Fine mapping and comparative genomics integration of two quantitative trait loci controlling resistance to powdery mildew in a Spanish barley landrace

The intervals containing two major quantitative trait loci (QTL) from a Spanish barley landrace conferring broad spectrum resistance to Blumeria graminis were subjected to marker saturation. First, all the available information on recently developed marker resources for barley was exploited. Then, a comparative genomic analysis of the QTL regions with other sequenced grass model species was performed. As a result of the first step, 32 new markers were added to the previous map and new flanking markers closer to both QTL were identified. Next, syntenic integration revealed that the barley target regions showed homology with regions on chromosome 6 of rice (Oryza sativa), chromosome 10 of Sorghum bicolor and chromosome 1 of Brachypodium distachyon. A nested insertion of ancestral syntenic blocks on Brachypodium chromosome 1 was confirmed. Based on sequence information of the most likely candidate orthologous genes, 23 new barley unigene-derived markers were developed and mapped within the barley target regions. The assessment of colinearity revealed an inversion on chromosome 7HL of barley compared to the other three grass species, and nearly perfect colinearity on chromosome 7HS. This two-step marker enrichment allowed for the refinement of the two QTL into much smaller intervals. Inspection of all predicted proteins for the barley unigenes identified within the QTL intervals did not reveal the presence of resistance gene candidates. This study demonstrates the usefulness of sequenced genomes for fine mapping and paves the way for the use of these two loci in barley breeding programs.     

Resistance to powdery mildew in Spanish barley landraces is controlled by different sets of quantitative trait loci

Twenty-two landrace-derived inbred lines from the Spanish Barley Core Collection (SBCC) were found to display high levels of resistance to a panel of 27 isolates of the fungus Blumeria graminis that exhibit a wide variety of virulences. Among these lines, SBCC145 showed high overall resistance and a distinctive spectrum of resistance compared with the other lines. Against this background, the main goal of the present work was to investigate the genetic basis underlying such resistance using a doubled haploid population derived from a cross between SBCC145 and the elite spring cultivar Beatrix. The population was genotyped with the 1,536-SNP Illumina GoldenGate Oligonucleotide Pool Assay (Barley OPA-1 or BOPA1 for short), whereas phenotypic analysis was performed using two B. graminis isolates. A major quantitative trait locus (QTL) for resistance to both isolates was identified on the long arm of chromosome 6H (6HL) and accounted for ca. 60% of the phenotypic variance. Depending on the B. graminis isolate tested, three other minor QTLs were detected on chromosomes 2H and 7H, which explained less than 5% of the phenotypic variation each. In all cases, the alleles for resistance derived from the Spanish parent SBCC145. The position, the magnitude of the effect observed and the proportion of phenotypic variation accounted for by the QTL on 6HL suggest this is a newly identified locus for broad-based resistance to powdery mildew.     

Mapping quantitative trait loci for downy mildew resistance in pearl millet

Quantitative trait loci (QTLs) for resistance to pathogen populations of Scelerospora graminicola from India, Nigeria, Niger and Senegal were mapped using a resistant x susceptible pearl millet cross. An RFLP map constructed using F₂ plants was used to map QTLs for traits scored on F₄ families. QTL analysis was carried out using the interval mapping programme Mapmaker/QTL. Independent inheritance of resistance to pathogen populations from India, Senegal, and populations from Niger and Nigeria was shown. These results demonstrate the existence of differing virulences in the pathogen populations from within Africa and between Africa and India. QTLs of large effect, contributing towards a large porportion of the variation in resistance, were consistently detected in repeated screens. QTLs of smaller and more variable effect were also detected. There was no QTLs that were effective against all four pathogen populations, demonstrating that pathotype-specific resistance is a major mechanism of downy mildew resistance in this cross. For all but one of the QTLs, resistance was inherited from the resistant parent and the inheritance of resistance tended to be the result of dominance or over-dominance. The implications of this research for pearl millet breeding are discussed.     

