The transfer of a powdery mildew resistance gene from Hordeum bulbosum L to barley (H. vulgare L.) chromosome 2 (2I)

Hordeum bulbosum L. is a source of disease resistance genes that would be worthwhile transferring to barley (H. vulgare L.). To achieve this objective, selfed seed from a tetraploid H. vulgare x H. bulbosum hybrid was irradiated. Subsequently, a powdery mildew-resistant selection of barley phenotype (81882/83) was identified among field-grown progeny. Using molecular analyses, we have established that the H. bulbosum DNA containing the powdery mildew resistance gene had been introgressed into 81882/83 and is located on chromosome 2 (2I). Resistant plants have been backcrossed to barley to remove the adverse effects of a linked factor conditioning triploid seed formation, but there remains an association between powdery mildew resistance and non-pathogenic necrotic leaf blotching. The dominant resistance gene is allelic to a gene transferred from H. bulbosum by co-workers in Germany, but non-allelic to all other known powdery mildew resistance genes in barley. We propose Mlhb as a gene symbol for this resistance.     

AB-QTL analysis in spring barley: III. Identification of exotic alleles for the improvement of malting quality in spring barley (<Emphasis Type="Italic">H. vulgare</Emphasis> ssp. <Emphasis Type="Italic">spontaneum</Emphasis>)

Malting quality is genetically determined by the complex interaction of numerous traits which are expressed prior to and, in particular, during the malting process. Here, we applied the advanced backcross quantitative trait locus (AB-QTL) strategy (Tanksley and Nelson, Theor Appl Genet 92:191–203, 1996), to detect QTLs for malting quality traits and, in addition, to identify favourable exotic alleles for the improvement of malting quality. For this, the BC₂DH population S42 was generated from a cross between the spring barley cultivar Scarlett and the wild barley accession ISR42-8 (Hordeum vulgare ssp. spontaneum). A QTL analysis in S42 for seven malting parameters measured in two different environments yielded 48 QTLs. The exotic genotype improved the trait performance at 18 (37.5%) of 48 QTLs. These favourable exotic alleles were detected, in particular, on the chromosome arms 3HL, 4HS, 4HL and 6HL. The exotic allele on 4HL, for example, improved α-amylase activity by 16.3%, fermentability by 0.8% and reduced raw protein by 2.4%. On chromosome 6HL, the exotic allele increased α-amylase by 16.0%, fermentability by 1.3%, friability by 7.3% and reduced viscosity by 2.9%. Favourable transgressive segregation, i.e. S42 lines exhibiting significantly better performance than the recurrent parent Scarlett, was recorded for four traits. For α-amylase, fermentability, fine-grind extract and VZ45 20, 16, 2 and 26 S42 lines, respectively, surpassed the recurrent parent Scarlett. The present study hence demonstrates that wild barley does harbour valuable alleles, which can enrich the genetic basis of cultivated barley and improve malting quality traits.     

Chemical-induced resistance against powdery mildew in barley: the effects of chitosan and benzothiadiazole

Chitosan (CHT), a deacetylated chitin derivative, and benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a non toxic synthetic functional analogue of salicylic acid, were applied as foliar spray to barley plants (Hordeum vulgare L.), to compare their effectiveness in inducing resistance against Blumeria graminis f. sp. hordei and to investigate the underlying defence response. After an induction phase of 3 days (IP, time elapsed between treatment and fungal inoculation) both compounds reduced significantly the infection on the primary leaf, namely of 55.5% for CHT and of 68.9% for BTH, showing the induction of a good level of local resistance (LAR). A 5-day IP further reduced the infected areas in BTH treated plants (−77.2%) but not in CHT treated ones (−47.1%). Furthermore, both CHT and BTH also induced SAR, being the infection in the second non treated leaves reduced of 57% and 76.2%, respectively, as evaluated at 10-day IP. Both BTH and CHT induced oxidative burst and phenolic compound deposition in treated leaves, creating an hostile environment that slowed down the fungal spreading by impairing haustorium development. However, the greater efficacy of BTH was possibly due to: i) a greater reinforcement of papilla; ii) a higher level and the more homogeneous diffusion of H₂O₂ in the treated leaf tissues and iii) an induced hypersensitive-like response in many penetrated cells.     

