QTL Conferring Fusarium Crown Rot Resistance in the Elite Bread Wheat Variety EGA Wylie

Fusarium crown rot (FCR) is one of the most damaging cereal diseases in semi-arid regions worldwide. The genetics of FCR resistance in the bread wheat (Triticum eastivum L.) variety EGA Wylie, the most resistant commercial variety available, was studied by QTL mapping. Three populations of recombinant inbred lines were developed with this elite variety as the resistant parent. Four QTL conferring FCR resistance were detected and resistance alleles of all of them were derived from the resistant parent EGA Wylie. One of these loci was located on the short arm of chromosome 5D (designated as Qcrs.cpi-5D). This QTL explains up to 31.1% of the phenotypic variance with an LOD value of 9.6. The second locus was located on the long arm of chromosome 2D (designated as Qcrs.cpi-2D) and explained up to 20.2% of the phenotypic variance with an LOD value of 4.5. Significant effects of both Qcrs.cpi-5D and Qcrs.cpi-2D were detected in each of the three populations assessed. Another two QTL (designated as Qcrs.cpi-4B.1 and Qcrs.cpi-4B.2, respectively) were located on the short arm of chromosome 4B. These two QTL explained up to 16.9% and 18.8% of phenotypic variance, respectively. However, significant effects of Qcrs.cpi-4B.1 and Qcrs.cpi-4B.2 were not detected when the effects of plant height was accounted for by covariance analysis. The elite characteristics of this commercial variety should facilitate the incorporation of the resistance loci it contains into breeding programs.     

Major QTL for Fusarium crown rot resistance in a barley landrace

Fusarium crown rot (FCR) is a serious cereal disease in semi-arid regions worldwide. In assisting the effort of breeding cultivars with enhanced resistance, we identified several barley genotypes with high levels of FCR resistance. One of these genotypes, AWCS079 which is a barley landrace originating from Japan, was investigated by developing and assessing three populations of recombinant inbred lines. Two QTL, one located on the long arm of chromosome 1H (designated as Qcrs.cpi-1H) and the other on 3HL (designated as Qcrs.cpi-3H), were found to be responsible for the FCR resistance of this genotype. Qcrs.cpi-1H is novel as no other FCR loci have been reported on this chromosome arm. Qcrs.cpi-3H co-located with a reduced height (Rht) locus and the effectiveness of the former was significantly affected by the latter. The total phenotypic variance explained by these two QTL was over 60 %. Significant effects were detected for each of the QTL in each of the three populations assessed. The existence of these loci with major effects should not only facilitate breeding and exploitation of FCR-resistant barley cultivars but also their further characterization based on fine mapping and map-based gene cloning.     

A major QTL conferring crown rot resistance in barley and its association with plant height

Crown rot (CR) is one of the most destructive diseases of barley and wheat. Fusarium species causing CR survive in crop residue and a growing acceptance of stubble retention practices has exacerbated disease severity and yield loss. Growing resistant cultivars has long been recognised as the most effective way to reduce CR damage but these are not available in barley. In a routine screening of germplasm, a barley landrace from China gave the best CR resistance among the genotypes tested. Using a doubled haploid population derived from this landrace crossed to Franklin, we demonstrate that the CR resistance of TX9425 was conditioned by a major QTL. The QTL, designated as Qcrs.cpi-3H, was mapped near the centromere on the long arm of chromosome 3H. Its effect is highly significant, accounting for up to 63.3% of the phenotypic variation with a LOD value of 14.8. The location of Qcrs.cpi-3H was coincident with a major QTL conferring plant height (PH) and the effect of PH on CR reaction was also highly significant. When the effect of PH was accounted for by covariance analysis, the Qcrs.cpi-3H QTL remained highly significant, accounting for over 40% of the phenotypic variation. The existence of such a major QTL implies that breeding barley cultivars with enhanced CR resistance should be feasible.     

