All-stage Resistance Research Articles

Evaluation of Stripe Rust Resistance in Hungarian Winter Wheat Cultivars in China

Stripe or yellow rust (Yr), caused by Puccinia striiformis Westend. (Pst), is one of the most important wheat diseases worldwide. New aggressive Pst races can spread quickly, even between countries and continents. To identify and exploit stripe rust resistance genes, breeders must characterize first the Pst resistance and genotypes of their cultivars. To find new sources of resistances it is important to study how wheat varieties respond to Pst races that predominate in other continents. In this study we evaluated stripe rust resistance in 53 Hungarian winter wheat cultivars in China. Twenty-four cultivars (45.3%) had all stage resistance (ASR) and 1 (1.9%) had adult-plant resistance (APR), based on seedling tests in growth chambers and adult-plant tests in fields. We molecularly genotyped six Yr resistance genes: Yr5, Yr10, Yr15, Yr17, Yr18, and Yr36. Yr18, an APR gene, was present alone in five cultivars, and in ‘GK Kapos’, that also had seedling resistance. The other five Yr genes were absent in all cultivars tested.

Review of Concepts and Methods of Genetic Characterization and Breeding Wheat for Resistance to Rusts

Rust diseases are economically the most important constraints to global wheat production. Breeding for resistance to diseases involves the understanding of genetic systems of both the host and pathogen simultaneously. Knowledge of host-pathogen interaction is essential for breeding disease resistant crops, because both host and pathogen populations are genetically diverse. Resistance to rust diseases can be classified into two broad categories; seedling/ all stage resistance (ASR) and adult plant resistance (APR). All stage resistance genes confer resistance at all growth stages against avirulent pathogen isolates. However, ASR genes often succumb to evolution in pathogen populations, whereas APR genes are expressed at later growth stages of the plant, show uniform response to all races of the pathogen and do not encourage pathotypic evolution and the nature of resistance they offer is usually durable. Characterization of resistance to rusts involves gene postulation (multi-pathotype test), genetic analyses and identifying the number and location of the gene(s) conferring resistance. Chromosomal location and mapping genes have been done using cytogenetic analysis; however, since 1990s the advancement of molecular genetics and biotechnology enabled to replace the tedious, time, labor and resource consuming cytogenetic analyses method of locating genes using molecular mapping techniques.

Identifying Loci Conferring Resistance to Leaf and Stripe Rusts in a Spring Wheat Population (Triticum aestivum) via Genome-Wide Association Mapping.

A previous genome-wide association study (GWAS) for leaf rust (caused by Puccinia triticina) resistance identified 46 resistance quantitative trait loci (QTL) in an elite spring wheat leaf rust resistance diversity panel. With the aim of characterizing the pleiotropic resistance sources to both leaf rust and stripe rust (caused by P. striiformis f. sp. tritici), stripe rust responses were tested in five U.S. environments at the adult-plant stage and to five U.S. races at the seedling stage. The data revealed balanced phenotypic distributions in this population except for the seedling response to P. striiformis f. sp. tritici race PSTv-37. GWAS for stripe rust resistance discovered a total of 21 QTL significantly associated with all-stage or field resistance on chromosomes 1B, 1D, 2B, 3B, 4A, 5A, 5B, 5D, 6A, 6B, 7A, and 7B. Previously documented pleiotropic resistance genes Yr18/Lr34 and Yr46/Lr67 and tightly linked genes Yr17-Lr37 and Yr30-Sr2-Lr27 were also detected in this population. In addition, stripe rust resistance QTL Yrswp-2B.1, Yrswp-3B, and Yrswp-7B colocated with leaf rust resistance loci 2B_3, 3B_t2, and 7B_4, respectively. Haplotype analysis uncovered that Yrswp-3B and 3B_t2 were either tightly linked genes or the same gene for resistance to both stripe and leaf rusts. Single nucleotide polymorphism markers IWB35950, IWB74350, and IWB72134 for the 3B QTL conferring resistance to both rusts should be useful in incorporating the resistance allele(s) in new cultivars.