Identification of a QTL decreasing yield in barley linked to Mlo powdery mildew resistance

A molecular marker map, including Mlo mildew resistance, of the spring barley cross Derkado (Mlo-resistant) × B83-12/21/5 (Mlo-susceptible) was scanned for yield QTLs to determine whether the association of Mlo resistance with reduced yield was due to linkage or pleiotropy. Over the mapped portion of the genome of the cross, the QTL with the greatest effect upon yield was located within a 22 cM region between mlo and the simple sequence repeat HVM67 on chromosome 4(4H). The association of Mlo resistance with lower yield was therefore due to a repulsion linkage. Analysis of yield component characters revealed QTL alleles for reduced grain number and earlier heading date in the same region, also associated with Mlo resistance. Genotyping of a range of cultivars and sources of Mlo resistance with the HVM67 simple sequence repeat showed that the Derkado HVM67 allele was rare as it was found only in one other cultivar and four land-races or sources of disease resistance. Grannenlose Zweizeilige, the source, and Salome, the carrier of Mlo resistance in Derkado, have the same HVM67 genotype, although Salome was a mixture of two genotypes. The entire mlo-HVM67 chromosomal segment from Grannenlose Zweizeilige is therefore thought to have been transmitted to Derkado, possibly through joint selection for Mlo resistance and early heading. L92, synonym EP79, was another source of Mlo resistance with the same HVM67 allele as Derkado but recombination must have occurred during the breeding of Atem as it possesses a different HVM67 allele which is present in all the other Mlo sources and cultivars surveyed. Abbreviations: GN, grains per main stem ear; HD, heading date; MSTGW, thousand grain weight derived from GN and MSY; MSY, yield of grain on the main stem; PY, yield of grain from the whole plot; sCIM, simplified compound interval mapping; SIM, simple interval mapping; SPY, single plant yield; S-SAP, sequence-specific amplification polymorphism; TGW, thousand grain weight derived from bulk of plot seed; TN, number of fertile stems per plant.     

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.     

Identification and mapping of a new powdery mildew resistance allele in the Chinese wheat landrace Hongyoumai

Powdery mildew (Pm) caused by Blumeria graminis f. sp. tritici (Bgt) is one of the world’s major wheat diseases and results in large grain yield losses. Discovery and utilization of Pm resistance genes constitute the most common strategy for wheat Pm control. Hongyoumai, a wheat landrace from Henan Province in China, has excellent resistance to infection by Bgt. In order to identify the basis of such Pm resistance, a segregating population was submitted to genetic analysis, which showed that Pm resistance in Hongyoumai was conferred by a single recessive resistance gene. This gene was temporarily named pmHYM. Molecular marker analysis, chromosomal location, resistance spectrum analysis, and an allelism test showed that pmHYM was located on the long arm of chromosome 7B (7BL), most likely representing a new recessive resistance gene allelic with Pm5e and mlXBD. By using 90-kb single-nucleotide polymorphism sequences (SNP) in the BLASTn analysis against the wheat 7BL genome sequence, 12 new simple sequence repeat (SSR) markers linked with pmHYM were developed to map pmHYM co-segregating with the marker Xmp1207 and between markers Xmp925 and Xmp1158, at genetic distances of 2.8 and 2.7 cM, respectively. In addition, physical mapping of the markers linked with pmHYM using Chinese Spring deletion lines indicated a location in the 0.86–1.00 bin of 7BL.     

Mapping resistance to powdery mildew in barley reveals a large-effect nonhost resistance QTL

Key message

Resistance factors against non-adapted powdery mildews were mapped in barley. Some QTLs seem effective only to non-adapted mildews, while others also play a role in defense against the adapted form.The durability and effectiveness of nonhost resistance suggests promising practical applications for crop breeding, relying upon elucidation of key aspects of this type of resistance. We investigated which genetic factors determine the nonhost status of barley (Hordeum vulgare L.) to powdery mildews (Blumeria graminis). We set out to verify whether genes involved in nonhost resistance have a wide effectiveness spectrum, and whether nonhost resistance genes confer resistance to the barley adapted powdery mildew. Two barley lines, SusBgt_SC and SusBgt_DC, with some susceptibility to the wheat powdery mildew B. graminis f.sp. tritici (Bgt) were crossed with cv Vada to generate two mapping populations. Each population was assessed for level of infection against four B. graminis ff.spp, and QTL mapping analyses were performed. Our results demonstrate polygenic inheritance for nonhost resistance, with some QTLs effective only to non-adapted mildews, while others play a role against adapted and non-adapted forms. Histology analyses of nonhost interaction show that most penetration attempts are stopped in association with papillae, and also suggest independent layers of defence at haustorium establishment and conidiophore formation. Nonhost resistance of barley to powdery mildew relies mostly on non-hypersensitive mechanisms. A large-effect nonhost resistance QTL mapped to a 1.4 cM interval is suitable for map-based cloning.

     

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.     