Identification and validation of a major QTL conferring crown rot resistance in hexaploid wheat

Crown rot (CR), caused by various Fusarium species, is a chronic wheat disease in Australia. As part of our objective of improving the efficiency of breeding CR resistant wheat varieties, we have been searching for novel sources of resistance. This paper reports on the genetic control of one of these newly identified resistant genotypes, ‘CSCR6’. A population derived from a cross between CSCR6 and an Australian variety ‘Lang’ was analyzed using two Fusarium isolates belonging to two different species, one Fusarium pseudograminearum and the other Fusarium graminearum. The two isolates detected QTL with the same chromosomal locations and comparable magnitudes, indicating that CR resistance is not species-specific. The resistant allele of one of the QTL was derived from ‘CSCR6’. This QTL, designated as Qcrs.cpi-3B, was located on the long arm of chromosome 3B and explains up to 48.8% of the phenotypic variance based on interval mapping analysis. Another QTL, with resistant allele from the variety ‘Lang’, was located on chromosome 4B. This QTL explained up to 22.8% of the phenotypic variance. A strong interaction between Qcsr.cpi-3B and Qcsr.cpi-4B was detected, reducing the maximum effect of Qcrs.cpi-3B to 43.1%. The effects of Qcrs.cpi-3B were further validated in four additional populations and the presence of this single QTL reduced CR severity by up to 42.1%. The fact that significant effects of Qcrs.cpi-3B were detected across all trials with different genetic backgrounds and with the use of isolates belonging to two different Fusarium species make it an ideal target for breeding programs as well as for further characterization of the gene(s) involved in its resistance.     

Does function follow form? Principal QTLs for Fusarium head blight (FHB) resistance are coincident with QTLs for inflorescence traits and plant height in a doubled-haploid population of barley

Fusarium head blight (FHB), an important disease of barley in many areas of the world, causes losses in grain yield and quality. Deoxynivalenol (DON) mycotoxin residues, produced by the primary pathogen Fusarium graminearum, pose potential health risks. Barley producers may not be able to profitably market FHB-infected barley, even though it has a low DON level. Three types of FHB resistance have been described in wheat: Type I (penetration), Type II (spread), and Type III (mycotoxin degradation). We describe putative measures of these three types of resistance in barley. In wheat, the three resistance mechanisms show quantitative inheritance. Accordingly, to study FHB resistance in barley, we used quantitative trait locus (QTL) mapping to determine the number, genome location, and effects of QTLs associated with Type-I and -II resistance and the concentration of DON in the grain. We also mapped QTLs for plant height, heading date, and morphological attributes of the inflorescence (seeds per inflorescence, inflorescence density, and lateral floret size). QTL analyses were based on a mapping population of F₁-derived doubled-haploid (DH) lines from the cross of the two-rowed genotypes Gobernadora and CMB643, a linkage map constructed with RFLP marker loci, and field evaluations of the three types of FHB resistance performed in China, Mexico, and two environments in North Dakota, USA. Resistance QTLs were detected in six of the seven linkage groups. Alternate favorable alleles were found at the same loci when different inoculation techniques were used to measure Type-I resistance. The largest-effect resistance QTL (for Type-II resistance) was mapped in the centromeric region of chromosome 2. All but two of the resistance QTLs coincided with QTLs determining morphological attributes of the inflorescence and/or plant height. Additional experiments are needed to determine if these coincident QTLs are due to linkage or pleiotropy and to more clearly define the biological basis of the FHB resistance QTLs. Plant architecture should be considered in FHB resistance breeding efforts, particularly those directed at resistance QTL introgression and/or pyramiding. Received: 22 November 1998 / Accepted: 2 June 1999     

Comparative mapping of Raphanus sativus genome using Brassica markers and quantitative trait loci analysis for the Fusarium wilt resistance trait

