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.     

Identification and mapping of net form of net blotch resistance in South African barley

Net form of net blotch (NFNB) caused by the fungus Pyrenophora teres f. teres is an economically important foliar disease of barley (Hordeum vulgare) in southern and eastern Africa. Little attention has been given to disease resistance breeding, and knowledge about the presence of NFNB resistance in breeding lines is limited. Deploying resistance into varieties used in this region is important for future control of the disease. We have identified NFNB disease resistance in existing South African breeders’ lines and have mapped the resistance in line UVC8. Six different trials, three conducted in South Africa and another three in Australia, were used to identify resistance QTL. A major QTL was identified on chromosome 6H having a LOD score of 40.5 and 55% of the phenotypic variance explained. Kompetitive Allele Specific PCR (KASP?) markers were designed for this QTL region. These and microsatellite markers can now be used to routinely select for NFNB resistance.     

Inheritance and RAPD tagging of multiple genes for resistance to net blotch in barley.

A doubled haploid barley (Hordeum vulgare L.) population that was created from a cross between cultivars 'Léger' and 'CI 9831' was characterized by RAPD (random amplified polymorphic DNA) markers for resistance to isolate WRS857 of Pyrenophora teres Drechs. f. sp. maculata Smedeg., the causal agent of the spot form of net blotch. Resistance, which initially appeared to be conferred by a single gene from the approximate 1:1 (resistant : susceptible) segregation ratio of the doubled-haploid (DH) progeny, was found to be associated with three different genomic regions by RAPD analysis. Of 500 RAPD random primers that were screened against the parents, 195 revealed polymorphic bands, seven showed an association to the resistance in bulks, and these seven markers were mapped to three unlinked genomic regions. Two of these regions, one of which was mapped to chromosome 2, have major resistance genes. The third region has some homology to the chromosome 2 region. This study demonstrates the simultaneous location of markers for more than one gene governing a trait by using RAPD and bulked segregant analysis (BSA).     

Identification of quantitative trait loci associated with resistance to net form net blotch in a collection of Nordic barley germplasm

Key message

Association mapping of resistance to Pyrenophora teres f. teres in a collection of Nordic barley germplasm at different developmental stages revealed 13 quantitative loci with mostly small effects.

Abstract

Net blotch, caused by the necrotrophic fungus Pyrenophora teres, is one of the major diseases in barley in Norway causing quantitative and qualitative yield losses. Resistance in Norwegian cultivars and germplasm is generally insufficient and resistance sources have not been extensively explored yet. In this study, we mapped quantitative trait loci (QTL) associated with resistance to net blotch in Nordic germplasm. We evaluated a collection of 209 mostly Nordic spring barley lines for reactions to net form net blotch (NFNB; Pyrenophora teres f. teres) in inoculations with three single conidia isolates at the seedling stage and in inoculated field trials at the adult stage in 4 years. Using 5669 SNP markers genotyped with the Illumina iSelect 9k Barley SNP Chip and a mixed linear model accounting for population structure and kinship, we found a total of 35 significant marker-trait associations for net blotch resistance, corresponding to 13 QTL, on all chromosomes. Out of these QTL, seven conferred resistance only in adult plants and four were only detectable in seedlings. Two QTL on chromosomes 3H and 6H were significant during both seedling inoculations and adult stage field trials. These are promising candidates for breeding programs using marker-assisted selection strategies. The results elucidate the genetic background of NFNB resistance in Nordic germplasm and suggest that NB resistance is conferred by a number of genes each with small-to-moderate effects, making it necessary to pyramid these genes to achieve sufficient levels of resistance.

     

Identification and chromosomal location of major genes for resistance to Pyrenophora teres in a doubled-haploid barley population.

Net blotch, caused by Pyrenophora teres, is one of the most economically important diseases of barley worldwide. Here, we used a barley doubled-haploid population derived from the lines SM89010 and Q21861 to identify major quantitative trait loci (QTLs) associated with seedling resistance to P. teres f. teres (net-type net blotch (NTNB)) and P. teres f. maculata (spot-type net blotch (STNB)). A map consisting of simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers was used to identify chromosome locations of resistance loci. Major QTLs for NTNB and STNB resistance were located on chromosomes 6H and 4H, respectively. The 6H locus (NTNB) accounted for as much as 89% of the disease variation, whereas the 4H locus (STNB resistance) accounted for 64%. The markers closely linked to the resistance gene loci will be useful for marker-assisted selection.     

