling stage and a mixture of prevalent pathotypes of both rust pathogens at the adult plant stage. Genomic in situ hybridization in some of the selected introgression lines detected two lines with complete Ae. umbellulata chromosomes. Depending on the rust reactions and allelism tests, the introgression lines could be classified into two groups, comprising of lines with seedling leaf rust resistance gene Lr9 and with new seedling leaf rust and stripe rust resistance genes. Inheritance studies detected an additional adult plant leaf rust resistance gene in one of the introgression lines. A minimum of three putatively new genes—two for leaf rust resistance (LrU1 and LrU2) and one for stripe rust resistance (YrU1) have been introgressed into wheat from Ae. umbellulata. Two lines with no apparent linkage drag have been identified. These lines could serve as sources of resistance to leaf rust and stripe rust in breeding programs.

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

Leaf rust, caused by Puccinia triticina Eriks., is the most common disease of wheat (Triticum aestivum L.) in the United States and worldwide. The objective of this study was to characterize seedling and adult plant leaf rust resistance in hard red spring wheat cultivars grown in Minnesota, North Dakota, and South Dakota, and postulate the identity of the seedling leaf rust resistance genes in the cultivars. Twenty-six cultivars, near-isogenic lines of Thatcher wheat that differ for single leaf rust resistance genes, and three wheat cultivars with known leaf rust resistance genes, were tested with 11 different isolates of leaf rust collected from the United States and Canada. The leaf rust infection types produced on seedling plants of the cultivars in greenhouse tests were compared with the infection types produced by the same isolates on the Thatcher near-isogenic lines to postulate which seedling leaf rust resistance genes were present. Seedling leaf rust resistance genes Lr1, Lr2a, Lr10, Lr16, Lr21, and Lr24 were postulated to be present in spring wheat cultivars. Seedling genes Lr3, Lr14a, and Lr23 likely were present in some cultivars but could not be clearly identified in this study. Most of the cultivars had some level of adult plant leaf rust resistance, most likely due to Lr34. Cultivars that had seedling resistance genes Lr1, Lr2a, Lr10, or Lr16 had poor to intermediate levels of leaf rust resistance in field plots. Cultivars with combinations of seedling resistance genes Lr16 and Lr24 with additional adult plant resistance were highly resistant to leaf rust.

Cytogenetic studies in wheat XIX. Chromosome location and linkage studies of a gene for leaf rust resistance in the Australian cultivar ‘Harrier’

AbstractMonosomic analysis indicated that a seedling leaf rust resistance gene present in the Australian wheat cultivar ‘Harrier’(tentatively designated LrH) is located on chromosome 2A. LrH segregated independently of the stripe rust resistance gene Yr1 located in the long arm of that chromosome, but failed to recombine with Lr17 located in the short arm. LrH was therefore designated Lr17b and the allele formerly known as Lr17 was redesignated as Lr17a. The genes Lr17b and Lr37 showed close repulsion linkage. Tests of allelism indicated that Lr1 7b is also present in the English wheats ‘Dwarf A’(‘Hobbit Sib’), ‘Maris Fundin’ and ‘Norman’. Virulence for Lr17b occurs in Australia, and pathogenicity studies have also demonstrated virulence in many western European isolates of the leaf rust pathogen. Despite this, it is possible that the gene may be of value in some regions if used in combination with other leaf rust resistance genes.

Resistance to wheat leaf rust and stem rust in Triticum tauschii and inheritance in hexaploid wheat of resistance transferred from T. tauschii

Twelve accessions of Triticum tauschii (Coss.) Schmal. were genetically analyzed for resistance to leaf rust (Puccinia recondita Rob. ex Desm.) and stem rust (Puccinia graminis Pers. f.sp. tritici Eriks. and E. Henn.) of common wheat (Triticum aestivum L.). Four genes conferring seedling resistance to leaf rust, one gene conferring seedling resistance to stem rust, and one gene conferring adult-plant resistance to stem rust were identified. These genes were genetically distinct from genes previously transferred to common wheat from T. tauschii and conferred resistance to a broad spectrum of pathogen races. Two of the four seedling leaf rust resistance genes were not expressed in synthetic hexaploids, produced by combining tetraploid wheat with the resistant T. tauschii accessions, probably owing to the action of one or more intergenomic suppressor loci on the A or B genome. The other two seedling leaf rust resistance genes were expressed at the hexaploid level as effectively as in the source diploids. One gene was mapped to the short arm of chromosome 2D more than 50 cM from the centromere and the other was mapped to chromosome 5D. Suppression of seedling resistance to leaf rust in synthetic hexaploids derived from three accessions of T. tauschii allowed the detection of three different genes conferring adult-plant resistance to a broad spectrum of leaf rust races. The gene for seedling resistance to stem rust was mapped to chromosome ID. The degree of expression of this gene at the hexaploid level was dependent on the genetic background in which it occurred and on environmental conditions. The expression of the adult-plant gene for resistance to stem rust was slightly diminished in hexaploids. The production of synthetic hexaploids was determined to be the most efficient and flexible method for transferring genes from T. tauschii to T. aestivum, but crossing success was determined by the genotypes of both parents. Although more laborious, the direct introgression method of crossing hexaploid wheat with T. tauschii has the advantages of enabling selection for maximum expression of resistance in the background hexaploid genotype and gene transfer into an agronomically superior cultivar.

Effect of gene Lr34 in the enhancement of resistance to leaf rust of wheat

Leaf rust resistance gene Lr34 is present in many wheat cultivars throughout the world that have shown durable resistance to leaf rust. Fourteen pair-wise combinations of Lr34 and seedling leaf rust resistance genes were developed by intercrossing near isogenic 'Thatcher' lines. In both seedling and adult plant tests homozygous paired combinations of specific resistance genes with Lr34 had enhanced resistance relative to either parent to different numbers of isolates that were avirulent to the additional resistance genes. The TcLr34, 18 line also expressed enhanced resistance to specific isolates virulent to Lr18 in seedling and adult plant stages. In rust nursery tests, homozygous lines were more resistant than either parent, if the additional leaf rust gene conditioned an effective of resistance when present singly. The ability of Lr34 to interact with other genes conditioning effective resistance may contribute to the durability of leaf rust resistance in cultivars with Lr34.

Enhanced leaf rust resistance in wheat conditioned by resistance gene pairs with Lr13

Leaf rust resistance gene Lr13 is present in many North American hard red spring wheat cultivars that have shown durable resistance to leaf rust. Fifteen pair-wise combinations of Lr13 and seedling leaf rust resistance genes were developed by intercrossing near isogenic Thatcher lines. In both seedling and adult plant tests, homozygous paired combinations of specific resistance genes with Lr13 had enhanced resistance relative to either parent to rust isolates that had intermediate avirulent infection types to the additional genes. In field tests, homozygous lines were more resistant than either parent if the additional leaf rust gene conditioned an effective level of resistance when present singly.