Fusarium wilt (FW), caused by the soil-borne fungal pathogen Fusarium oxysporum is a serious disease in cruciferous plants, including the radish (Raphanus sativus). To identify quantitative trait loci (QTL) or gene(s) conferring resistance to FW, we constructed a genetic map of R. sativus using an F₂ mapping population derived by crossing the inbred lines ‘835’ (susceptible) and ‘B2’ (resistant). A total of 220 markers distributed in 9 linkage groups (LGs) were mapped in the Raphanus genome, covering a distance of 1,041.5 cM with an average distance between adjacent markers of 4.7 cM. Comparative analysis of the R. sativus genome with that of Arabidopsis thaliana and Brassica rapa revealed 21 and 22 conserved syntenic regions, respectively. QTL mapping detected a total of 8 loci conferring FW resistance that were distributed on 4 LGs, namely, 2, 3, 6, and 7 of the Raphanus genome. Of the detected QTL, 3 QTLs (2 on LG 3 and 1 on LG 7) were constitutively detected throughout the 2-year experiment. QTL analysis of LG 3, flanked by ACMP0609 and cnu_mBRPGM0085, showed a comparatively higher logarithm of the odds (LOD) value and percentage of phenotypic variation. Synteny analysis using the linked markers to this QTL showed homology to A. thaliana chromosome 3, which contains disease-resistance gene clusters, suggesting conservation of resistance genes between them.     

Mapping multiple disease resistance genes using a barley mapping population evaluated in Peru, Mexico, and the USA

We used a well-characterized barley mapping population (BCD 47 × Baronesse) to determine if barley stripe rust (BSR) resistance quantitative trait loci (QTL) mapped in Mexico and the USA were effective against a reported new race in Peru. Essentially the same resistance QTL were detected using data from each of the three environments, indicating that these resistance alleles are effective against the spectrum of naturally occurring races at these sites. In addition to the mapping population, we evaluated a germplasm array consisting of lines with different numbers of mapped BSR resistance alleles. A higher BSR disease severity on CI10587, which has a single qualitative resistance gene, in Peru versus Mexico suggests there are differences in pathogen virulence between the two locations. Confirmation of a new race in Peru will require characterization using a standard set of differentials, an experiment that is underway. The highest levels of resistance in Peru were observed when the qualitative resistance gene was pyramided with quantitative resistance alleles. We also used the mapping population to locate QTL conferring resistance to barley leaf rust and barley powdery mildew. For mildew, we identified resistance QTL under field conditions in Peru that are distinct from the Mla resistance that we mapped using specific isolates under controlled conditions. These results demonstrate the long-term utility of a reference mapping population and a well-characterized germplasm array for locating and validating genes conferring quantitative and qualitative resistance to multiple pathogens.     

Association mapping of resistance to Puccinia hordei in Australian barley breeding germplasm

Key message

“To find stable resistance using association mapping tools, QTL with major and minor effects on leaf rust reactions were identified in barley breeding lines by assessing seedlings and adult plants.”

Abstract

Three hundred and sixty (360) elite barley (Hordeum vulgare L.) breeding lines from the Northern Region Barley Breeding Program in Australia were genotyped with 3,244 polymorphic diversity arrays technology markers and the results used to map quantitative trait loci (QTL) conferring a reaction to leaf rust (Puccinia hordei Otth). The F_3:5 (Stage 2) lines were derived or sourced from different geographic origins or hubs of international barley breeding ventures representing two breeding cycles (2009 and 2011 trials) and were evaluated across eight environments for infection type at both seedling and adult plant stages. Association mapping was performed using mean scores for disease reaction, accounting for family effects using the eigenvalues from a matrix of genotype correlations. In this study, 15 QTL were detected; 5 QTL co-located with catalogued leaf rust resistance genes (Rph1, Rph3/19, Rph8/14/15, Rph20, Rph21), 6 QTL aligned with previously reported genomic regions and 4 QTL (3 on chromosome 1H and 1 on 7H) were novel. The adult plant resistance gene Rph20 was identified across the majority of environments and pathotypes. The QTL detected in this study offer opportunities for breeding for more durable resistance to leaf rust through pyramiding multiple genomic regions via marker-assisted selection.     