Barley 4H QTL confers NFNB resistance to a global set of <Emphasis Type="Italic">P. teres</Emphasis> f. <Emphasis Type="Italic">teres</Emphasis> isolates

Net form net blotch (NFNB), caused by Pyrenophora teres f. teres Drechs., is prevalent in barley-growing regions worldwide. A population of 132 recombinant inbred lines (RILs) developed from a cross of the barley varieties ‘Falcon’ and ‘Azhul’ were used to evaluate resistance to NFNB due to their differential reactions to isolates of P. teres f. teres from Australia, Canada, Japan, and the USA. Falcon is a six-rowed, hulless feed barley harboring resistance to NFNB, while Azhul is a six-rowed, hulless food barley with high levels of susceptibility to many P. teres f. teres isolates. Seedling disease resistance data were collected on seedlings of parents, RILs, and checks in a growth chamber. The population was genotyped using Illumina’s GoldenGate assay, and quantitative trait loci (QTL) were detected on chromosomes 2H, 3H, 4H, and 6H. We identified a single genetic region on barley chromosome 4H that provided varying levels of resistance to all P. teres f. teres isolates evaluated.     

Association mapping of spot blotch resistance in wild barley

Spot blotch, caused by Cochliobolus sativus, is an important foliar disease of barley. The disease has been controlled for over 40 years through the deployment of cultivars with durable resistance derived from the line NDB112. Pathotypes of C. sativus with virulence for the NDB112 resistance have been detected in Canada; thus, many commercial cultivars are vulnerable to spot blotch epidemics. To increase the diversity of spot blotch resistance in cultivated barley, we evaluated 318 diverse wild barley accessions comprising the Wild Barley Diversity Collection (WBDC) for reaction to C. sativus at the seedling stage and utilized an association mapping (AM) approach to identify and map resistance loci. A high frequency of resistance was found in the WBDC as 95% (302/318) of the accessions exhibited low infection responses. The WBDC was genotyped with 558 Diversity Array Technology (DArT^®) and 2,878 single nucleotide polymorphism (SNP) markers and subjected to structure analysis before running the AM procedure. Thirteen QTL for spot blotch resistance were identified with DArT and SNP markers. These QTL were found on chromosomes 1H, 2H, 3H, 5H, and 7H and explained from 2.3 to 3.9% of the phenotypic variance. Nearly half of the identified QTL mapped to chromosome bins where spot blotch resistance loci were previously reported, offering some validation for the AM approach. The other QTL mapped to unique genomic regions and may represent new spot blotch resistance loci. This study demonstrates that AM is an effective technique for identifying and mapping QTL for disease resistance in a wild crop progenitor.     

Identification and mapping of a gene conferring resistance to the spot form of net blotch (Pyrenophora teres f maculata) in barley

 Spot form of net blotch (SFNB) (Pyrenophora teres f maculata) is an economically damaging foliar disease of barley in many of the world’s cereal growing areas. The development of SFNB-resistant cultivars may be accelerated through the use of molecular markers. A screen for SFNB resistance in 96 lines identified four new sources of resistance, including a feed variety, ‘Galleon’, for which a fully mapped doubled haploid population was available. Segregation data indicated SFNB resistance was conferred by a single gene in the ‘Galleon’בHaruna Nijo’ cross, positioned on the long arm of chromosome 7H. This gene is designated Rpt4 and is flanked by the RFLP loci Xpsr117(D) and Xcdo673 at distances of 6.9 cM and 25.9 cM, respectively. The marker Xpsr117(D) was validated using another population segregating for Rpt4, correctly predicting SFNB resistance with more than 90% accuracy. Received: 24 September 1998 / Accepted: 19 December 1998     

Seedling and adult stage resistance to net form of net blotch (NFNB) in spring barley and stability of adult stage resistance to NFNB in Morocco