Dynamic QTL for adult plant resistance to powdery mildew in common wheat (Triticum aestivum L.).

Agriculture will benefit from a rigorous characterization of genes for adult plant resistance (APR) since this gene class was recognized to provide more durable protection from plant diseases. The present study reports the identification of APR loci to powdery mildew in German winter wheat cultivars Cortez and Atlantis. Cortez was previously shown to carry all-stage resistance gene Pm3e. To avoid interference of Pm3e in APR studies, line 6037 that lacked Pm3e but showed field resistance from doubled-haploid (DH) population Atlantis/Cortez was used in two backcrosses to Atlantis for the establishment of DH population 6037/Atlantis//Atlantis. APR was assessed in the greenhouse 10, 15, and 20days after inoculation (dai) from the 4-leaf stage onwards and combined with single-nucleotide polymorphism data in a genome-wide association study (GWAS) and a linkage map-based quantitative trait loci (QTL) analysis. In GWAS, two QTL were detected: one on chromosome 1BL 10dai, the other on chromosome 2BL 20dai. In conventional QTL analysis, both QTL were detected with all three disease ratings: the QTL on chromosome 1BL explained a maximum of 35.2% of the phenotypic variation 10dai, whereas the QTL on chromosome 2BL explained a maximum of 43.5% of the phenotypic variation 20dai. Compared with GWAS, linkage map-based QTL analysis allowed following the dynamics of QTL action. The two large-effect QTL for APR to powdery mildew with dynamic gene action can be useful for the enhancement of wheat germplasm.

Evaluating stripe rust resistance in Indian wheat genotypes and breeding lines using molecular markers

Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici (Pst), is a serious disease of wheat worldwide, including India. Growing resistant cultivars is the most cost-effective and eco-friendly approach to manage the disease. In this study, 70 publically available molecular markers were used to identify the distribution of 35 Yr genes in 68 wheat genotypes. Out of 35 Yr genes, 25 genes amplified the loci associated with Yr genes. Of the 35, 18 were all-stage resistance ASR (All-stage resistance) genes and 7 (Yr16, Yr18, Yr29, Yr30, Yr36, Yr46 &Yr59) were APR (Adult-plant resistance) genes. In the field tests, evaluation for stripe rust was carried out under artificial inoculation of Pst. Fifty-three wheat genotypes were found resistant to yellow rust (ITs 0), accounting for 77.94% of total entries. Coefficients of infection ranged from 0 to 60 among all wheat genotypes. Two genotypes (VL 1099 & VL 3002) were identified with maximum 15 Yr genes followed by 14 genes in VL 3010 and HI8759, respectively. Maximum number of all-stage resistance genes were identified in RKD 292 (11) followed by ten genes in DBW 216, WH 1184 and VL 3002. Maximum number of adult-plant resistance gene was identified in VL 3009 (6), HI 8759 (5) and Lassik (4) respectively. Genes Yr26 (69.2%), Yr2 (69.1%), Yr64 (61.7%), Yr24 (58.9%), Yr7 (52.9%), Yr10 (50%) and Yr 48 (48.5%) showed high frequency among selected wheat genotypes, while Yr9 (2.94%), Yr36 (2.94%), Yr60 (1.47%) and Yr32 (8.8%) were least frequent in wheat genotypes. In future breeding programs, race specific genes and non-race specific genes should be utilised to pyramid with other effective genes to develop improved wheat cultivars with high-level and durable resistance to stripe rust. Proper deployment of Yr genes and utilizing the positive interactions will be helpful for resistance breeding in wheat.

Whole-Genome Mapping of Stripe Rust Resistance Quantitative Trait Loci and Race Specificity Related to Resistance Reduction in Winter Wheat Cultivar Eltan.

