Quantitative Trait Loci For Adult-plant Resistance Research Articles

Identification and Mapping of QTLs for Adult Plant Resistance in Wheat Line XK502.

Stripe rust is a serious wheat disease occurring worldwide. At present, the most effective way to control it is to grow resistant cultivars. In this study, a population of 221 recombinant inbred lines (RILs) derived via single-seed descent from a hybrid of a susceptible wheat line, SY95-71, and a resistant line, XK502, was tested in three crop seasons from 2022 to 2024 in five environments. A genetic linkage map was constructed using 12,577 single-nucleotide polymorphisms (SNPs). Based on the phenotypic data of infection severity and the linkage map, five quantitative trait loci (QTL) for adult plant resistance (APR) were detected using the inclusive composite interval mapping (ICIM) method. These five loci are QYrxk502.swust-1BL, QYrxk502.swust-2BL, QYrxk502.swust-3AS, QYrxk502.swust-3BS, and QYrxk502.swust-7BS, explaining 5.67-19.64%, 9.63-36.74%, 9.58-11.30%, 9.76-23.98%, and 8.02-12.41% of the phenotypic variation, respectively. All these QTL originated from the resistant parent XK502. By comparison with the locations of known stripe rust resistance genes, three of the detected QTL, QYrxk502.swust-3AS, QYrxk502.swust-3BS, and QYrxk502.swust-7BS, may harbor new, unidentified genes. From among the tested RILs, 16 lines were selected with good field stripe rust resistance and acceptable agronomic traits for inclusion in breeding programs.

Bread wheat cultivar Popo harbors QTLs for seedling and adult plant resistance to leaf rust in South African and Argentine environments

Leaf rust, caused by the fungus Puccinia triticina Eriks (Pt), is a destructive disease affecting wheat (Triticum aestivum L.) production in many countries, and a serious threat to food security. As a result, several breeding programs have included leaf rust resistance as an important trait. The discovery and identification of new resistance genes that could aid in incorporating durable or long-lasting leaf rust resistance into wheat is fundamental in these breeding programs. The present study aimed to identify quantitative trait loci (QTLs) for leaf rust resistance in 127 recombinant inbred lines (RIL) developed from the cross between the resistant wheat cultivar Popo and the susceptible cultivar Kariega. The RIL population and parental lines were phenotyped for leaf rust infection type and severity at seedling and adult plant stage, respectively. The former in the greenhouse (in Argentina) and the latter in multiple field test environments comprising 3 locations in South Africa (in Tygerhoek in the Western Cape Province during the 2014, 2015, 2017 and 2018 cropping seasons; Clarens during 2014, 2016 and 2017 cropping seasons and in Bethlehem in the Free State Province during 2017 cropping season) and in 1 location in Argentina (during the 2017 and 2018 cropping seasons in Marcos Juárez, Córdoba Province). The population was genotyped using genotyping-by-sequencing. A total of 12,080 silicoDArT and 2,669 SNP markers were used for QTL analysis. In total, 25 putative QTLs for resistance to leaf rust at seedling and adult plant stages were identified, including 5 QTLs for seedling and 20 QTLs for adult plant resistance (APR). Interestingly, both Popo and Kariega contributed with alleles for resistance. Significant loci for reducing leaf rust infection at seedling stage were designated QLr.arc-1A, QLr.arc-2B, QLr.arc-5B, QLr.arc-6A and QLr.arc-6D. Three minor QTLs derived from Popo designated as QLr.arc-1B1, QLr.arc-2D and QLr.arc-3D were also detected from the field tests, explaining 5–10%, 10–16% and 5–7% of the phenotypic variance, respectively. The identified QTLs and their closely linked silicoDArT and SNP-based markers can be used for fine mapping and candidate gene discovery in wheat breeding programs targeting durable leaf rust resistance.