This study was conducted to identify stable resistance to net form of net blotch (NFNB) in spring barley in Moroccan environments. Seedling resistance to NFNB was evaluated by inoculating 336 barley genotypes with two NFNB isolates LDNH04Ptt-19 and TD-10 in the greenhouse. These genotypes were evaluated for adult plant resistance to NFNB under seven environments in Morocco in 2015 and 2016. The disease severity was estimated at GS 77–87 on barley leaves using a double-digit scale. To investigate stability of resistance, 149 barley genotypes were subjected to AMMI analysis. At the seedling stage, differential responses of barley genotypes to different NFNB isolates were identified, whereas genotypes had variable stability to NFNB resistance at the adult stages. Five genotypes, AM-68, AM-95, AM-250, AM-267 and AM-322, were resistant to both NFNB isolates at the seedling stage. There were significant (p < .001) effects of genotype (G) and G × E interaction on NFNB severity for barley genotypes at the adult stage. The principal components, IPCA1 and IPCA2, accounted for 48.4% and 18.7% variation for NFNB severity, respectively. The AMMI stability values (ASVs) ranged from 0.01 to 15.5, and fifty-nine barley genotypes had stable responses (ASV ≤ 0.05) across all seven environments. Specifically, two stable genotypes, AM-187 and AM-244, had lower mean NFNB severities across all environments, suggesting a quantitative resistance in these genotypes. Divergent environmental responses of NFNB severity were measured in Sidi El Ayedi 2015 and Sidi Allal Tazi 2016, suggesting that these environments may be suitable to capture resistance to diverse pathotypes. These stable genotypes are valuable resources for introgression of both qualitative resistance and quantitative resistance to NFNB in future.     

Genome-wide association studies of net form of net blotch resistance at seedling and adult plant stages in spring barley collection

Net form of net blotch (NFNB) of barley (Hordeum vulgare L.), caused by Pyrenophora teres f. teres (Ptt) Drechsler (anamorph: Drechslera teres [Sacc.] Shoem.), is considered one of the major constraints of successful barley production in major barley growing regions of the world. Resistance to NFNB was evaluated in a barley collection of 336 genotypes (AM-2014), at seedling stage using isolates LGDPtt.19 and TD10 in the USA, and adult stage in seven hotspot environments in Morocco. The AM-2014 panel was genotyped with 9K SNP markers and genome-wide association studies (GWAS) were carried out using mixed linear model (MLM: Q?+?K) accounting for population structure (Q) and kinship (K) as covariates. Significant (P?<?0.001) marker trait associations were corrected for false discovery rate (FDR) at the q?<?0.05. Four genotypes showed an average infection response (IRs ≤ 2) to both isolates, LGDPttt.19 and TD10, at the seedling stage, and 30 genotypes showed resistance in all environments in the field while three genotypes exhibited the highest resistance at both stages. The GWAS of NFNB identified 31 distinct QTLs on all seven barley chromosomes, of which 8 with resistance at seedling stage, 21 were associated with resistance at the adult stage, and two QTLs, QRptt.2H-132.15 and QPtt.6H-54-55, conferred resistance at both stages. Of 31 resistance QTLs reported in this study, 10 QTLs coincided with previously mapped QTL while 21 are novel, thereby validating the GWAS approach used in this study. The resistance sources identified in AM-2014 and QTL mapped in this study are valuable resources for marker-assisted breeding for NFNB resistance in the future.     

Comprehensive genetic analyses reveal differential expression of spot blotch resistance in four populations of barley

Spot blotch, caused by Cochliobolus sativus, is an important disease of barley in the Upper Midwest region of the United States. The resistance of six-rowed malting cultivars like Morex has remained effective for over 40 years and is considered durable. Previous research on Steptoe/Morex (S/M), a 6×6-rowed doubled haploid (DH) population, showed that seedling resistance is controlled by a single gene (Rcs5) on chromosome 1(7H) and adult plant resistance by two quantitative trait loci (QTL): one of the major effect on chromosome 5(1H) explaining 62% of the phenotypic variance and a second of minor effect on chromosome 1(7H) explaining 9% of the phenotypic variance. To corroborate these results in a 2×6-rowed DH population, composite interval mapping (CIM) was performed on Harrington/Morex (H/M). As in the S/M population, a single major gene (presumably Rcs5) on chromosome 1(7H) conferred resistance at the seedling stage. However, at the adult plant stage, the results were markedly different as no chromosome 5(1H) effect whatsoever was detected. Instead, a QTL at or near Rcs5 on chromosome 1(7H) explained nearly all of the phenotypic variance (75%) for disease severity. To determine whether this result might be due to the genetic background of the two-rowed susceptible parent Harrington, we analyzed another DH population that included the same resistance donor (Morex) and another six-rowed susceptible cultivar Dicktoo (D/M). Three QTL conferred seedling resistance in the D/M population: one near Rcs5 on chromosome 1(7H) explaining 30%, a second near the centromere of chromosome 1(7H) explaining 9%, and a third on the short arm of chromosome 3(3H) explaining 19% of the phenotypic variation. As in the H/M population, no chromosome 5(1H) QTL was detected for adult plant resistance in the D/M population. Instead, three QTL on other chromosomes explained most of the variation: one on the short arm of chromosome 3(3H) explaining 36%, a second on the long arm of chromosome 3(3H) explaining 11%, and a third at or near Rcs5 on chromosome 1(7H) explaining 20% of the phenotypic variation. These data demonstrate the complexity of expression of spot blotch resistance in different populations and have important implications in breeding for durable resistance.     

