Molecular characterization of a novel powdery mildew resistance gene Pm30 in wheat originating from wild emmer

Powdery mildew caused by Erysiphe graminis f. sp. tritici is one of the most important wheat diseases in many regions of theworld. A powdery mildew resistance gene, originating from wild emmerwheat (Triticum dicoccoides) accession `C20', from Rosh Pinna, Israel,was successfully transferred to hexaploid wheat through crossing andbackcrossing. Genetic analysis indicated that a single dominant genecontrols the powdery mildew resistance at the seedling stage. SegregatingBC₁F₂ progenies of the cross 87-1/C20//2*8866 wereused for bulked segregant analysis (BSA). The PCR approach was used togenerate polymorphic DNA fragments between the resistant and susceptibleDNA pools by use of 10-mer random primers, STS primers, and wheatmicrosatellite primers. Three markers, Xgwm159/430,Xgwm159/460, and Xgwm159/500, were found to be linked tothe resistance gene. After evaluating the polymorphic markers in twosegregating populations, the distance between the markers and the mildewresistance gene was estimated to be 5–6 cM. By means of ChineseSpring nullisomic-tetrasomics and ditelosomics, the polymorphic markersand the resistance gene were assigned to chromosome arm 5BS and werephysically mapped on the gene rich regions of fragment length (FL) 0.41–0.43 by Chinese Spring deletion lines. As no powdery mildew resistancegene has been reported on chromosome arm 5BS, the mildew resistancegene originating from C20 should be a new gene and is designated Pm30.     

Development and characterization of a new 1BL.1RS translocation line with resistance to stripe rust and powdery mildew of wheat

The 1BL.1RS wheat-rye translocation from Petkus rye has contributed substantially to the world wheat production. However, following the breakdown of disease resistance genes in 1RS, its importance for wheat improvement decreased. We have developed a new 1BL.1RS line, R14, by means of crossing rye inbred line L155, selected from Petkus rye to several wheat cultivars. One new gene each, for stripe rust and powdery mildew resistance, located on 1RS of the line R14, are tentatively named YrCn17 and PmCn17. YrCn17 and PmCn17 confer resistance to Puccinia striiformis f. sp. tritici pathotypes that are virulent on Yr9, and Blumeria graminis f. sp. tritici pathotypes virulent on Pm8. These two new resistances, YrCn17 and PmCn17, are now available for wheat improvement programs. The present study indicates that rye cultivars may carry yet untapped variations as potential sources of resistance.     

Hordeum chilense resistance to powdery mildew and its potential use in cereal breeding

Summary Hordeum chilense is a wild barley with high crossability with Triticum, Hordeum and Secale. Its amphiploid with wheat, tritordeum, has potential as a new crop. H. chilense is highly resistant to the powdery mildew diseases of both wheat and barley. Whereas tritordeum is resistant to barley powdery mildew, its reaction to wheat powdery mildew is similar to that of its wheat parent. However H. chilense contributes to a reduced density of mildew colonies. This quantitative resistance of tritordeum is diluted at higher ploidy levels.     

Molecular mapping of powdery mildew resistance genes in wheat: A review

Powdery mildew, caused by Blumeria graminis f. sp. tritici (syn. Erysiphe graminis f. sp. tritici), is one of the most important diseases of common wheat (Triticum aestivum L.) worldwide. Molecular mapping and cloning of genes for resistance to powdery mildew in hexaploid wheat will facilitate the study of molecular mechanisms underlying resistance to powdery mildew diseases and help understand the structure and function of powdery mildew resistance genes, and permit marker-assisted selection in breeding programs. So far, 48 genes/alleles for resistance to powdery mildew at 32 loci have been identified and located on 16 different chromosomes, of which 21 resistance genes/alleles have been tagged by restriction fragment length polymorphisms (RFLPs), random-amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), sequence characterized amplified regions (SCARs), sequence-tagged sites (STS) or simple sequence repeats (SSRs). Several quantitative trait loci (QTLs) for adult plant resistance (APR) to powdery mildew have been associated with molecular markers. The detailed information on chromosomal location and molecular mapping of these genes has been reviewed. Isolation of powdery mildew resistance genes and development of valid molecular markers for pyramiding resistance genes in breeding programs is also discussed.     