Susceptibility to <Emphasis Type="Italic">Fusarium </Emphasis>head blight is associated with the <Emphasis Type="Italic">Rht-D1b</Emphasis> semi-dwarfing allele in wheat

Fusarium head blight (FHB) is an important disease of wheat worldwide. The cultivar Spark is more resistant than most other UK winter wheat varieties but the genetic basis for this is not known. A mapping population from a cross between Spark and the FHB susceptible variety Rialto was used to identify quantitative trait loci (QTL) associated with resistance. QTL analysis across environments revealed nine QTL for FHB resistance and four QTL for plant height (PH). One FHB QTL was coincident with the Rht-1D locus and accounted for up to 51% of the phenotypic variance. The enhanced FHB susceptibility associated with Rht-D1b is not an effect of PH per se as other QTL for height segregating in this population have no influence on susceptibility. Experiments with near-isogenic lines supported the association between susceptibility and the Rht-D1b allele conferring the semi-dwarf habit. Our results demonstrate that lines carrying the Rht-1Db semi-dwarfing allele are compromised in resistance to initial infection (type I resistance) while being unaffected in resistance to spread within the spike (type II resistance).     

Histological Evidence for Different Spread of Fusarium crown rot in Barley Genotypes with Different Heights

Based on visual assessment of disease severity, previous studies reported that tall genotypes tend to be more severely affected by Fusarium crown rot (FCR) in wheat and barley. To clarify whether tall and dwarf genotypes have different susceptibility to FCR or whether it takes longer for Fusarium pathogens to infect dwarf genotypes, histological analyses were conducted with two pairs of near isogenic lines (NILs) for a semi‐dwarfing gene in barley. This analysis showed that F. pseudograminearum hyphae were detected earlier and proliferated more rapidly during the time‐course of FCR development in the tall isolines. Histological analysis showed that cell densities of the dwarf isolines were significantly higher than those of the tall isolines due to reduced lengths and widths of cells, and FCR severity was strongly correlated with cell density. An analysis with real‐time quantitative polymerase chain reaction detected a higher amount of F. pseudograminearum in the tall isolines at each of the time points assessed during FCR development. These results support the hypothesis that the increased cell density associated with dwarf genes could act as a physical barrier to the spread of FCR in cereals.     

Genome-wide association mapping of spot blotch resistance to three different pathotypes of <Emphasis Type="Italic">Cochliobolus sativus</Emphasis> in the USDA barley core collection

Spot blotch, caused by Cochliobolus sativus, is an economically important disease of barley. To identify genetic loci conferring resistance to three different pathotypes of C. sativus, a worldwide barley core collection (BCC) consisting of 1480 accessions from the USDA National Small Grains Collection were genotyped with the barley 9k Illumina Infinium iSELECT assay and phenotyped at the seedling stage with three C. sativus isolates ND85F (pathotype 1), ND90Pr (pathotype 2), and ND4008 (pathotype 7). Association mapping analysis was performed with the Whole_Panel containing 1480 barley accessions, as well as Two-rowed_Panel and Six-rowed_Panel consisting of 621 two-rowed and 857 six-rowed barley accessions, respectively. For resistance to isolate ND4008, one quantitative trait locus (QTL, QRcs-6H-P7) was detected in all three panels. Three other QTL (QRcs-1H-P7, QRcs-2H-P7, and QRcs-3H-P7) were detected in Whole_Panel, Six-rowed_Panel, and Two-rowed_Panel, respectively. For resistance to isolate ND90Pr, one QTL (QRcs-1H-P2) was identified in the Whole_Panel and the Two-rowed_Panel, and the other QTL (QRcs-6H-P2) was only identified in the Six-rowed_Panel. For resistance to isolate ND85F, three QTL (QRcs-1H-P1, QRcs-3H-P1, QRcs-7H-2-P1) were detected in all three panels, and one QTL (QRcs-7H-1-P1) was only detected in the Two-rowed_Panel. Among the ten QTL detected, four (QRcs-1H-P1, QRcs-3H-P1, QRcs-7H-2-P1, and QRcs-1H-P2) were mapped to chromosome regions containing previously identified QTL for spot blotch resistance, while six (QRcs-1H-P7, QRcs-2H-P7, QRcs-3H-P7, QRcs-6H-P7, QRcs-6H-P2, and QRcs-7H-1-P1) were novel. The SNP markers associated with the QTL identified in this study will be useful for breeding barley cultivars with resistance to multiple pathotypes of C. sativus.     