The complex quantitative barley–<Emphasis Type="Italic">Rhynchosporium secalis</Emphasis> interaction: newly identified QTL may represent already known resistance genes

Two barley populations, i.e. 135 doubled haploid (DH) lines of the cross 'Igri' (rrs1) x 'Triton' (Rrs1) (I x T) and 76 DH lines of the cross 'Post' x 'Vixen' (both rrs1) (P x V), were analysed to identify QTL for Rhynchosporium secalis resistance independent of the Rrs1 locus by using the single spore R. secalis isolate 271 (Rrs1-virulent). A major QTL with its positive allele derived from cv. 'Triton' was detected in the I x T population on chromosome 2HS explaining almost 80% of the phenotypic variance. Thus, it can be considered as an R-gene corresponding to the already described Rrs15(CI8288) on chromosome 2HS. In addition, two minor QTL were identified, one in the centromeric region of 6H in a highly polymorphic region with already several mapped R-genes and a second one at the end of the short arm of chromosome 7H which may be an allele of Rrs2 because of its chromosomal position. Regarding the DH population P x V different minor QTL were identified on chromosomes 6H and 7H. The first one is corresponding to the genomic region of the Rrs13 gene whereas the QTL on chromosome 7H maps in a genomic region where several R-genes against different pathogens have been localized. A comparison of both QTL analyses reveals no R. secalis isolate 271-specific resistance locus but leads to the hypothesis that two of the identified QTL may be alleles of the R-genes Rrs15(CI8288) and Rrs2.     

Genetics of seedling and adult plant resistance to net blotch (Pyrenophora teres f. teres) and spot blotch (Cochliobolus sativus) in barley

Net blotch (caused by Pyrenophora teres f. teres) and spot blotch (Cochliobolus sativus) are important foliar diseases of barley in the midwestern region of the USA. To determine the number and chromosomal location of Mendelian and quantitative trait loci (QTL) controlling resistance to these diseases, a doubled haploid population (Steptoe/Morex) was evaluated to the pathogens at the seedling stage in the greenhouse and at the adult plant stage in the field. Alleles at two or three unlinked loci were found to confer resistance to the net blotch pathogen at the seedling stage depending on how progeny exhibiting an intermediate infection response were classified. This result was corroborated in the quantitative analysis of the raw infection response data as 2 major QTL were identified on chromosomes 4 and 6M. A third QTL was also identified on chromosome 6P. Seven QTL were identified for net blotch resistance at the adult plant stage and mapped to chromosomes 1P, 2P, 3P, 3M, 4, 6P, and 7P. The 7 QTL collectively accounted for 67.6% of the phenotypic variance under a multiple QTL model. Resistance to the spot blotch pathogen was conferred by a single gene at the seedling stage. This gene was mapped to the distal region of chromosome 1P on the basis of both qualitative and quantitative data analyses. Two QTL were identified for spot blotch resistance at the adult plant stage: the largest QTL effect mapped to chromosome 5P and the other mapped to chromosome 1P near the seedling resistance locus. Together, the 2 QTL explained 70.1% of the phenotypic variance under a multiple QTL model. On the basis of the chromosomal locations of resistance alleles detected in this study, it should be feasible to combine high levels of resistance to both P. teres f. teres and C. sativus in barley cultivars.     

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.     

Genetic analysis of net form net blotch resistance in barley lines CIho 5791 and Tifang against a global collection of <Emphasis Type="Italic">P. teres</Emphasis> f. <Emphasis Type="Italic">teres</Emphasis> isolates

Key message

A CIho 5791 × Tifang recombinant inbred mapping population was developed and used to identify major dominant resistance genes on barley chromosomes 6H and 3H in CI5791 and on 3H in Tifang.