Transfer of a dominant gene for powdery mildew resistance and DNA from Hordeum bulbosum into cultivated barley (H. vulgare)

Summary In an attempt to transfer traits of agronomic importance from H. bulbosum into H. vulgare we carried out crosses between four diploid barley cultivars and a tetraploid H. bulbosum. Eleven viable triploid F₁ plants were produced by means of embryo rescue techniques. Meiotic pairing between H. vulgare and H. bulbosum chromosomes was evidenced by the formation of trivalents at a mean frequency of 1.3 with a maximum of five per cell. The resulting triploid hybrids were backcrossed to diploid barley, and nine DC₁ plants were obtained. Three of the BC₁ plants exhibited H. bulbosum DNA or disease resistance. A species specific 611-bp DNA probe, pSc119.2, located in telomeres of the H. bulbosum genome, clearly detected five H. bulbosum DNA fragments of about 2.1, 2.4, 3.4, 4.0 and 4.8 kb in size present in one of the BC₁ plants (BC₁-5) in BamHI-digésted genomic Southern blots. Plant BC₁-5 also contained a heterozygous chromosomal interchange involving chromosomes 3 and 4 as identified by N-banding. One of the two translocated chromosomes had the H. bulbosum sequence in the telomeric region as detected using in situ hybridization with pSc119.2. Two other BC₁ plants (BC₁-1 and BC₁-2) were resistant to the powdery mildew isolates to which the barley cultivars were susceptible. Seventy-nine BC₂ plants from plant BC₁-2 segregated 32 mildew resistant to 47 susceptible, which fits a ratio of 11, indicating that the transferred resistance was conditioned by a single dominant gene. Reciprocal crosses showed a tendency towards gametoselection that was relative to the resistance. Mildew resistant plant BC₁-2 also had a 1-kb H. bulbosum DNA fragment identified with a ten-base random primer using polymerase chain reaction (PCR). Forty-three BC₁ plants, randomly sampled from the 79 BC₁ plants, also segregated 2320 for the presence versus absence of this 1-kb H. bulbosum DNA fragment, thereby fitting a 11 ratio and indicating that the PCR product originated from a single locus. The 1-kb DNA fragment and disease resistance were independently inherited as detected by PCR analysis of bulked DNA from 17 resistant and 17 susceptible plants as well as by trait segregation in the 43 individual plants. The progenies produced could serve as an important resistant source in plant breeding. This is the first conclusive report of the stable transfer of disease resistance and DNA from H. bulbosum to H. vulgare.     

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.     

RFLP mapping of three new loci for resistance genes to powdery mildew (Erysiphe graminis f. sp. hordei) in barley

Three new major, race-specific, resistance genes to powdery mildew (Erysiphe graminis f. sp. hordei) were identified in three barley lines, RS42-6*O, RS137-28*E, and HSY-78*A, derived from crosses with wild barley (Hordeum vulgare ssp. spontaneum). The resistance gene origining from wild barley in line RS42-6*O, showed a recessive mode of inheritance, whereas the other wild barley genes were (semi)-dominant. RFLP mapping of these three genes was performed in segregating F₂ populations. The recessive gene in line RS42-6*O, was localized on barley chromosome 1S (7HS), while the (semi)-dominant genes in lines RS137-28*E, and HSY-78*A, were localized on chromosomes 1L (7HL) and 7L (5HL), respectively. Closely linked RFLP clones mapped at distances between 2.6cM and 5.3 cM. Hitherto, specific loci for powdery mildew resistance in barley had not been located on these chromosomes. Furthermore, tests for linkage to the unlocalized resistance gene Mlp revealed free segregation. Therefore, these genes represent new loci and new designations are suggested: mlt (RS42-6*O), Mlf (RS137-28*E), and Mlj (HSY-78*A). Comparisons with mapped QTLs for mildew resistance were made and are discussed in the context of homoeology among the genomes of barley (H-vulgare), wheat (Triticum aestivum), and rye (Secale cereale). Duplications of RFLP bands detected in the neighbourhood of Mlf and mlt might indicate an evolutionary interrelationship to the Mla locus for mildew resistance.     