Mapping Rph20: a gene conferring adult plant resistance to Puccinia hordei in barley

A doubled haploid (DH) barley (Hordeum vulgare L.) population of 334 lines (ND24260?×?Flagship) genotyped with DArT markers was used to map genes for adult plant resistance (APR) to leaf rust (Puccinia hordei Otth) under field conditions in Australia and Uruguay. The Australian barley cultivar Flagship carries an APR gene (qRphFlag) derived from the cultivar Vada. Association analysis and composite interval mapping identified two genes conferring APR in this DH population. qRphFlag was mapped to the short arm of chromosome 5H (5HS), accounting for 64?C85% of the phenotypic variation across four field environments and 56% under controlled environmental conditions (CEC). A second quantitative trait locus (QTL) from ND24260 (qRphND) with smaller effect was mapped to chromosome 6HL. In the absence of qRphFlag, qRphND conferred only a low level of resistance. DH lines displaying the highest level of APR carried both genes. Sequence information for the critical DArT marker bPb-0837 (positioned at 21.2?cM on chromosome 5HS) was used to develop bPb-0837-PCR, a simple PCR-based marker for qRphFlag. The 245?bp fragment for bPb-0837-PCR was detected in a range of barley cultivars known to possess APR, which was consistent with previous tests of allelism, demonstrating that the qRphFlag resistant allele is common in leaf rust resistant cultivars derived from Vada and Emir. qRphFlag has been designated Rph20, the first gene conferring APR to P. hordei to be characterised in barley. The PCR marker will likely be effective in marker-assisted selection for Rph20.     

Mapping quantitative trait loci associated with spot blotch and net blotch resistance in a doubled-haploid barley population

Spot blotch and net blotch are important foliar barley (Hordeum vulgare L.) diseases in Canada and elsewhere. These diseases result in significant yield reduction and, more importantly, loss of grain quality, downgrading barley from malt to feed. Combining resistance to these diseases is a breeding priority but is a significant challenge using conventional breeding methodology. In the present investigation, an evaluation of the inheritance of resistance to spot and net blotch was conducted in a doubled-haploid barley population from the cross CDC Bold (susceptible)?×?TR251 (resistant). The population was screened at the seedling stage in the Phytotron and at the adult-plant stage in the field for several years. Chi-squared analysis indicated one- to four-gene segregation depending on disease, isolate, plant development stage, location and year. A major seedling and adult-plant resistance quantitative trait locus (QTL), designated QRpt6, was re-confirmed for net-form net blotch resistance, explaining 32?C61% of phenotypic variation in different experiments. Additional QTL for seedling and adult-plant resistance to net blotch were identified. For spot blotch resistance, a major seedling resistance QTL (QRcss1) was detected on chromosome 1H for isolate WRS1909, explaining 79% of the phenotypic variation. A highly significant QTL on 3H (QRcs3) was identified for seedling resistance to isolate WRS1908 and adult-plant resistance at Brandon, MB, Canada in 2008. The identification of QTL at only one location or from 1?year suggests spot blotch resistance is complex and highly influenced by the environment. Efforts are being made to combine spot and net blotch resistance in elite barley lines using molecular marker-assisted selection.     

Genetic relationships between resistances to Fusarium head blight and crown rot in bread wheat (Triticum aestivum L.)

Fusarium head blight (FHB) and crown rot (CR) are two wheat diseases caused by the same Fusarium pathogens. Progress towards CR resistance could benefit from FHB-resistant germplasm if the same genes are involved in resistance to these two different diseases. Two independent studies were conducted to investigate the relationship between host resistances to these two diseases. In the first study 32 genotypes were assessed and no significant correlation between their reactions to FHB and CR was detected. The second study was based on a QTL analysis of a doubled haploid population derived from a variety with resistance to both diseases. Results from this study showed that loci conferring resistance to FHB and CR are located on different chromosomes. Together, these results suggest that, despite a common aetiology, different host genes are involved in the resistance against FHB and CR in wheat. Thus, although it is possible that genes affecting both diseases may exist in other germplasm or under different conditions, separate screening seems to be needed in identifying sources of CR resistance.     