Registration of CDL‐111 and CDL‐167 oat germplasm lines with pyramided adult plant crown rust resistance

AbstractOat (Avena sativa) production worldwide is constrained by crown rust (caused by Puccinia coronata f. sp. avenae), which can cause significant yield losses. The disease is often controlled by spraying fungicides or planting resistant cultivars. Developing host resistance, however, is a challenge due to the high genetic variability of the pathogen. Race‐specific resistance usually succumbs to new races in just a few years. As such, the USDA‐ARS Cereal Disease Laboratory developed mapping populations to identify adult plant resistance (APR) loci from Avena sativa donors. Resistant lines from the mapping populations were selected and crossed with buckthorn (Rhamnus cathartica L.) nursery selection lines BT1020‐1‐1 and BT1021‐1‐1, which possess a different gene for crown rust resistance derived from Avena strigosa. From the crosses, CDL‐111 (Reg. no. GP‐122, PI 702639) and CDL‐167 (Reg. no. GP‐123, PI 702640), both containing three APR quantitative trait loci, were selected as germplasm for resistance breeding. High‐throughput markers for selection were developed and implemented in pyramiding the APR loci.

Mapping of quantitative trait loci for leaf rust resistance in the wheat population 'Xinmai 26/Zhoumai 22'.

Leaf rust, caused by the fungal pathogen Puccinia triticina (Pt), is one of the major and dangerous diseases of wheat, and has caused serious yield loss of wheat worldwide. Here, we investigated adult-plant resistance (APR) to leaf rust in a recombinant inbred line (RIL) population derived from 'Xinmai 26' and 'Zhoumai 22' over 3 years. Linkage mapping for APR to leaf rust revealed four quantitative trait loci (QTL) in this RIL population. Two QTL, QLr.hnau-2BS and QLr.hnau-3BS were contributed by 'Zhoumai22', whereas QLr.hnau-2DS and QLr.hnau-5AL were contributed by 'Xinmai 26'. The QLr.hnau-2BS covering a race-specific resistance gene Lr13 showed the most stable APR to leaf rust. Overexpression of Lr13 significantly increased APR to leaf rust. Interestingly, we found that a CNL(coiled coil-nucleotide-binding site-leucine-rich repeat)-like gene, TaCN, in QLr.hnau-2BS completely co-segregated with leaf rust resistance. The resistant haplotype TaCN-R possessed half the sequence of the coiled-coil domain of TaCN protein. Lr13 strongly interacted with TaCN-R, but did not interact with the full-length TaCN (TaCN-S). In addition, TaCN-R was significantly induced after Pt inoculation and changed the sub-cellular localization of Lr13 after interaction. Therefore, we hypothesized that TaCN-R mediated leaf rust resistance possibly by interacting with Lr13. This study provides important QTL for APR to leaf rust, and new insights into understanding how a CNL gene modulates disease resistance in common wheat.

Genome-Wide Association Study Reveals Novel Genetic Loci for Quantitative Resistance to Septoria Tritici Blotch in Wheat (Triticum aestivum L.).