Winter wheat cultivar Eltan has been one of the most widely grown cultivars in the U.S. Pacific Northwest. It has shown variable levels of resistance to stripe rust in different years since it was released in 1990. To map all currently effective and defeated resistance genes in Eltan and understand the factors causing the resistance changes, 112 F2:5 recombinant inbred lines (RILs) were developed from a cross of Eltan with cultivar Avocet S. The RILs were evaluated in fields of Pullman, Washington in 2015, 2016, 2017, and 2018 and Mount Vernon, Washington in 2016 and 2017 under natural infections; they were also evaluated in the greenhouse with races PSTv-4 and PSTv-40 of Puccinia striiformis f. sp. tritici. The RILs were genotyped with the 90K Illumina iSelect wheat single-nucleotide polymorphism chip. A total of five quantitative trait loci (QTLs) were identified in Eltan. Two major QTLs on chromosome arms 2BS and 4AL were detected in the greenhouse tests, explaining up to 28.0 and 42.0% of phenotypic variation, respectively. The two race-specific QTLs were also detected in some field experiments but with reduced effects. A minor QTL on 5BS was detected in the greenhouse and field tests, explaining 10.0 to 14.8% of the phenotypic variation. The other two minor QTLs were mapped on 6AS and 7BL and detected only in field experiments, explaining up to 20.5 and 13.5% of phenotypic variation, respectively. All stripe rust samples collected in the experimental fields in 2015 and 2016 were identified as P. striiformis f. sp. tritici races virulent on seedlings of Eltan. The resistance reduction of Eltan was caused by changes of the P. striiformis f. sp. tritici population from avirulent to virulent, overcoming the race-specific all-stage resistance in Eltan.

Stripe rust resistance to a burgeoning Puccinia striiformis f. sp. tritici race CYR34 in current Chinese wheat cultivars for breeding and research

Stripe (yellow) rust is one of the most destructive diseases in wheat production. More than 80 stripe rust resistance (Yr) genes have been officially named, however Yr26 gene has lost resistance to CYR34 (V26) since 2011. In this study, we evaluated resistance of 692 elite wheat cultivars from China to stripe rust in adult plant stage and resistance to CYR32, CYR33, and CYR34 Pst races in seedling stags. Yr26 was deduced in 692 cultivars by WE173 and WE33 molecular marks. The result showed that 45 (7%) entries had all-stage resistance, 79 (11%) entries had adult-plant resistance, and 568 (82%) entries were susceptible to one or more stripe rust races. Besides, 48 (81%) entries in over-summering region were resistant to CYR34, 4 (10%) in over-wintering region, 121 (20%) in spring epidemic region. And 43 entries across China were detected to have Yr26 gene.

Genetic analysis of leaf rust resistance in three common wheat cultivars MP 3288, HI 784 and HI 1418 under field conditions

This study aimed to investigate the genetic basis of leafrust resistance in three bread wheat cultivars viz., MP 3288, HI 1418 and HI 784 which have been maintaining high levels of resistance to leaf rust since their release in 2011, 2000, and 1983, respectively. These cultivars also possess leaf tip necrosis phenotype. These were crossed with a susceptible bread wheat cultivar Lal Bahadur and also among themselves in non-reciprocal manner.The F1 , F2 and F3 populations were raised and the inheritance of leaf rust resistance was studied using prevalent and highly virulent Puccinia triticina pathotype 77-5 (121R63-1) during 2014- 17. These studies showed that the field (adult-plant) resistance of these cultivars is governed by two dominant genes each. Closely linked molecular markers L34DINT9F and L34PLUSR revealed the presence of non-race specific adult-plant leaf rust resistance gene Lr34 in all cultivars of present study. Absence of the other documented race nonspecific APR genes viz., Lr46, Lr67 and Lr68 was indicated in all the three test cultivars based on genotyping with closely linked molecular markers WMC44, CFD71 and csgs, respectively. The other dominant gene appears to be an allstage resistance gene since all the three cultivars displayed high levels of seedling resistance to the test pathotype. Stable resistance of these cultivars could be due to synergistic/additive or complementary effects resulting from the combination of Lr34 and the all-stage resistance gene.

Identification and validation of a novel major QTL for all-stage stripe rust resistance on 1BL in the winter wheat line 20828.