Patterns of post-infectional protein synthesis in barley carrying different genes for resistance to the powdery mildew fungus

Summary Pairs of susceptible and resistant, near-isogenic cultivars ofHordeum vulgare which differ for the Mla, Mlk and Mlp genes for resistance toErysiphe graminis f. sp.hordei were inoculated with race 3 of this pathogen and patterns of protein synthesis associated with primary infection mapped using pulse-labelling with L-[³⁵S]methionine and 2-dimensional electrophoresis. Extraction of proteins with buffer containing detergent revealed the enhanced synthesis of 5 and 8 polypeptides at 25 and 30 h respectively after inoculation of barley carrying the Mla gene (cvMla). The enhanced synthesis of these same polypeptides together with 11 additional polypeptides was observed at 48 h and 72 h after inoculation of barley carrying either the Mlp (cvMlp) or Mlk (cvMlk) genes. The labelling of several major constitutive polypeptides was suppressed in cvMla at 24 h after inoculation; the labelling of six of these polypeptides was also suppressed in both cvMlp and cvMlk but not until 48 and 72 h after inoculation. These results indicate that changes occur in the synthesis of some common polypeptides following infection of cultivars carrying different resistance genes but the timing and extent of these changes varies with the resistance gene in the host.     

Sources of powdery mildew resistance in barley landraces from morocco

A total of forty eight accessions of barley landraces from Morocco were screened for resistance to powdery mildew. Twenty two (46%) of tested landraces showed resistance reactions and thirty four single plant lines were selected. Eleven of these lines were tested in seedling stage with seventeen and another twenty three lines with twenty three isolates of powdery mildew respectively. The isolates were chosen according to the virulence spectra observed on the ‘Pallas’ isolines differential set. Line 229–2–2 was identified with resistance to all prevalent in Europe powdery mildew virulence genes. Lines 230–1–1, 248–1–3 showed susceptible reaction for only one and lines 221–3–2, 227–1–1, 244–3–4 for only two isolates respectively. Three different resistance alleles (Mlat, Mla6, and MLA14) were postulated to be present in tested lines alone or in combination. In thirty (88%) tested lines it was impossible to determine which specific gene or genes for resistance were present. Most probably these lines possessed alleles not represented in the ‘Pallas’ isolines differential set. The distribution of reaction type indicated that about 71% of all reaction types observed were classified as powdery mildew resistance (scores 0, 1 and 2). Majority (79%) of resistance reaction types observed in tested lines was intermediate resistance reaction type two and twenty three lines (68%) showed this reaction for inoculation with more than 50% isolates used. The use of new effective sources of resistance from Moroccan barley landraces for diversification of resistance genes for powdery mildew in barley cultivars was discussed.     

Mapping of Rym14 Hb,a gene introgressed from Hordeum bulbosum and conferring resistance to BaMMV and BaYMV in barley

Hordeum bulbosum represents the secondary gene pool of barley and constitutes a potential source of various disease resistances in barley breeding. Interspecific crosses of H. vulgare × H. bulbosum resulted in recombinant diploid-barley progeny with immunity to BaMMV after mechanical inoculation. Tests on fields contaminated with different viruses demonstrated that resistance was effective against all European viruses of the soil-borne virus complex (BaMMV, BaYMV-1, -2). Genetic analysis revealed that resistance was dominantly inherited. Marker analysis in a F5 mapping family was performed to map the introgression in the barley genome and to estimate its size after several rounds of recombination. RFLP anchor-marker alleles indicative of an H. bulbosum introgression were found to cover an interval 2.9 cM in length on chromosome 6HS. The soil-borne virus resistance locus harboured by this introgressed segment was designated Rym14^Hb. For marker-assisted selection of Rym14^Hb carriers, a diagnostic codominant STS marker was derived from an AFLP fragment amplified from leaf cDNA of homozygous-resistant genotypes inoculated with BaMMV.Communicated by F. Salamini     

Allele-specific amplification of polymorphic sites for the detection of powdery mildew resistance loci in cereals