Septoria tritici blotch, caused by the fungus Zymoseptoria titici, poses serious and persistent challenges to wheat cultivation in Ethiopia and worldwide. Deploying resistant cultivars is a major component of controlling septoria tritici blotch (STB). Thus, the objective of this study was to elucidate the genomic architecture of STB resistance in an association panel of 178 bread wheat genotypes. The association panel was phenotyped for STB resistance, phenology, yield, and yield-related traits in three locations for 2 years. The panel was also genotyped for single nucleotide polymorphism (SNP) markers using the genotyping-by-sequencing (GBS) method, and a total of 7,776 polymorphic SNPs were used in the subsequent analyses. Marker-trait associations were also computed using a genome association and prediction integrated tool (GAPIT). The study then found that the broad-sense heritability for STB resistance ranged from 0.58 to 0.97 and 0.72 to 0.81 at the individual and across-environment levels, respectively, indicating the presence of STB resistance alleles in the association panel. Population structure and principal component analyses detected two sub-groups with greater degrees of admixture. A linkage disequilibrium (LD) analysis in 338,125 marker pairs also detected the existence of significant (p ≤ 0.01) linkage in 27.6% of the marker pairs. Specifically, in all chromosomes, the LD between SNPs declined within 2.26–105.62 Mbp, with an overall mean of 31.44 Mbp. Furthermore, the association analysis identified 53 loci that were significantly (false discovery rate, FDR, <0.05) associated with STB resistance, further pointing to 33 putative quantitative trait loci (QTLs). Most of these shared similar chromosomes with already published Septoria resistance genes, which were distributed across chromosomes 1B, 1D, 2A, 2B, 2D, 3A,3 B, 3D, 4A, 5A, 5B, 6A, 7A, 7B, and 7D. However, five of the putative QTLs identified on chromosomes 1A, 5D, and 6B appeared to be novel. Dissecting the detected loci on IWGSC RefSeq Annotation v2.1 revealed the existence of disease resistance-associated genes in the identified QTL regions that are involved in plant defense responses. These putative QTLs explained 2.7–13.2% of the total phenotypic variation. Seven of the QTLs (R2 = 2.7–10.8%) for STB resistance also co-localized with marker-trait associations (MTAs) for agronomic traits. Overall, this analysis reported on putative QTLs for adult plant resistance to STB and some important agronomic traits. The reported and novel QTLs have been identified previously, indicating the potential to improve STB resistance by pyramiding QTLs by marker-assisted selection.

Genome-Wide Wheat 55K SNP-Based Mapping of Stripe Rust Resistance Loci in Wheat Cultivar Shaannong 33 and Their Alleles Frequencies in Current Chinese Wheat Cultivars and Breeding Lines.

Wheat cultivar Shaannong 33 (SN33) has remained highly resistant to stripe rust in the field since its release in 2009. To unravel the genetic architecture of stripe rust resistance, seedlings of 161 recombinant inbred lines (RILs) from the cross Avocet S × SN33 were evaluated with two isolates (PST-Lab.1 and PST-Lab.2) of the stripe rust pathogen (Puccinia striiformis f. sp. tritici) in the greenhouse, and the RILs were evaluated in naturally or artificially inoculated field sites during two cropping seasons. The RILs and parents were genotyped with the wheat 55K single-nucleotide polymorphism array. Three genomic regions conferring seedling resistance were mapped on chromosomes 1DS, 2AS, and 3DS, and four consistent quantitative trait loci (QTL) for adult-plant resistance (APR) were detected on 1BL, 2AS, 3DL, and 6BS. The 2AS locus conferring all-stage resistance was identified as the resistant gene Yr17 located on 2NS translocation. The QTL identified on 1BL and 6BS likely correspond to Yr29 and Yr78, respectively. An APR QTL on 3DL explaining 5.8 to 12.2% of the phenotypic variation is likely to be new. Molecular marker detection assays with the 2NS segment (Yr17), Yr29, Yr78, and QYrsn.nwafu-3DL on a panel of 420 current Chinese wheat cultivars and breeding lines indicated that these genes were present in 11.4, 7.6, 14.8, and 7.4% of entries, respectively. The interactions among these genes and QTL were additive, suggesting their potential value in enhancing stripe rust resistance breeding materials as observed in the resistant parent. In addition, we also identified two leaf necrosis genes, Ne1 and Ne2; however, the F1 plants from cross Avocet S × SN33 survived, indicating that SN33 probably has another allele of Ne1 which allows seed to be harvested.

Mapping Powdery Mildew (Blumeria graminis f. sp. tritici) Resistance in Wild and Cultivated Tetraploid Wheats.