A major, likely novel stripe rust resistance QTL for all-stage resistance on chromosome arm 1BL identified in a 1.76-cM interval using a saturated linkage map was validated in four populations with different genetic backgrounds. Stripe rust is a globally important disease of wheat. Identification and utilization of new resistance genes are essential for breeding resistant cultivars. Wheat line 20828 has exhibited high levels of stripe rust resistance for over a decade. However, the genetics of stripe rust resistance in this line has not been studied. A set of 199 recombinant inbred lines (RILs) were developed from a cross between 20828 and a susceptible cultivar Chuannong 16. The RIL population was genotyped with the Wheat55K SNP (single nucleotide polymorphism) array and SSR (simple sequence repeat) markers and evaluated in four environments with current predominant Puccinia striiformis f. sp. tritici t races including CYR32, CYR33 and CYR34. Four stable QTL were located on chromosomes 1B (2 QTL), 4A and 6A. Among them, the major QTL, QYr.sicau-1B.1 (LOD = 23-28, PVE = 16-39%), was localized to a 1.76-cM interval flanked by SSR markers Xwmc216 and Xwmc156 on chromosome 1BL. Eight resistance genes were previously identified in the physical interval of QYr.sicau-1B.1. Compared with previous studies, QYr.sicau-1B.1 is a new gene for resistant to stripe rust. It was further verified by analysis of the closely linked SSR markers Xwmc216 and Xwmc156 in four other populations with different genetic backgrounds. QYr.sicau-1B.1 reduced the stripe rust disease index by up to 82.8%. Three minor stable QTL (located on chromosomes 1B, 4A and 6A, respectively) also added to the resistance level of QYr.sicau-1B.1. Our results provide valuable information for further fine mapping and cloning as well as molecular-assisted breeding with QYr.sicau-1B.1.

Pathogenesis-related protein genes involved in race-specific all-stage resistance and non-race specific high-temperature adult-plant resistance to Puccinia striiformis f. sp. tritici in wheat

Interactions of the stripe rust pathogen (Puccinia striiformis f. sp. tritici) with wheat plants activate a wide range of host responses. Among various genes involved in the plant-pathogen interactions, the expressions of particular pathogenesis-related (PR) protein genes determine different defense responses. Different types of resistance have been recognized and utilized for developing wheat cultivars for resistance to stripe rust. All-stage resistance can be detected in seedling stage and remains at high levels throughout the plant growth stages. This type of resistance is race-specific and not durable. In contrast, plants with only high-temperature adult-plant (HTAP) resistance are susceptible in seedling stage, but become resistant when plants grow older and the weather becomes warmer. HTAP resistance controlled by a single gene is partial, but usually non-race specific and durable. The objective of this study was to analyze the expression of PR protein genes involved in different types of wheat resistance to stripe rust. The expression levels of 8 PR protein genes (PR1, PR1.2, PR2, PR3, PR4, PR5, PR9 and PR10) were quantitatively evaluated at 0, 1, 2, 7 and 14 days after inoculation in single resistance gene lines of wheat with all-stage resistance genes YrTr1, Yr76, YrSP and YrExp2 and lines carrying HTAP resistance genes Yr52, Yr59, Yr62 and Yr7B. Races PSTv-4 and PSTv-37 for compatible and incompatible interactions were used in evaluation of PR protein gene expression in wheat lines carrying all-stage resistance genes in the seedling-stage experiment while PSTv-37 was used in the HTAP experiment. Analysis of quantitative real-time polymerase chain reaction (qRT-PCR) revealed that all of the PR protein genes were involved in the different types of resistance controlled by different Yr genes. However, these genes were upregulated at different time points and at different levels during the infection process among the wheat lines with different Yr genes for either all-stage resistance or HTAP resistance. Some of the genes were also induced in compatible interactions, but the levels were almost always higher in the incompatible interaction than in the compatible interaction at the same time point for each Yr gene. These results indicate that both allsalicylic acid and jasmonate signaling pathways are involved in both race-specific all-stage resistance and non-race specific HTAP resistance. Although expressing at different stages of infection and at different levels, these PR protein genes work in concert for contribution to different types of resistance controlled by different Yr genes.