Primers for the polymerase chain reaction (PCR) were tailored to selectively amplify RFLP marker alleles associated with resistance and susceptibility for powdery mildew in cereals. The differentiation between marker alleles for susceptible and resistant genotypes is based on the discrimination of a single nucleotide by using allele-specific oligonucleotides as PCR primers. The PCR assays developed are diagnostic for RFLP alleles at the loci MWG097 in the barley genome and Whs350 in the wheat genome. The first marker locus is closely linked to MlLa resistance in barley, while the latter is linked to Pm2 resistance locus in wheat. PCR analysis of 31 barley and 30 wheat cultivars, with some exceptions, verified the presence or absence of the resistance loci investigated. These rapid PCR-based approaches are proposed as an efficient alternative to conventional procedures for selecting powdery mildew-resistant genotypes in breeding programs.     

Inheritance of partial resistance to powdery mildew in spring wheat

Summary Four spring wheat (Triticum aestivum L.) cultivars exhibiting partial resistance to powdery mildew induced by Erysiphe graminis f.sp. tritici were crossed to a common susceptible cultivar to study the inheritance of resistance. The genetic parameters contributing to resistance were estimated by generation means analyses. Additive gene action was the most important genetic component of variation among generation means in all four crosses. Additive by additive effects were significant in one cross and both additive by additive and additive by dominance effects were significant in another. Dominance effects were not significant. The F2/F3 correlations in three crosses ranged from 0.27 to 0.43. Three additional crosses among resistant cultivars were employed to study the effectiveness of selection in improving resistance. By selecting the most resistant plants from the F2 and evaluating the progenies in the F4, increases in resistance ranging from 21% to 31% were obtained. In all crosses, there was transgressive segregation in both directions indicating that the genes conferring resistance to these cultivars differ and exhibit additive effects.     

Genetic characterization of powdery mildew resistance in U.S. hard winter wheat

Powdery mildew significantly affects grain yield and end-use quality of winter wheat in the southern Great Plains. Employing resistance resources in locally adapted cultivars is the most effective means to control powdery mildew. Two types of powdery mildew resistance exist in wheat cultivars, i.e., qualitative and quantitative. Qualitative resistance is controlled by major genes, is race-specific, is not durable, and is effective in seedlings and in adult plants. Quantitative resistance is controlled by minor genes, is non-race-specific, is durable, and is predominantly effective in adult plants. In this study, we found that the segregation of powdery mildew resistance in a population of recombinant inbred lines developed from a cross between the susceptible cultivar Jagger and the resistant cultivar 2174 was controlled by a major QTL on the short arm of chromosome 1A and modified by four minor QTLs on chromosomes 1B, 3B, 4A, and 6D. The major QTL was mapped to the genomic region where the Pm3 gene resides. Using specific PCR markers for seven Pm3 alleles, 2174 was found to carry the Pm3a allele. Pm3a explained 61% of the total phenotypic variation in disease reaction observed among seedlings inoculated in the greenhouse and adult plants grown in the field and subjected to natural disease pressure. The resistant Pm3a allele was present among 4 of 31 cultivars currently being produced in the southern Great Plains. The genetic effects of several minor loci varied with different developmental stages and environments. Molecular markers associated with these genetic loci would facilitate incorporating genetic resistance to powdery mildew into improved winter wheat cultivars.     

C-banding pattern and powdery mildew resistance of Triticum ovatum and four T. aestivum-T. ovatum chromosome addition lines

Summary C-banding patterns of T. ovatum (Ae. ovata) and four T. aestivum cv Poros-T. ovatum chromosome addition lines are presented, and the added chromosomes of T. ovatum have been identified. Furthermore, nucleolar activity and powdery mildew resistance were analyzed in the Poros-ovatum addition lines and compared to that of T. ovatum and T. aestivum cv Poros. The addition lines II, III and IV and Poros were highly susceptible to powdery mildew isolates nos. 8 and 9, whereas the addition lines VI₁ and VI₂ showed high resistance. Even for an Ml-k virulent isolate, these two lines were highly resistant. By combining the cytological results and those of the powdery mildew analysis, the added chromosomes of T. ovatum can be excluded from responsibility for the high powdery mildew resistance of the addition lines VI₁ and VI₂. The same is true for a modified chromosome 6B, which is present in the Poros-ovataum addition lines II, III and VI. The high variation in C-banding pattern observed in the A-, B- and D-genome complement of the addition lines is believed to be the result of crossing different lines of T. aestivum instead of Poros alone. Thus, we cannot trace the powdery mildew resistance back to a specific chromosome.     