Wheat is the most widely grown crop and represents the staple food for one third of the world’s population. Wheat is attacked by a large variety of pathogens and the use of resistant cultivars is an effective and environmentally safe strategy for controlling diseases and eliminating the use of fungicides. In this study, a collection of wild and cultivated tetraploid wheats (Triticum turgidum) were evaluated for seedling resistance (SR) and adult plant resistance (APR) to powdery mildew (Blumeria graminis) and genotyped with a 90K single nucleotide polymorphism (SNP) array to identify new sources of resistance genes. The genome-wide association mapping detected 18 quantitative trait loci (QTL) for APR and 8 QTL for SR, four of which were identical or at least closely linked to four QTL for APR. Thirteen candidate genes, containing nucleotide binding sites and leucine-rich repeats, were localized in the confidence intervals of the QTL-tagging SNPs. The marker IWB6155, associated to QPm.mgb-1AS, was located within the gene TRITD1Av1G004560 coding for a disease resistance protein. While most of the identified QTL were described previously, five QTL for APR (QPm.mgb-1AS, QPm.mgb-2BS, QPm.mgb-3BL.1, QPm.mgb-4BL, QPm.mgb-7BS.1) and three QTL for SR (QPm.mgb-3BL.3, QPm.mgb-5AL.2, QPm.mgb-7BS.2) were mapped on chromosome regions where no resistance gene was reported before. The novel QTL/genes can contribute to enriching the resistance sources available to breeders.

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.

Unlocking new alleles for leaf rust resistance in the Vavilov wheat collection.

Thirteen potentially new leaf rust resistance loci were identified in a Vavilov wheat diversity panel. We demonstrated the potential of allele stacking to strengthen resistance against this important pathogen. Leaf rust (LR) caused by Puccinia triticina is an important disease of wheat (Triticum aestivum L.), and the deployment of genetically resistant cultivars is the most viable strategy to minimise yield losses. In this study, we evaluated a diversity panel of 295 bread wheat accessions from the N. I. Vavilov Institute of Plant Genetic Resources (St Petersburg, Russia) for LR resistance and performed genome-wide association studies (GWAS) using 10,748 polymorphic DArT-seq markers. The diversity panel was evaluated at seedling and adult plant growth stages using three P. triticina pathotypes prevalent in Australia. GWAS was applied to 11 phenotypic data sets which identified a total of 52 significant marker-trait associations representing 31 quantitative trait loci (QTL). Among them, 29 QTL were associated with adult plant resistance (APR). Of the 31 QTL, 13 were considered potentially new loci, whereas 4 co-located with previously catalogued Lr genes and 14 aligned to regions reported in other GWAS and genomic prediction studies. One seedling LR resistance QTL located on chromosome 3A showed pronounced levels of linkage disequilibrium among markers (r 2=0.7), suggested a high allelic fixation. Subsequent haplotype analysis for this region found seven haplotype variants, of which two were strongly associated with LR resistance at seedling stage. Similarly, analysis of an APR QTL on chromosome 7B revealed 22 variants, of which 4 were associated with resistance atthe adult plant stage. Furthermore, most of the tested lines in the diversity panel carried 10 or more combined resistance-associated marker alleles, highlighting the potential of allele stacking for long-lasting resistance.

Genetic architecture of adult plant resistance to leaf rust in a wheat association mapping panel

Leaf rust caused by Puccina triticina is one of the most destructive fungal diseases of wheat (Triticum aestivum). Adult plant resistance (APR) is an effective strategy to achieve long-term protection from the disease. In this study, findings are reported from a genome-wide association study (GWAS) using a panel of 96 wheat cultivars genotyped with 874 Diversity Arrays Technology (DArT) markers and tested for adult leaf rust response in six field trials. A total of 13 quantitative trait loci (QTL) conferring APR to leaf rust were identified on chromosome arms 1BL, 1DS, 2AS, 2BL, 2DS, 3BS, 3BL, 4AL, 6BS (two), 7DS, 5BL/7BS and 6AL/6BS. Of these, seven QTLs mapped close to known resistance genes and QTLs, while the remaining six are novel and can be used as additional sources of resistance. Accessions with a greater number of combined QTLs for APR showed lower levels of disease severity, demonstrating additive and significant pyramiding effects. All QTLs had stable main effects and they did not exhibit a significant interaction with the experiments. These findings could help to achieve adequate levels of durable resistance through marker-assisted selection and pyramiding resistance QTLs in local germplasm.