Identification of quantitative trait loci for race-nonspecific resistance to tan spot in wheat

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an economically important foliar disease in the major wheat growing areas throughout the world. Multiple races of the pathogen have been characterized based on their ability to cause necrosis and/or chlorosis on differential wheat lines. In this research, we evaluated a population of recombinant inbred lines derived from a cross between the common wheat varieties Grandin and BR34 for reaction to tan spot caused by Ptr races 1–3 and 5. Composite interval mapping revealed QTLs on the short arm of chromosome 1B and the long arm of chromosome 3B that were significantly associated with resistance to all four races. The effects of the two QTLs varied for the different races. The 1B QTL explained from 13% to 29% of the phenotypic variation, whereas the 3B QTL explained from 13% to 41% of the variation. Additional minor QTLs were detected but not associated with resistance to all races. The host-selective toxin Ptr ToxA, which is produced by races 1 and 2, was not a significant factor in the development of disease in this population. The race-nonspecific resistance derived from BR34 may take precedence over the gene-for-gene interaction known to be associated with the wheat–Ptr system.     

Field resistance of spring barley cultivars to powdery mildew in Lithuania

During vegetative period 2004–2005 powdery mildew (Erysiphe graminis DC. f. sp. hordei Em. Marchal) field resistance of spring barley cultivars was investigated at the Lithuanian Institute of Agriculture. The spring barley genotypes tested were Lithuania-registered cultivars, cultivars from genetic resources collection, and the new cultivars used for initial breeding. In total, 23 resistance genes were present in the 84 cultivars studied. Among mono-genes only mlo and 1-B-53 showed very high resistance. Slight powdery mildew necroses (up to 3 scores) formed on cultivars possessing these genes. The maximal powdery mildew (PM) severity reached a score of 8.5 and the area under disease progress curve (AUDPC) a value of 1216.8. The cultivars ‘Primus’, ‘Astoria’, ‘Power’, ‘Harrington’ and ‘Scarlett’ were the most resistant among the non mlo cultivars. Severity of PM on ‘Primus’ reached a score of 3.5 (3.0 of PM necrosis) in average, the other cultivars were diseased from 4.5 (3.0) to 5.0 (2.0). The AUDPC values for these cultivars except ‘Scarlett’ were the lowest (85.0–145.3) among the other cultivars. The highest contrast in development of the other leaf diseases was between highly resistant and susceptible to PM cultivar groups. The fast development of PM depressed development of the other diseases 4.7 times.     

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.

     

Molecular marker analyses of powdery mildew resistance in barley

Powdery mildew, caused byEryisphe graminis f. sp.hordei, is one of the most important diseases of barley (Hordeum vulgare). A number of loci conditioning resistance to this disease have been reported previously. The objective of this study was to use molecular markers to identify chromosomal regions containing genes for powdery mildew resistance and to estimate the resistance effect of each locus. A set of 28 F₁ hybrids and eight parental lines from a barley diallel study was inoculated with each of five isolates ofE. graminis. The parents were surveyed for restriction fragment length polymorphisms (RFLPs) at 84 marker loci that cover about 1100 cM of the barley genome. The RFLP genotypes of the F₁s were deduced from those of the parents. A total of 27 loci, distributed on six of the seven barley chromosomes, detected significant resistance effects to at least one of the five isolates. Almost all the chromosomal regions previously reported to carry genes for powdery mildew resistance were detected, plus the possible existence of 1 additional locus on chromosome 7. The analysis indicated that additive genetic effects are the most important component in conditioning powdery mildew resistance. However, there is also a considerable amount of dominance effects at most loci, and even overdominance is likely to be present at a number of loci. These results suggest that quantitative differences are likely to exist among alleles even at loci which are considered to carry major genes for resistance, and minor effects may be prevalent in cultivars that are not known to carry major genes for resistance.     

Quantitative trait loci for aluminum resistance in Chinese wheat landrace FSW

Aluminum (Al) toxicity is a major constraint for wheat production in acid soils worldwide. Chinese landrace FSW demonstrates a high level of Al resistance. A population of recombinant inbred lines (RILs) was developed from a cross between FSW and an Al-sensitive Chinese line, ND35, using single seed descent, to map quantitative trait loci (QTLs) for Al resistance. Wheat reaction to Al stress was measured by net root growth (NRG) in a nutrient solution culture containing Al(3+) and hematoxylin staining score (HSS) of root after Al stress. After 1,437 simple sequence repeats (SSRs) were screened using bulk segregant analysis, three QTLs were identified to control Al resistance in FSW. One major QTL (Qalt.pser-4DL) was mapped on chromosome 4DL that co-segregated with Xups4, a marker for the promoter of the Al-activated malate transporter (ALMT1) gene. The other two QTLs (Qalt.pser-3BL, Qalt.pser-2A) were located on chromosomes 3BL and 2A, respectively. Together, the three QTLs accounted for up to 81.9% of the phenotypic variation for HSS and 78.3% of the variation for NRG. The physical positions of flanking markers for Qalt.pser-4DL and Qalt.pser-3BL were determined by analyzing these markers in corresponding nulli-tetrasomic, ditelosomic, and 3BL deletion lines of Chinese Spring. Qalt.pser-3BL is a novel QTL with a major effect on Al resistance discovered in this study. The two major QTLs on 4DL and 3BL demonstrated an additive effect. The SSR markers closely linked to the QTLs have potential to be used for marker-assisted selection (MAS) to improve Al resistance of wheat cultivars in breeding programs.     