Discovering new alleles for yellow spot resistance in the Vavilov wheat collection.

GWAS detected 11 yellow spot resistance QTL in the Vavilov wheat collection. Promising adult-plant resistance loci could provide a sustainable genetic solution to yellow spot in modern wheat varieties. Yellow spot, caused by the fungal pathogen Pyrenophora tritici-repentis (Ptr), is the most economically damaging foliar disease of wheat in Australia. Genetic resistance is considered to be the most sustainable means for disease management, yet the genomic regions underpinning resistance to Ptr, particularly adult-plant resistance (APR), remain vastly unknown. In this study, we report results of a genome-wide association study using 295 accessions from the Vavilov wheat collection which were extensively tested for response to Ptr infections in glasshouse and field trials at both seedling an adult growth stages. Combining phenotypic datasets from multiple experiments in Australia and Russia with 25,286 genome-wide, high-quality DArTseq markers, we detected a total of 11 QTL, of which 5 were associated with seedling resistance, 3 with all-stage resistance, and 3 with APR. Interestingly, the novel APR QTL were effective even in the presence of host sensitivity gene Tsn1. These genomic regions could offer broad-spectrum yellow spot protection, not just to ToxA but also other pathogenicity or virulence factors. Vavilov wheat accessions carrying APR QTL combinations displayed enhanced levels of resistance highlighting the potential for QTL stacking through breeding. We propose that the APR genetic factors discovered in our study could be used to improve resistance levels in modern wheat varieties and contribute to the sustainable control of yellow spot.

Genetic analysis and molecular mapping of stripe rust resistance in an excellent wheat line Sanshumai1

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici (Pst), is one of the most widespread diseases in wheat (Triticum aestivum L.). Planting resistant cultivars has been constantly practiced for decades as the best strategy to defense against the variations of prevalent Pst races. The wheat line Sanshumai1 was developed from an interspecific hybridization of Haynaldia villosa (L) Schur. (2n = 2× = 14, VV)/Triticum turgidum L. var. durum (4n = 4× = 28, AABB)///Yanxiaomai. Sanshumai1 has all-stage resistance to most of known stripe rust races in China, including three widely virulent races CYR31, CYR32, and CYR33. To identify stripe rust resistance gene in this line, Sanshumai1 was crossed with the susceptible genotype, Mingxian169, and the F1, F2, F3, and BC1 generations were inoculated with Pst races under the controlled greenhouse conditions. The genetic results indicated that two stripe rust resistance genes in Sanshumai1, temporarily designated as YrS1 and YrS2, confer resistance to CYR31 and Su11–11, respectively. Using bulked segregant analysis (BSA) methodology, we identified five simple sequence repeat (SSR) markers and two expressed sequence tag-sequence tagged site (EST-STS) markers associated with YrS1 on the short arm of chromosome 3D. The genetic distances of the two closest flanking markers, namely Xcfd79 and Xwmc674, were 4.1 and 8.7 centiMorgans, respectively. In addition, we identified four SSR markers associated with YrS2 on the long arm of chromosome 4D. The genetic distances of the two closest flanking markers, namely Xcfd84 and Xgwm194, were 6.8 and 7.1 centiMorgans, respectively. Based on the chromosomal location, reaction patterns, and pedigree analysis, these two genes are likely novel resistance genes. These two genes and the flanking markers developed from this study are expected to be useful in pyramiding YrS1 and YrS2 with other Yr genes to develop wheat cultivars with high-level and durable resistance to stripe rust and may also benefit marker assisted selection (MAS) in breeding programs.