Novel QTL for Stripe Rust Resistance on Chromosomes 4A and 6B in Soft White Winter Wheat Cultivars

Stripe rust (caused by Puccinia striiformis f. sp. tritici) of wheat (Triticum aestivum) is a devastating disease in temperate regions when susceptible varieties are grown and environmental conditions sustain high disease pressures. With frequent and severe outbreaks, disease resistance is a key tool for controlling stripe rust on wheat. The goal of this research was to identify quantitative trait loci (QTL) involved in stripe rust resistance from the important US Pacific Northwest soft white winter wheat varieties “Eltan” and “Finch”. An F2:5 recombinant inbred line (RIL) mapping population of 151 individuals derived from the Finch × Eltan cross was developed through single seed descent. A linkage map comprising 683 unique single nucleotide polymorphism (SNP) loci and 70 SSR markers were used to develop 22 linkage groups consisting of 16 of the 21 chromosomes. Stripe rust data were collected on the RILs during the summers of 2012 to 2014. QTL analysis identified two genomic regions on chromosomes 4A (QYrel.wak-4A) and 6B (QYrfi.wak-6B) associated with resistance from Eltan and Finch, respectively. The results of the QTL analysis showed that QYrel.wak-4A and QYrfi.wak-6B reduced infection type and disease severity. Based upon both molecular and phenotypic differences, QYrel.wak-4A is a novel QTL for adult plant resistance (APR) to stripe rust.

Mapping of QTL conferring leaf rust resistance in Chinese wheat lines W014204 and Fuyu 3 at adult plant stage

Wheat leaf rust is a destructive foliar disease of common wheat (Triticum aestivum L.) worldwide. The most effective, economical s to control the disease is growing resistant cultivars with adult plant resistance (APR). The Chinese wheat lines W014204 and Fuyu 3 showed high leaf rust resistance in the field. To identify leaf rust APR genes in the two lines, two mapping populations with 215 and 163 F2:3 lines from the crosses W014204/Zhengzhou 5389 and Fuyu 3/Zhengzhou 5389, respectively, were phenotyped for leaf rust severities during the 2010–2011, 2011–2012 and 2012–2013 cropping seasons in the field at Baoding, Hebei Province, China. A total of 1 215 SSR markers were used to identify the quantitative trait loci (QTLs) for leaf rust APR in the two populations. In the W014204/Zhengzhou 5389 population, three QTLs were detected and designated as QLr.hbu-1BL.1, QLr.hbu-2BS.1 and QLr.hbu-7DS, and explained 2.9–8.4, 11.5–38.3 and 8.5–44.5% of the phenotypic variance, respectively; all the resistance alleles at these loci were derived from W014204. In the Fuyu 3/Zhengzhou 5389 population, three QTLs, QLr.hbu-1BL.2, QLr.hbu-2BS.2 and QLr.hbu-7BL, explained 12.0–19.2, 22.3–38.9 and 4.1–4.3% of the phenotypic variance, respectively, and all resistance alleles were contributed by Fuyu 3. Based on chromosome positions of closely linked markers, both QLr.hbu-1BL.1 and QLr.hbu-1BL.2 are Lr46, and QLr.hbu-7DS is Lr34. QLr.hbu-7BL was mapped on chromosome 7BL near to Lr68 and they are likely the same gene. Based on chromosome positions, pedigree and field reactions, the two 2BS QTLs are different from all the known APR genes and are likely to be new APR QTL for leaf rust. These QTLs and their closely linked markers are potentially useful for improving leaf rust resistance in wheat breeding.

QTL mapping of adult‐plant resistance to stripe rust in a ‘Lumai 21 × Jingshuang 16’ wheat population