Abstract

The barley line CIho 5791 confers high levels of resistance to Pyrenophora teres f. teres, causal agent of net form net blotch (NFNB), with few documented isolates overcoming this resistance. Tifang barley also harbors resistance to P. teres f. teres which was previously shown to localize to barley chromosome 3H. A CIho 5791 × Tifang F₆ recombinant inbred line (RIL) population was developed using single seed descent. The Illumina iSelect SNP platform was used to identify 2562 single nucleotide polymorphism (SNP) markers across the barley genome, resulting in seven linkage maps, one for each barley chromosome. The CIho 5791 × Tifang RIL population was evaluated for NFNB resistance using nine P. teres f. teres isolates collected globally. Tifang was resistant to four of the isolates tested whereas CIho 5791 was highly resistant to all nine isolates. QTL analysis indicated that the CIho 5791 resistance mapped to chromosome 6H whereas the Tifang resistance mapped to chromosome 3H. Additionally, CIho 5791 also harbored resistance to two Japanese isolates that mapped to a 3H region similar to that of Tifang. SNP markers and RILs harboring both 3H and 6H resistance will be useful in resistance breeding against NFNB.

     

Necrotrophic effector‐triggered susceptibility (NETS) underlies the barley–Pyrenophora teres f. teres interaction specific to chromosome 6H

Barley net form net blotch (NFNB), caused by the necrotrophic fungus Pyrenophora teres f. teres, is a destructive foliar disease in barley‐growing regions worldwide. Little is known about the genetic and molecular basis of this pathosystem. Here, we identified a small secreted proteinaceous necrotrophic effector (NE), designated PttNE1, from intercellular wash fluids of the susceptible barley line Hector after inoculation with P. teres f. teres isolate 0–1. Using a barley recombinant inbred line (RIL) population developed from a cross between the sensitive/susceptible line Hector and the insensitive/resistant line NDB 112 (HN population), sensitivity to PttNE1, which we have named SPN1, mapped to a common resistance/susceptibility region on barley chromosome 6H. PttNE1–SPN1 interaction accounted for 31% of the disease variation when the HN population was inoculated with the 0–1 isolate. Strong accumulation of hydrogen peroxide and increased levels of electrolyte leakage were associated with the susceptible reaction, but not the resistant reaction. In addition, the HN RIL population was evaluated for its reactions to 10 geographically diverse P. teres f. teres isolates. Quantitative trait locus (QTL) mapping led to the identification of at least 10 genomic regions associated with disease, with chromosomes 3H and 6H harbouring major QTLs for resistance/susceptibility. SPN1 was associated with all the 6H QTLs, except one. Collectively, this information indicates that the barley–P. teres f. teres pathosystem follows, at least partially, an NE‐triggered susceptibility (NETS) model that has been described in other necrotrophic fungal disease systems, especially in the Dothideomycete class of fungi.     

Marker-trait associations in Virginia Tech winter barley identified using genome-wide mapping

Genome-wide association studies (GWAS) provide an opportunity to examine the genetic architecture of quantitatively inherited traits in breeding populations. The objectives of this study were to use GWAS to identify chromosome regions governing traits of importance in six-rowed winter barley (Hordeum vulgare L.) germplasm and to identify single-nucleotide polymorphisms (SNPs) markers that can be implemented in a marker-assisted breeding program. Advanced hulled and hulless lines (329 total) were screened using 3,072 SNPs as a part of the US. Barley Coordinated Agricultural Project (CAP). Phenotypic data collected over 4 years for agronomic and food quality traits and resistance to leaf rust (caused by Puccinia hordei G. Otth), powdery mildew [caused by Blumeria graminis (DC.) E.O. Speer f. sp. hordei Em. Marchal], net blotch (caused by Pyrenophora teres), and spot blotch [caused by Cochliobolus sativus (Ito and Kuribayashi) Drechsler ex Dastur] were analyzed with SNP genotypic data in a GWAS to determine marker-trait associations. Significant SNPs associated with previously described quantitative trait loci (QTL) or genes were identified for heading date on chromosome 3H, test weight on 2H, yield on 7H, grain protein on 5H, polyphenol oxidase activity on 2H and resistance to leaf rust on 2H and 3H, powdery mildew on 1H, 2H and 4H, net blotch on 5H, and spot blotch on 7H. Novel QTL also were identified for agronomic, quality, and disease resistance traits. These SNP-trait associations provide the opportunity to directly select for QTL contributing to multiple traits in breeding programs.     