Characterization and molecular mapping of stripe rust resistance in wheat – Psathyrostachys huashanica introgression line H9015-17-1-9-6

Stripe rust (yellow rust), caused by Puccinia striiformis Westend. f. sp. tritici (Pst), is one of the most destructive fungal disease in wheat(Triticum aestivum L.). Growing resistant cultivars is the best strategy to control stripe rust. The wheat introgression line H9015-17–1-9–6 was developed from an interspecific hybridization between the common wheat cultivar 7182 and Psathrostachys huashanica Keng (2n = 2x = 14, NsNs). H9015-17–1-9–6 has all-stage resistance to all known Chinese stripe rust races, including three widely virulent races: CYR32, CYR33 and CYR34. To identify stripe rust resistance genes in the introgression line, H9015-17–1-9–6 was crossed with the susceptible genotype ‘Mingxian169' and the F1, F2, F3 and BC1 generations were inoculated with Pst races under controlled greenhouse conditions. The results indicated that a single dominant gene in H9015-17–1-9–6, temporarily designated as YrH9015, confers resistance to race CYR32. Using bulked segregant analysis methods, we identified seven resistance gene analogue polymorphism (RGAP) markers and three simple sequence repeat (SSR) markers associated with YrH9015 on the long arm of chromosome 5D. The genetic distances of the two closest flanking markers, namely M4 and M5, were 2.4 and 3.1 centimorgans, respectively. Based on the chromosomal location, reaction patterns and pedigree analysis, YrH9015 is likely a novel resistance gene. This gene and the flanking markers obtained from this study are expected to be useful in pyramiding YrH9015 with other Yr genes to develop wheat cultivars with high-level and durable resistance to stripe rust and may also benefit marker assisted selection (MAS) in breeding programmes.

Combination of all-stage and high-temperature adult-plant resistance QTL confers high-level, durable resistance to stripe rust in winter wheat cultivar Madsen.

Wheat cultivar Madsen has a new gene on the short arm of chromosome 1A and two QTL for all-stage resistance and three QTL for high-temperature adult-plant resistance that in combination confer high-level, durable resistance to stripe rust. Wheat cultivar Madsen has maintained a high-level resistance to stripe rust over 30years. To map quantitative trait loci (QTL) underlying the high-level, durable resistance, 156 recombinant inbred lines (RILs) developed from cross Avocet S × Madsen were phenotyped with selected races of Puccinia striiformis f. sp. tritici in the greenhouse seedling tests, and in naturally infected fields during 2015-2017. The RILs were genotyped by SSR and SNP markers from genotyping by sequencing and the 90K wheat SNP chip. Three QTL for all-stage resistance were mapped on chromosomes 1AS, 1BS and 2AS, and two QTL for high-temperature adult-plant (HTAP) resistance were mapped on 3BS and 6BS. The most effective QTL on 2AS, explaining 8.97-23.10% of the phenotypic variation in seedling tests and 8.60-71.23% in field tests, contained Yr17 for all-stage resistance and an additional gene for HTAP resistance. The 6BS QTL, detected in all field tests, was identified as Yr78. The 1AS QTL, conferring all-stage resistance, was identified as a new gene, which explained 20.45 and 30.23% of variation in resistance to races PSTv-37 and PSTv-40, respectively, and contributed significantly to field resistance at Pullman in 2015-2017, but was not detected at Mount Vernon. The interactions among QTL were mostly additive, and RILs with all five QTL had the highest level of resistance in the field, similar to Madsen. Genotyping 148 US Pacific Northwest wheat cultivars with markers for the 1AS, 2AS and 6BS QTL validated the genes and markers, and indicated their usefulness for marker-assisted selection.

Characterizing and Validating Stripe Rust Resistance Loci in US Pacific Northwest Winter Wheat Accessions (Triticum aestivum L.) by Genome-wide Association and Linkage Mapping.