AbstractStripe rust, caused by Puccinia striiformis f. sp. tritici, is a devastating fungal disease in common wheat (Triticum aestivum L.) worldwide. Chinese wheat cultivars ‘Lumai 21’ and ‘Jingshuang 16’ show moderate levels of adult‐plant resistance (APR) to stripe rust in the field, and they showed a mean maximum disease severity (MDS) ranging from 24 to 56.7% and 26 to 59%, respectively, across different environments. The aim of this study was to identify quantitative trait loci (QTL) for resistance to stripe rust in an F3 population of 199 lines derived from ‘Lumai 21’ × ‘Jingshuang 16’. The F3 lines were evaluated for MDS in Qingshui, Gansu province, and Chengdu, Sichuan province, in the 2009–2010 and 2010–2011 cropping seasons. Five QTL for APR were detected on chromosomes 2B (2 QTL), 2DS, 4DL and 5DS based on mean MDS in each environment and averaged values from all three environments. These QTL were designated QYr.caas‐2BS.2, QYr.caas‐2BL.2, QYr.caas‐2DS.2, QYr.caas‐4DL.2 and QYr.caas‐5DS, respectively. QYr.caas‐2DS.2 and QYr.caas‐5DS were detected in all three environments, explaining 2.3–18.2% and 5.1–18.0% of the phenotypic variance, respectively. In addition, QYr.caas‐2BS.2 and QYr.caas‐2BL.2 colocated with QTL for powdery mildew resistance reported in a previous study. These APR genes and their linked molecular markers are potentially useful for improving stripe rust and powdery mildew resistances in wheat breeding.

Identification of QTL for adult plant resistance to stripe rust in Chinese wheat landrace Caoxuan 5

Yellow (or stripe) rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an important wheat disease worldwide. The development of wheat cultivars with adult plant resistance (APR) has been given increasing emphasis in recent years because of the reputed durability of APR compared to all-stage resistance. Wheat landrace Caoxuan 5 showed a high level of APR in field nurseries from 2008 to 2014. Bulked segregant analysis using resistance gene analog polymorphism, simple sequence repeat and sequence-tagged site markers identified molecular markers linked to resistance genes. A total of 176 F3 lines were evaluated for stripe rust response in field nurseries at two sites for 2 years. Broad sense heritability of APR in the population was estimated as 0.81. Inclusive composite interval mapping identified quantitative trait loci (QTL) QYrcx.nafu-3BS and QYrcx.nafu-6AL conferring adult plant resistance and they explained 17.3–38.1 % and 9.7–19.8 % of the phenotypic variations, respectively. Based on molecular markers, both QTL appear to be different from previously identified stripe rust resistance genes. The resistance QTL and their molecular markers identified in Caoxuan 5 should be useful in breeding for stripe rust resistance.

Overview and Application of QTL for Adult Plant Resistance to Leaf Rust and Powdery Mildew in Wheat

ABSTRACTLeaf rust and powdery mildew, caused by Puccinia triticina and Blumeria graminis f. sp. tritici, respectively, are widespread fungal diseases of wheat (Triticum aestivum L.). Development of cultivars with durable resistance is crucially important for global wheat production. This paper reviews the progress of genetic study and application of adult plant resistance (APR) to wheat leaf rust and powdery mildew. Eighty leaf rust and 119 powdery mildew APR quantitative trait loci (QTL) have been reported on 16 and 21 chromosomes, respectively, in over 50 publications during the last 15 yr. More important, we found 11 loci located on chromosomes 1BS, 1BL, 2AL, 2BS (2), 2DL, 4DL, 5BL, 6AL, 7BL, and 7DS showing pleiotropic effects on resistance to leaf rust, stripe rust, and powdery mildew. Among these, QTL on chromosomes 1BL, 4DL, and 7DS also correlate with leaf tip necrosis. Fine mapping and cloning of these QTL will be achieved with the advent of cheaper high‐throughput genotyping technologies. Germplasm carrying these potential resistance genes will be useful for developing cultivars with durable multidisease resistance. In addition to its non‐NBS–LRR (nucleotide binding site–leucine rich repeat) structure, the senescence‐like processes induced by Lr34 could be the reason for durability of resistance; however, more information is needed for a full understanding of the molecular mechanism related to durability. Adult plant resistance genes have been used by CIMMYT for more than 30 yr and have also been transferred to many Chinese wheat varieties through shuttle breeding.