A barley RFLP map: alignment of three barley maps and comparisons to Gramineae species

Several gene linkage maps have been produced for cultivated barley. We have produced a new linkage map for barley, based on a cross between Hordeum vulgare subsp. spontaneum and Hordeum vulgare subsp. vulgare (Hvs x Hvv), having a higher level of polymorphism than most of the previous barley crosses used for RFLP mapping. Of 133 markers mapped in the Hvs x Hvv F₂ population, 69 were previously mapped on other barley maps, and 26 were mapped in rice, maize, or wheat. Two known gene clones were mapped as well as two morphological markers. The distributions of previously mapped markers were compared with their respective barley maps to align the different maps into one consensus map. The distributions of common markers among barley, wheat, rice and maize were also compared, indicating colinear linkage groups among these species.To be considered dual first authorsPublished with the approval of the Director of the Colorado State University/Agricultural Experiment Station.     

Correlation between hordatine accumulation, environmental factors and genetic diversity in wild barley (Hordeum spontaneum C. Koch) accessions from the Near East Fertile Crescent

Wild barley shows a large morphological and phenotypic variation, which is associated with ecogeographical factors and correlates with genotypic differences. Diversity of defense related genes and their expression in wild barley has been recognized and has led to attempts to exploit genes from H. spontaneum in breeding programs. The aim of this study was to determine the variation in the accumulation of hordatines, which are Hordeum-specific preformed secondary metabolites with strong and broad antimicrobial activity in vitro, in 50 accessions of H. spontaneum from different habitats in Israel. Differences in the accumulation of hordatines in the seedling stage were significant between different H. spontaneum genotypes from different regional locations and micro-sites. Variation in the hordatine accumulation within genotypes was between 9% and 45%, between genotypes from the same location between 13% and 38%, and between genotypes from different locations up to 121%. Principal component analysis showed that water related factors explain 39%, temperature related factors explain 33% and edaphic factors account for 11% of the observed variation between the populations of H. spontaneum. Genetic analysis of the tested accessions with LP-PCR primers that are specific for genes involved in the biosynthetic pathway of hordatines showed tight correlations between hordatine abundance and genetic diversity of these markers. Multiple regression analyses indicated associations between genetic diversity of genes directly involved in hordatine biosynthesis, ecogeographical factors and the accumulation of hordatines.     

Plastid genome characterisation in Brassica and Brassicaceae using a new set of nine SSRs

The objective of the present study was to identify favourable exotic Quantitative Trait Locus (QTL) alleles for the improvement of agronomic traits in the BC₂DH population S42 derived from a cross between the spring barley cultivar Scarlett and the wild barley accession ISR42-8 (Hordeum vulgare ssp. spontaneum). QTLs were detected as a marker main effect and/or a marker × environment interaction effect (M × E) in a three-factorial ANOVA. Using field data of up to eight environments and genotype data of 98 SSR loci, we detected 86 QTLs for nine agronomic traits. At 60 QTLs the marker main effect, at five QTLs the M × E interaction effect, and at 21 QTLs both the effects were significant. The majority of the M × E interaction effects were due to changes in magnitude and are, therefore, still valuable for marker assisted selection across environments. The exotic alleles improved performance in 31 (36.0%) of 86 QTLs detected for agronomic traits. The exotic alleles had favourable effects on all analysed quantitative traits. These favourable exotic alleles were detected, in particular on the short arm of chromosome 2H and the long arm of chromosome 4H. The exotic allele on 4HL, for example, improved yield by 7.1%. Furthermore, the presence of the exotic allele on 2HS increased the yield component traits ears per m² and thousand grain weight by 16.4% and 3.2%, respectively. The present study, hence, demonstrated that wild barley does harbour valuable alleles, which can enrich the genetic basis of cultivated barley and improve quantitative agronomic traits.     