Stripe rust resistance is a critical need for wheat cultivars in the US Pacific Northwest (PNW). Our previous genome-wide association study (GWAS) for stripe rust resistance in a set of PNW winter wheat accessions (Panel-2) identified multiple marker-trait associations (MTAs) for both all-stage and field resistance. In this study, we conducted additional GWAS using a different set of PNW winter wheat accessions (Panel-1) that contained recently bred soft white winter wheat breeding lines and cultivars. A total of 12 all-stage resistance MTAs and eight field resistance MTAs were identified. Within these MTAs, nine MTAs for all-stage resistance and two MTAs for field resistance were located distinctly from previously characterized genes and likely represent novel loci. Markers IWB60567 (1B), IWB24342 (2A), and IWB46564 (2B) explained the largest phenotypic variances for disease responses. The analysis confirmed that MTAs on chromosome 1B were indeed the same as identified in Panel-2 and that MTAs on chromosome 2A were likely and closely linked to another field resistance QTL, (Panel-2). Haplotypes for MTAs on chromosome 1B, , and linked loci on chromosome 2A provide useful information for marker development and introgression of these QTL into wheat breeding programs.

Characterization of Novel Gene Yr79 and Four Additional Quantitative Trait Loci for All-Stage and High-Temperature Adult-Plant Resistance to Stripe Rust in Spring Wheat PI 182103.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is an important disease of wheat worldwide. Exploring new resistance genes is essential for breeding resistant wheat cultivars. PI 182103, a spring wheat landrace originally from Pakistan, has shown a high level of resistance to stripe rust in fields for many years, but genes for resistance to stripe rust in the variety have not been studied. To map the resistance gene(s) in PI 182103, 185 recombinant inbred lines (RILs) were developed from a cross with Avocet Susceptible (AvS). The RIL population was genotyped with simple sequence repeat (SSR) and single nucleotide polymorphism markers and tested with races PST-100 and PST-114 at the seedling stage under controlled greenhouse conditions and at the adult-plant stage in fields at Pullman and Mt. Vernon, Washington under natural infection by the stripe rust pathogen in 2011, 2012, and 2013. A total of five quantitative trait loci (QTL) were detected. QyrPI182103.wgp-2AS and QyrPI182103.wgp-3AL were detected at the seedling stage, QyrPI182103.wgp-4DL was detected only in Mt. Vernon field tests, and QyrPI182103.wgp-5BS was detected in both seedling and field tests. QyrPI182103.wgp-7BL was identified as a high-temperature adult-plant resistance gene and detected in all field tests. Interactions among the QTL were mostly additive, but some negative interactions were detected. The 7BL QTL was mapped in chromosomal bin 7BL 0.40 to 0.45 and identified as a new gene, permanently designated as Yr79. SSR markers Xbarc72 and Xwmc335 flanking the Yr79 locus were highly polymorphic in various wheat genotypes, indicating that the molecular markers are useful for incorporating the new gene for potentially durable stripe rust resistance into new wheat cultivars.

Genome-Wide Association Mapping of Loci for Resistance to Stripe Rust in North American Elite Spring Wheat Germplasm

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a major yield-limiting foliar disease of wheat (Triticum aestivum) worldwide. In this study, the genetic variability of elite spring wheat germplasm from North America was investigated to characterize the genetic basis of effective all-stage and adult plant resistance (APR) to stripe rust. A genome-wide association study was conducted using 237 elite spring wheat lines genotyped with an Illumina Infinium 90K single-nucleotide polymorphism array. All-stage resistance was evaluated at seedling stage in controlled conditions and field evaluations were conducted under natural disease pressure in eight environments across Washington State. High heritability estimates and correlations between infection type and severity were observed. Ten loci for race-specific all-stage resistance were confirmed from previous mapping studies. Three potentially new loci associated with race-specific all-stage resistance were identified on chromosomes 1D, 2A, and 5A. For APR, 11 highly significant quantitative trait loci (QTL) (false discovery rate < 0.01) were identified, of which 3 QTL on chromosomes 3A, 5D, and 7A are reported for the first time. The QTL identified in this study can be used to enrich the current gene pool and improve the diversity of resistance to stripe rust disease.

Genetic and Physical Mapping of a Putative Leymus mollis-Derived Stripe Rust Resistance Gene on Wheat Chromosome 4A.