Identification of QTL associated with durable adult plant resistance to stem rust race Ug99 in wheat cultivar ‘Pavon 76’

Stem rust of wheat, caused by Puccinia graminis f. sp. tritici, was under control worldwide for over 30 years by utilizing genetic resistance. The emergence of stem rust in 1998 in eastern Africa in form of race Ug99 and its evolving variants with virulence to many resistance genes were recognized as potential threats to wheat production. In this study we identified genomic regions contributing to putatively durable, adult plant resistance (APR) to wheat stem rust. A recombinant inbred line (RIL) population of 298 lines was previously developed at CIMMYT from a cross between ‘Avocet S’ and ‘Pavon 76’. Pavon 76 has been described to carry APR to stem rust. Avocet S carries the race-specific resistance gene Sr26. A subset of RILs without Sr26 segregated for APR to stem rust race Ug99 when evaluated in Kenya for three years. Single year and joint year analysis by inclusive composite interval mapping using 450 DArT markers identified five quantitative trait loci (QTL) that contributed to the resistance of wheat to stem rust race Ug99. Three of these, including QSr.cim-3B, which probably represents the Sr2 gene, were contributed by Pavon 76 whereas the remaining two QTL were contributed by Avocet S. QSr.cim-3B, or putatively Sr2, on chromosome arm 3BS explained 32 % of the phenotypic variation while the additional QTL in Pavon contributed 24 and 20 %, respectively. Two QTL from Avocet S explained 8 and 6 % of phenotypic variance, respectively. A combination of APR QTL from the two parents resulted in transgressive segregants expressing higher levels of resistance than Pavon 76. Our results indicate that it is possible to accumulate several minor resistance genes each with a small to intermediate effect resulting in a variety that exhibits negligible disease levels even under high stem rust pressure.

Pyramiding adult-plant powdery mildew resistance QTLs in bread wheat

Pyramiding of quantitative trait loci (QTLs) can be an effective approach for developing durable resistance to powdery mildew in wheat (Triticum aestivum L.). The Chinese wheat cultivars Bainong 64 and Lumai 21, with outstanding agronomic traits, possess four and three QTLs, respectively, for adult-plant resistance (APR) to powdery mildew. To achieve optimal durable resistance, 21 F6 lines combining two–five powdery mildew APR QTLs were developed from the cross Bainong 64/Lumai 21 using a modified pedigree selection. These lines were planted in a randomised complete block design with two replicates in Beijing during the 2009–10 and 2010–11 cropping seasons, and were evaluated for powdery mildew response using the highly virulent Blumeria graminis f. sp. tritici isolate E20. Based on the phenotypic data of both maximum disease severity (MDS) and area under the disease progress curve (AUDPC), analysis of variance indicated that there were highly significant effects of QTL combinations on reducing powdery mildew MDS and AUDPC. Six pyramided QTL combinations possessing QPm.caas-1A and QPm.caas-4DL in common along with one or more of the others expressed better APR to powdery mildew than the more resistant parent, Bainong 64. Thus, pyramiding these two QTLs with one or more of QPm.caas-2BS, QPm.caas-2BL, and QPm.caas-2DL from Lumai 21 could be a desirable strategy to breed cultivars with high levels of durable resistance to powdery mildew. Experienced breeders with a good knowledge of minor genes can achieve APR by phenotypic selection, and selection by molecular markers will still require uniform field testing for powdery mildew and disease phenotype to validate the resistance. These results provided very useful information for pyramiding APR QTLs in wheat breeding programs.

Molecular mapping of quantitative trait loci for adult-plant resistance to powdery mildew in Italian wheat cultivar Libellula