Detection of Fusarium head blight resistance QTLs using five populations of top-cross progeny derived from two-row × two-row crosses in barley

Fusarium head blight (FHB) resistance was evaluated in five recombinant inbred (RI) populations. The RI populations consisted of top-cross progeny derived from a diallel set of crosses. Each of five two-row barley lines differing in response to FHB were crossed with ‘Harbin 2-row’. FHB severity was scored on an 11-point scale, where resistant = 0 and susceptible = 10, based on the ‘cut-spike test’. Disease data were obtained for each population for 2 or 3 years. Linkage maps comprised of expressed sequence tag (EST) markers were developed for each population and used for quantitative trait locus (QTL) detection. Thirty two QTLs were detected using all data sets (individual populations and years). Thirteen QTLs were detected using averages across years; 10 of these were consistent across the individual year and average data sets. These QTLs clustered at 14 regions, with clusters on all chromosomes. At 11 of these clusters, Harbin 2-row contributed FHB resistance alleles. No QTLs were detected near the row type (vrs1) locus in any of the five RI populations, suggesting that the FHB resistance QTL in this region reported in two-row × six-row crosses may be pleiotropic effect of vrs1. QTL were coincident with the flowering type locus (cly1/Cly2) on chromosome 2H in every population. Some QTL × QTL interactions were significant, but these were smaller than QTL main effects. Considering the pleiotropic effect of spike morphology on FHB resistance, future FHB resistance mapping efforts in barley should focus on cross combinations in which alleles at vrs1 are not segregating. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Introgression of quantitative trait loci (QTLs) determining stripe rust resistance in barley: an example of marker-assisted line development

 Genome-analysis tools are useful for dissecting complex phenotypes and manipulating determinants of these phenotypes in breeding programs. Quantitative trait locus (QTL)-analysis tools were used to map QTLs conferring adult plant resistance to stripe rust (caused by Puccinia striiformis f.sp. hordei) in barley. The resistance QTLs were introgressed into a genetic background unrelated to the mapping population with one cycle of marker-assisted backcrossing. Doubled-haploid lines were derived from selected backcross lines, phenotyped for stripe-rust resistance, and genotyped with an array of molecular markers. The resistance QTLs that were introgressed were significant determinants of resistance in the new genetic background. Additional resistance QTLs were also detected. The susceptible parent contributed resistance alleles at two of these new QTLs. We hypothesize that favorable alleles were fixed at these new QTLs in the original mapping population. Genetic background may, therefore, have an important role in QTL-transfer experiments. A breeding system is described that integrates single-copy and multiplex markers with confirmation of the target phenotype in doubled-haploid lines phenotyped in field tests. This approach may be useful for simultaneously producing agronomically useful germplasm and contributing to an understanding of quantitatively inherited traits. Received: 6 May 1997 / Accepted: 1 September 1997     

Linkage mapping and identification of QTL affecting deoxynivalenol (DON) content (Fusarium resistance) in oats (Avena sativa L.)