Quantitative trait loci controlling barley powdery mildew and scald resistances in two different barley doubled haploid populations

Powdery mildew and scald can cause significant yield loss in barley. In order to identify new resistance genes for powdery mildew and scald in barley, two barley doubled haploid (DH) populations were screened for adult plant resistance in the field and glasshouse under natural infection. The mapping populations included 92 DH lines from the cross of TX9425 × Franklin and 177 DH lines from the cross of Yerong × Franklin. Two quantitative trait loci (QTL) for resistance to powdery mildew were identified in the TX9425 × Franklin population. These QTL were mapped to chromosomes 7H and 5H, respectively. The phenotypic variation explained by the two QTL detected in this population was 22 and 17%, respectively. Three significant QTL were identified from the Yerong × Franklin population for the resistance to powdery mildew; the major one, detected on the short arm of chromosome 1H, explained 66% of phenotypic variation. The major QTL for scald resistance, identified from two different populations which shared a common parent, Franklin, were mapped in the similar position on 3H. However, the Franklin allele provided resistance to one population but susceptibility to the other population. The Yerong allele on 3H showed much better resistance to scald than the Franklin allele, which has not been reported before. Using high-density maps for both populations, some markers which were very close to the resistance genes were identified. Transgression beyond the parents in disease resistances of the DH populations indicates that both small-effect QTLs and genetic background may also have significant contributions towards the resistance.     

Inheritance and localisation of resistance to <Emphasis Type="Italic">Mycosphaerella graminicola</Emphasis> causing septoria tritici blotch and plant height in the wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) genome with DNA markers

Resistance to the disease septoria tritici blotch of wheat (Triticum aestivum L.), caused by the fungus Mycosphaerella graminicola (Fuckel.) J. Schrot in Cohn (anamorph Septoria tritici Roberge in Desmaz.) was investigated in a doubled-haploid (DH) population of a cross between the susceptible winter wheat cultivar Savannah and the resistant cultivar Senat. A molecular linkage map of the population was constructed including 76 SSR loci and 244 AFLP loci. Parents and DH progeny were tested for resistance to single isolates of M. graminicola in a growth chamber at the seedling stage, and to an isolate mixture at the adult plant stage, in field trials. A gene located at or near the Stb6 locus mapping to chromosome 3A provided seedling resistance to IPO323. Two complementary genes, mapping to chromosome 3A, one of which was the IPO323 resistance gene, were needed for resistance to the Danish isolate Ris?97-86. In addition, a number of minor loci influenced the expression of resistance in the growth chamber. In the field, four QTLs for resistance to septoria tritici blotch were detected. Two QTLs, located on chromosomes 3A and 6B explained 18.2 and 67.9% of the phenotypic variance in the mean over two trials. Both these QTLs were also detected at the seedling stage with isolate Ris?97-86, whereas isolate IPO323 only detected the QTL on 3A. Additionally, two QTLs identified in adult plants on chromosomes 2B and 7B were not detected at the seedling stage. Four QTLs were detected for plant height located on chromosomes 2B, 3A, 3B and on a linkage group not assigned to a chromosome. The major QTLs on 3A and on the unassigned linkage group were consistent over two trials, and the QTL on 3A seemed to be linked to a QTL for septoria tritici blotch resistance.     

Genetic and molecular analyses of resistance to a variant of Puccinia striiformis in barley

Seedlings of 62 Australian barley cultivars and two exotic barley genotypes were assessed for resistance to a variant of Puccinia striiformis, referred to as “Barley Grass Stripe Rust” (BGYR), first detected in Australia in 1998, which is capable of infecting wild Hordeum species and some genotypes of cultivated barley. Fifty-three out of 62 cultivated barley cultivars tested were resistant to the pathogen. Genetic analyses of seedling resistance to BGYR in six Australian barley cultivars and one Algerian barley landrace indicated that they carried either one or two major resistance genes to the pathogen. A single recessive seedling resistance gene, rpsSa3771, identified in Sahara 3771, was located on the long arm of chromosome 1 (7 H), flanked by the restriction fragment length polymorphism (RFLP) markers Xwg420 and Xcdo347 at genetic distances of 12.8 and 21.9 cM, respectively. Mapping resistance to BGYR at adult plant growth stages using the doubled haploid (DH) population Clipper × Sahara 3771 identified two major quantitative trait loci (QTL), one on the long arm of chromosome 3 (3 H) and the second on the long arm of chromosome 1 (7 H), accounting for 26 % and 18 % of the total phenotypic variation, respectively. The QTL located on chromosome 7HL corresponded to seedling resistance gene rpsSa3771 and the second QTL was concluded to correspond to a single APR gene, designated rpsCl, contributed by cultivar Clipper.