Identification of quantitative trait loci linked to Ceratocystis wilt resistance in cacao

Ceratocystis wilt (CW) in cacao (Theobroma cacao L.), caused by Ceratocystis cacaofunesta, is a drastic disease that results in plant death. The pathogen was recently identified in the major cacao-producing region of Brazil?CBahia. The identification of genetic markers tightly linked to disease resistance loci is a valuable tool for the development of resistant cultivars using marker-assisted selection (MAS). Branches of 143 six-year-old individuals of an F2 Sca 6?×?ICS 1 population were wounded by making a 3-mm deep cut with a sterile scalpel, and inoculated with a 20-??l drop of a spore suspension of 3?×?10⁴?CFU/ml. The inoculation method used allowed the population to be quantitatively phenotyped. The length of the xylem discoloration followed a continuous distribution. These results imply that the resistance was quantitatively inherited. Quantitative trait loci (QTL) analysis revealed two genomic regions (in linkage groups 3 and 9) associated with CW resistance. The QTL explained individually from 6.9 to 8.6?% of the phenotypic variation. The QTL identified are crucial for identifying genes for resistance and can be applied in the genetic breeding of cacao using MAS.     

Quantitative trait loci for resistance to fusarium head blight in a Chinese wheat landrace Haiyanzhong

Fusarium head blight (FHB) of wheat causes not only significant reduction in grain yield and end-use quality, but also the contamination of the grain with mycotoxins that are detrimental to human and animal health after consumption of infected grain. Growing resistant varieties is an effective approach to minimize the FHB damage. The Chinese wheat landrace Haiyanzhong (HYZ) shows a high level of resistance to FHB. To identify quantitative trait loci (QTL) that contribute to FHB resistance in HYZ, 136 recombinant inbred lines (RIL) were developed from a cross of HYZ and Wheaton, a hard spring wheat cultivar from the USA. The RIL and their parents were evaluated for percentage of scabbed spikelets (PSS) in both greenhouse and field environments. Five QTL were detected for FHB resistance in HYZ with one major QTL on 7DL. The 7DL QTL peaked at SSR marker Xwmc121, which is flanked by the SSR markers Xcfd46 and Xwmc702. This QTL explained 20.4?C22.6% of the phenotypic variance in individual greenhouse experiments and 15.9% in a field experiment. Four other minor QTL on 6BS (two QTL), 5AS and 1AS each explained less than 10% of the phenotypic variance in individual experiments. HYZ carried the favorable alleles associated with FHB resistance at the QTL on 7DL, 6BS and 5AS, and the unfavorable allele at the QTL on 1AS. The major QTL on 7D can be used to improve the FHB resistance in wheat breeding programs and add diversity to the FHB resistance gene pool.     

Identification of quantitative trait loci controlling resistance to maize chlorotic dwarf virus

Ineffective screening methods and low levels of disease resistance have hampered genetic analysis of maize (Zea mays L.) resistance to disease caused by maize chlorotic dwarf virus (MCDV). Progeny from a cross between the highly resistant maize inbred line Oh1VI and the susceptible inbred line Va35 were evaluated for MCDV symptoms after multiple virus inoculations, using the viral vector Graminella nigrifrons. Symptom severity scores from three rating dates were used to calculate area under the disease progress curve (AUDPC) scores for vein banding, leaf twist and tear, and whorl chlorosis. AUDPC scores for the F₂ population indicated that MCDV resistance was quantitatively inherited. Genotypic and phenotypic analyses of 314 F₂ individuals were compared using composite interval mapping (CIM) and analysis of variance. CIM identified two major quantitative trait loci (QTL) on chromosomes 3 and 10 and two minor QTL on chromosomes 4 and 6. Resistance was additive, with alleles from Oh1VI at the loci on chromosomes 3 and 10 contributing equally to resistance.     