Wheat stripe rust is one of the most damaging diseases of wheat worldwide. The wheat-Leymus mollis introgression line M8664-3 exhibits all-stage resistance to Chinese stripe rust races. Genetic analysis of stripe rust resistance was performed by crossing M8664-3 with the susceptible line Mingxian169. Analysis of the disease resistance of F2 and F2:3 populations revealed that its resistance to Chinese stripe rust race 33 (CYR33) is controlled by a single dominant gene, temporarily designated as YrM8664-3. Genetic and physical mapping showed that YrM8664-3 is located in bin 4AL13-0.59-0.66 close to 4AL12-0.43-0.59 on chromosome 4AL and is flanked by single-nucleotide polymorphism markers AX111655681 and AX109496237 with genetic distances of 5.3 and 2.3 centimorgans, respectively. Resistance spectrum and position analyses indicated that YrM8664-3 may be a novel gene. Molecular detection using the markers linked to YrM8664-3 with wheat varieties commonly cultivated and wheat-L. mollis-derived lines showed that YrM8664-3 is also present in other wheat-L. mollis introgression lines but absent in commercial common wheat cultivars. Thus, YrM8664-3 is a potentially valuable source of stripe rust resistance for breeding.

Characterization and Mapping of Leaf Rust and Stripe Rust Resistance Loci in Hexaploid Wheat Lines UC1110 and PI610750 under Mexican Environments.

Growing resistant wheat varieties is a key method of minimizing the extent of yield losses caused by the globally important wheat leaf rust (LR) and stripe rust (YR) diseases. In this study, a population of 186 F8 recombinant inbred lines (RILs) derived from a cross between a synthetic wheat derivative (PI610750) and an adapted common wheat line (cv. “UC1110”) were phenotyped for LR and YR response at both seedling and adult plant stages over multiple seasons. Using a genetic linkage map consisting of single sequence repeats and diversity arrays technology markers, in combination with inclusive composite interval mapping analysis, we detected a new LR adult plant resistance (APR) locus, QLr.cim-2DS, contributed by UC1110. One co-located resistance locus to both rusts, QLr.cim-3DC/QYr.cim-3DC, and the known seedling resistance gene Lr26 were also mapped. QLr.cim-2DS and QLr.cim-3DC showed a marginally significant interaction for LR resistance in the adult plant stage. In addition, two previously reported YR APR loci, QYr.ucw-3BS and Yr48, were found to exhibit stable performances in rust environments in both Mexico and the United States and showed a highly significant interaction in the field. Yr48 was also observed to confer intermediate seedling resistance against Mexican YR races, thus suggesting it should be re-classified as an all-stage resistance gene. We also identified 5 and 2 RILs that possessed all detected YR and LR resistance loci, respectively. With the closely linked molecular markers reported here, these RILs could be used as donors for multiple resistance loci to both rusts in wheat breeding programs.

Evaluation of resistance of current wheat cultivars and breeding lines to stripe rust from three Gorges reservoir area

Nearly 100,000 ha in the Three Gorges Reservoir Area (TGRA) are in wheat production and the area is a junction where wheat stripe rust overwinters and causes epidemics the next spring; thus the area plays a pivotal role in wheat stripe rust epidemics in China. To better understand wheat resistance levels and the application of Yr genes in this area, 116 wheat cultivars (lines) were collected from the TGRA to investigate stripe rust resistance during the 2014–2016 cropping seasons. Seedling resistance evaluation results indicated that only nine accessions (7.8%) were immune or nearly immune to three predominant races of CYR32, CYR33 and PST-V26. In the field evaluation, 51 accessions (43.9%) showed adult-plant resistance, whereas 56 accessions (48.3%) were susceptible. The application of resistant sources focused on ineffective Yr9 (26.7%) and Yr17 (18.9%), and gradual ineffective Yr26 (34.5%), while effective Yr5, Yr10 and Yr15 were absent. Among them, 21 accessions (18.1%) were combined with two resistance genes. Both low resistance and more concentrated use of Yr genes indicated that this region faces a major risk for a wheat stripe rust epidemic. To improve the wheat resistance level in the TGRA, it is important to discover new all-stage resistance resources and diversify resistance resources for breeding.