Genetic transfer of resistance to powdery mildew and of an associated biochemical marker from Aegilops ventricosa to hexaploid wheat

Summary Resistance to powdery mildew, caused by the fungus Erysiphe graminis f.sp. tritici, has been transferred from Aegilops ventricosa (genomes D^vM^v) to hexaploid wheat (Triticum aestivum, ABD). In two transfer lines, H-93-8 and H-93-35, the resistance gene was linked to a gene encoding protein U-1, whereas one line, H-93-33, was resistant but lacked the molecular marker, and another line, H-93-1, was susceptible but carried the gene for U-1, indicating that the original M^v chromosome from Ae. ventricosa, carrying the two genes, had undergone recombination with a wheat chromosome in the last two lines.     

Seedling resistance of wheat to the powdery mildew fungus,Erysiphe graminis f. sp. tritici. I. Resistance of first leaves

During primary infection by conidia ofErysiphe graminis f. sp.tritici, three mechanisms of resistance operate in first leaves of 8-day-old seedlings of both resistant and susceptible wheats. The first mechanism, operating at the penetration site, is responsible for the failure of penetrations attempted by primary germ tubes (PGT). The second mechanism is concerned with the abortion of haustoria in normal-appearing host cells. The third mechanism relates to the abortion of haustoria and the hypersensitivity of the penetrated host cells.With the inoculum-level of 19–24 conidia/mm², the three mechanisms together prevented 89.3 % of the attempted penetrations by PGT from producing normal haustoria in resistant wheat Purdue 5752C1-7-5-1 and 37.4 % in the susceptible wheat Vermillion. The first mechanism accounted for the prevention of 73.3 % of the attempted PGT penetrations on Purdue 5752C1-7-5-1 and 36 % on Vermillion. The second mechanism was responsible for stopping 19 % of all the successful penetrations in Purdue 5752C1-7-5-1 and 0.8 % in Vermillion. The third mechanism accounted for the failure of 41 % of all the successful penetrations in Purdue 5752C1-7-5-1 and 1.4% in Vermillion. Thirty-six hours after inoculation, 10.7% of all the attempted PGT penetrations appeared to be developing normally in first leaves of 8-day-old seedlings of resistant wheat Purdue 5752C1-7-5-1 as compared to 62.6 % in the susceptible wheat Vermillion.This appears to be the first report showing the relative effectiveness of various mechanisms of resistance concerning any powdery mildew fungus.     

Molecular mapping of a new gene in wild barley conferring complete resistance to leaf rust (Puccinia hordei Otth)

 A dominant gene conferring resistance to all known races of Puccinia hordei Otth was identified in two accessions of Hordeum vulgare ssp. spontaneum. Using restriction fragment length polymorphism (RFLP) markers the gene was mapped on chromosome 2HS in doubled-haploid populations derived from crosses of both accessions to the susceptible cultivar L94. Until now, complete leaf rust resistance was not known to be conditioned by genetic factors on this barley chromosome. Therefore, the designation Rph16 is proposed for the gene described in this study. A series of sequence tagged site (STS) and cleaved amplified polymorphic sequence (CAPS) markers were generated by conversion of RFLP probes which originate from the chromosomal region carrying the resistance gene. Two PCR-based markers were shown to co-segregate with the Rph16 gene in both populations thus providing the basis for marker-assisted selection. Received: 20 May 1998 / Accepted: 9 June 1998     

Chromosomal location of a gene suppressing powdery mildew resistance genes Pm8 and Pm17 in common wheat (Triticum aestivum L. em. Thell.)

The chromosomal location of a suppressor for the powdery mildew resistance genes Pm8 and Pm17 was determined by a monosomic set of the wheat cultivar Caribo. This cultivar carries a suppressor gene inhibiting the expression of Pm8 in cv Disponent and of Pm17 in line Helami-105. In disease resistance assessments, monosomic F₁ hybrids (2n=41) of Caribo x Disponent and Caribo x Helami-105 lacking chromosome 7D were resistant, whereas monosomic F₁ hybrids involving the other 20 chromosomes, as well as disomic F₁ hybrids (2n=42) of all cross combinations, were susceptible revealing that the suppressor gene for Pm8 and Pm17 is localized on chromosome 7D. It is suggested that genotypes without the suppressor gene be used for the exploitation of genes Pm8 and Pm17 in enhancing powdery mildew resistance in common wheat.