Identification of resistance gene analogs linked to a powdery mildew resistance locus in grapevine

Oligonucleotide primers, designed to conserved regions of nucleotide binding site (NBS) motifs within previously cloned pathogen resistance genes, were used to amplify resistance gene analogs (RGAs) from grapevine. Twenty eight unique grapevine RGA sequences were identified and subdivided into 22 groups on the basis of nucleic acid sequence-identity of approximately 70% or greater. Representatives from each group were used in a bulked segregant analysis strategy to screen for restriction fragment length polymorphisms linked to the powdery mildew resistance locus, Run1, introgressed into Vitis vinifera L. from the wild grape species Muscadinia rotundifolia. Three RGA markers were found to be tightly linked to the Run1 locus. Of these markers, two (GLP1–12 and MHD145) cosegregated with the resistance phenotype in 167 progeny tested, whereas the third marker (MHD98) was mapped to a position 2.4 cM from the Run1 locus. The results demonstrate the usefulness of RGA sequences, when used in combination with bulked segregant analysis, to rapidly generate markers tightly linked to resistance loci in crop species. Received: 2 May 2001 / Accepted: 3 August 2001     

Quantitative trait loci analysis of powdery mildew disease resistance in the Arabidopsis thaliana accession kashmir-1.

Powdery mildew diseases are economically important diseases, caused by obligate biotrophic fungi of the Erysiphales. To understand the complex inheritance of resistance to the powdery mildew disease in the model plant Arabidopsis thaliana, quantitative trait loci analysis was performed using a set of recombinant inbred lines derived from a cross between the resistant accession Kashmir-1 and the susceptible accession Columbia glabrous1. We identified and mapped three independent powdery mildew quantitative disease resistance loci, which act additively to confer disease resistance. The locus with the strongest effect on resistance was mapped to a 500-kbp interval on chromosome III.     

Identification of molecular markers linked to quantitative trait loci for soybean resistance to corn earworm

 One hundred and thirty nine restriction fragment length polymorphisms (RFLPs) were used to construct a soybean (Glycine max L. Merr.) genetic linkage map and to identify quantitative trait loci (QTLs) associated with resistance to corn earworm (Helicoverpa zea Boddie) in a population of 103 F₂-derived lines from a cross of ‘Cobb’ (susceptible) and PI229358 (resistant). The genetic linkage map consisted of 128 markers which converged onto 30 linkage groups covering approximately 1325 cM. There were 11 unlinked markers. The F₂-derived lines and the two parents were grown in the field under a plastic mesh cage near Athens, Ga., in 1995. The plants were artificially infested with corn earworm and evaluated for the amount of defoliation. Using interval-mapping analysis for linked markers and single-factor analysis of variance (ANOVA), markers were tested for an association with resistance. One major and two minor QTLs for resistance were identified in this population. The PI229358 allele contributed insect resistance at all three QTLs. The major QTL is linked to the RFLP marker A584 on linkage group (LG) ‘M’ of the USDA/Iowa State University public soybean genetic map. It accounts for 37% of the total variation for resistance in this cross. The minor QTLs are linked to the RFLP markers R249 (LG ‘H’) and Bng047 (LG ‘D1’). These markers explain 16% and 10% of variation, respectively. The heritability (h²) for resistance was estimated as 64% in this population. Received: 15 October 1997 / Accepted: 4 November 1997     

Identification of QTLs for resistance to powdery mildew and SSR markers diagnostic for powdery mildew resistance genes in melon (Cucumis melo L.)

Powdery mildew caused by Podosphaera xanthii is an important foliar disease in melon. To find molecular markers for marker-assisted selection, we constructed a genetic linkage map of melon based on a population of 93 recombinant inbred lines derived from crosses between highly resistant AR 5 and susceptible ‘Earl’s Favourite (Harukei 3)’. The map spans 877 cM and consists of 167 markers, comprising 157 simple sequence repeats (SSRs), 7 sequence characterized amplified region/cleavage amplified polymorphic sequence markers and 3 phenotypic markers segregating into 20 linkage groups. Among them, 37 SSRs and 6 other markers were common to previous maps. Quantitative trait locus (QTL) analysis identified two loci for resistance to powdery mildew. The effects of these QTLs varied depending on strain and plant stage. The percentage of phenotypic variance explained for resistance to the pxA strain was similar between QTLs (R ² = 22–28%). For resistance to pxB strain, the QTL on linkage group (LG) XII was responsible for much more of the variance (41–46%) than that on LG IIA (12–13%). The QTL on LG IIA was located between two SSR markers. Using an independent population, we demonstrated the effectiveness of these markers. This is the first report of universal and effective markers linked to a gene for powdery mildew resistance in melon.