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a fungal disease that causes significant yield losses in many wheat-growing regions of the world. Previously, five quantitative trait loci (QTLs) for adult-plant resistance (APR) to stripe rust resistance were identified in Italian wheat cultivar Libellula. The objectives of this study were to map QTLs for APR to powdery mildew in 244 F2 : 3 lines of Libellula/Huixianhong, to analyse the stability of detected QTLs across environments, and to assess the association of these QTLs with stripe rust resistance. Powdery mildew response was evaluated for 2 years in Beijing and for 1 year in Anyang. The correlation between averaged maximum disease severity (MDS) and averaged area under disease progress curve (AUDPC) over 2 years in Beijing was 0.98, and heritabilities of MDS and AUDPC were 0.65 and 0.81, respectively, based on the mean values averaged across environments. SSR markers were used to screen the parents and mapping population. Five QTLs were identified by inclusive composite interval mapping, designated as QPm.caas-2DS, QPm.caas-4BL.1, QPm.caas-6BL.1, QPm.caas-6BL.2, and QPm.caas-7DS. Three QTLs (QPm.caas-2DS and QPm.caas-6BL.1, and QPm.caas-6BL.2) seem to be new resistance loci for powdery mildew. QTLs QPm.caas-2DS and QPm.caas-4BL.1 were identified at the same position as previously mapped QTLs for stripe rust resistance in Libellula. The QTL QPm.caas-7DS, derived from Libellula, coincided with the slow rusting and slow mildewing locus Lr34/Yr18/Pm38. These results and the identified markers could be useful for wheat breeders aiming for durable resistance to both powdery mildew and stripe rust.

Identification of Genomic Regions Controlling Adult-Plant Stripe Rust Resistance in Chinese Landrace Pingyuan 50 Through Bulked Segregant Analysis

ABSTRACT Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most widespread and destructive wheat diseases worldwide. Growing resistant cultivars with adult-plant resistance (APR) is an effective approach for the control of the disease. In this study, 540 simple sequence repeat markers were screened to map quantitative trait loci (QTL) for APR to stripe rust in a doubled haploid (DH) population of 137 lines derived from the cross Pingyuan 50 x Mingxian 169. The DH lines were planted in randomized complete blocks with three replicates in Gansu and Sichuan provinces during the 2005-06, 2006-07, and 2007-08 cropping seasons, providing data for four environments. Artificial inoculations were carried out in Gansu and Sichuan with the prevalent Chinese race CYR32. Broad-sense heritability of resistance to stripe rust for maximum disease severity was 0.91, based on the mean value averaged across four environments. Inclusive composite interval mapping detected three QTL for APR to stripe rust on chromosomes 2BS, 5AL, and 6BS, designated QYr.caas-2BS, QYr.caas-5AL, and QYr.caas-6BS, respectively, separately explaining from 4.5 to 19.9% of the phenotypic variation. QYr.caas-5AL, different from QTL previously reported, was flanked by microsatellite markers Xwmc410 and Xbarc261, and accounted for 5.0 to 19.9% of phenotypic variance. Molecular markers closely linked to the QTL could be used in marker-assisted selection for APR to stripe rust in wheat breeding programs.

Quantitative trait loci mapping of adult-plant resistance to powdery mildew in Chinese wheat cultivar Lumai 21

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a major fungal disease in common wheat (Triticum aestivum L.) worldwide. The Chinese winter wheat cultivar Lumai 21 has shown good and stable adult plant resistance for 19 years. The aim of this study was to map quantitative trait loci (QTLs) for resistance to powdery mildew in a population of 200 F3 lines from the cross Lumai 21/Jingshuang 16. The population was tested for powdery mildew reaction in Beijing and Anyang in the 2005–2006 and 2006–2007 cropping seasons, providing data for 4 environments. A total of 1,375 simple sequence repeat (SSR) markers were screened for associations with powdery mildew reactions, initially in bulked segregant analysis. Based on the mean disease values averaged across environments, broad-sense heritabilities of maximum disease severity and area under the disease progress curve were 0.96 and 0.77, respectively. Three QTLs for adult plant resistance were detected by inclusive composite interval mapping. These were designated QPm.caas-2BS, QPm.caas-2BL and QPm.caas-2DL, respectively, and explained from 5.4 to 20.6% of the phenotypic variance across environments. QPm.caas-2BS and QPm.caas-2DL were likely new adult plant resistance QTLs flanked by SSR markers Xbarc98–Xbarc1147 and Xwmc18–Xcfd233, respectively. These markers could be useful for improving wheat powdery mildew resistance in breeding programs.