Mycotoxins caused by Fusarium spp. is a major concern on food and feed safety in oats, although Fusarium head blight (FHB) is often less apparent than in other small grain cereals. Breeding resistant cultivars is an economic and environment-friendly way to reduce toxin content, either by the identification of resistance QTL or phenotypic evaluation. Both are little explored in oats. A recombinant-inbred line population, Hurdal × Z595-7 (HZ595, with 184 lines), was used for QTL mapping and was phenotyped for 3 years. Spawn inoculation was applied and deoxynivalenol (DON) content, FHB severity, days to heading and maturity (DH and DM), and plant height (PH) were measured. The population was genotyped with DArTs, AFLPs, SSRs and selected SNPs, and a linkage map of 1,132 cM was constructed, covering all 21 oat chromosomes. A QTL for DON on chromosome 17A/7C, tentatively designated as Qdon.umb-17A/7C, was detected in all experiments using composite interval mapping, with phenotypic effects of 12.2–26.6 %. In addition, QTL for DON were also found on chromosomes 5C, 9D, 13A, 14D and unknown_3, while a QTL for FHB was found on 11A. Several of the DON/FHB QTL coincided with those for DH, DM and/or PH. A half-sib population of HZ595, Hurdal × Z615-4 (HZ615, with 91 lines), was phenotyped in 2011 for validation of QTL found in HZ595, and Qdon.umb-17A/7C was again localized with a phenotypic effect of 12.4 %. Three SNPs closely linked to Qdon.umb-17A/7C were identified in both populations, and one each for QTL on 5C, 11A and 13A were identified in HZ595. These SNPs, together with those yet to be identified, could be useful in marker-assisted selection to pyramiding resistance QTL.     

Association mapping of stem rust race TTKSK resistance in US barley breeding germplasm

Key message

Loci conferring resistance to the highly virulent African stem rust race TTKSK were identified in advanced barley breeding germplasm and positioned to chromosomes 5H and 7H using an association mapping approach.

Abstract

African races of the stem rust pathogen (Puccinia graminis f. sp. tritici) are a serious threat to barley production worldwide because of their wide virulence. To discover and characterize resistance to African stem rust race TTKSK in US barley breeding germplasm, over 3,000 lines/cultivars were assessed for resistance at the seedling stage in the greenhouse and also the adult plant stage in the field in Kenya. Only 12 (0.3 %) and 64 (2.1 %) lines exhibited a resistance level comparable to the resistant control at the seedling and adult plant stage, respectively. To map quantitative trait loci (QTL) for resistance to race TTKSK, an association mapping approach was conducted, utilizing 3,072 single nucleotide polymorphism (SNP) markers. At the seedling stage, two neighboring SNP markers (0.8 cM apart) on chromosome 7H (11_21491 and 12_30528) were found significantly associated with resistance. The most significant one found was 12_30528; thus, the resistance QTL was named Rpg-qtl-7H-12_30528. At the adult plant stage, two SNP markers on chromosome 5H (11_11355 and 12_31427) were found significantly associated with resistance. This resistance QTL was named Rpg-qtl-5H-11_11355 for the most significant marker identified. Adult plant resistance is of paramount importance for stem rust. The marker associated with Rpg-qtl-5H-11_11355 for adult plant resistance explained only a small portion of the phenotypic variation (0.02); however, this QTL reduced disease severity up to 55.0 % under low disease pressure and up to 21.1 % under heavy disease pressure. SNP marker 11_11355 will be valuable for marker-assisted selection of adult plant stem rust resistance in barley breeding.     

Genome-wide association mapping reveals genetic architecture of durable spot blotch resistance in US barley breeding germplasm

Spot blotch, an economically important disease of both barley and wheat, is caused by Cochliobolus sativus (anamorph: Bipolaris sorokiniana). The disease has been reported in many regions of the world, but is particularly severe on barley in the Upper Midwest region of the USA and adjacent areas of Canada. For over 50 years, spot blotch has been effectively controlled through the deployment of durable resistance in six-rowed malting cultivars. To characterize loci conferring spot blotch resistance in US barley germplasm, we employed an association mapping approach using 3,840 breeding lines and cultivars. Three quantitative trait loci (QTL), Rcs-qtl-1H-11_10764, Rcs-qtl-3H-11_10565 and Rcs-qtl-7H-11_20162, were found to confer both seedling and adult plant resistance. Together, these three QTL comprise the Midwest Six-rowed Durable Resistant Haplotype (MSDRH), which is present in all Midwest six-rowed cultivars released since the 1960s. Each QTL alone only partially reduced disease levels, but combining all three together reduced the seedling infection response and adult plant disease severity by 47 and 83 %, respectively. The identified MSDRH will be valuable for marker-assisted selection of breeding lines to deploy spot blotch resistance and can also be incorporated into genomic selection as one of the disease resistance traits.