Genetic linkage of the Yr1 and Pm4 genes for stripe rust and powdery mildew resistances in wheat

Summary Genes Yr1 for resistance to stripe rust and Pm4a for resistance to powdery mildew showed linkage of 2.0±0.6 cM. Close repulsion linkage probably accounts for the absence in European wheats of genes Yr1 and Pm4b in combination.     

Conservation of powdery mildew resistance genes in three composite cross populations of barley

Summary Barley powdery mildew was used as a model to evaluate the potential of barley composite cross populations for conservation of disease resistance. The objective was to determine if increases in resistance to powdery mildew could be detected over periods of time in composite cross populations developed in California, where the disease might have had a selective influence on the populations, and the same populations grown in Montana, where no selective influence of powdery mildew was expected. Four isolates of Erysiphe graminis f. sp. hordei were used to monitor the frequencies of plants with specific mildew resistances through early, intermediate and late generations of three composite cross populations (CCII, CCV, CCXII) grown at Davis, California, and Bozeman and Moccasin, Montana. Changes in frequencies of plants resistant to the four isolates were observed between generations in all populations from the three locations. Trends in the frequencies of resistance are discussed in relation to selection pressure applied by E. graminis. It is suggested that associations with gene complexes other than resistance to E. graminis might help to explain the increased resistance observed in these studies.This research was funded in part by U.S. Agency for International Development Contract No. AID/DSAN-C-0024. The authors are grateful to Dr A. L. Kahler for seed of the composite cross populations and to Dr J. G. Moseman for the powdery mildew cultures.Cooperative investigations of the Agricultural Research Service, U.S. Department of Agriculture; the Department of Plant Pathology, Montana State University; and The Montana Agricultural Experiment Station. Journal Series Paper No. 1381 of the Montana Agricultural Experiment Station.     

Characterisation of new oat germplasm for resistance to powdery mildew

Summary Wild oat accessions from the Institute of Plant Genetics and Crop Plant Research (IPK) Gatersleben were selected as potential sources of resistance to powdery mildew (Erysiphe graminis f.sp. avenae). The Avena strigosa accessions AVE 128, AVE 488 and AVE 264, along with A. occidentalis accession CAV 3889 showed high levels of mildew resistance in tests at early and late growth stages over two years; the latter is a particularly useful source because it is a hexaploid species. Moderate levels of resistance were also found in A. fatua accession AVE 1981, AVE 2032 and A.sterilis accession AVE 1373 but this could be due to the later maturity of these genotypes.     

Assessment of partial resistance to powdery mildew in Chinese wheat varieties

Field trials in two cropping seasons and two locations in central China were conducted on 60 Chinese autumn‐sown wheat varieties to assess their partial resistance to powdery mildew. Mean levels of disease severity ranged from close to 0 to more than 90%. The method of inoculation and the location in which trials were conducted affected the relative performance of the varieties, but these effects were much smaller than the main effect of variety. The area under the disease progress curve was highly correlated with final disease severity, but both were poorly correlated with apparent infection rate. Disease severity was regressed against frequencies of virulence in the Blumeria graminis (syn. Erysiphe graminis) f sp. tritici populations in the trial plots. A vertical distance (D) from the mean mildew severity to the fitted line was calculated for each variety and was used to quantify partial resistance. Five of the 60 varieties, ‘Hx8541’, ‘E28547’, ‘Chuan1066’, ‘Zhe88pin6’ and ‘Lin5064’, consistently expressed relatively low levels of disease despite high frequencies of virulence in the pathogen and had consistently high D‐values. They may therefore have good levels of partial resistance.     

Identification of QTLs for powdery mildew and scald resistance in barley

A population of 103 recombinant inbred lines (RILs, F₉-derived lines) developed from the two-row spring barley cross L94 × ‘Vada’ was evaluated under field conditions for resistance against powdery mildew (Blumeria graminis f.sp. hordei) and scald (Rhynchosporium secalis). Apart from the major resistance gene mlo on chromosome 4 (4H), three QTLs (Rbgq1, Rbgq2 and Rbgq3) for resistance against powdery mildew were detected on chromosomes 2 (2H), 3 (3H), and 7 (5H), respectively. Rbgq1 and Rbgq2 have not been reported before, and did not map to a chromosome region where a major gene for powdery mildew had been reported. Four QTLs (Rrsq1, Rrsq2, Rrsq3 and Rrsq4) for resistance against scald were detected on chromosomes 3 (3H), 4 (4H) and 6 (6H). All four mapped to places where QTLs for scald resistance had been reported before in different populations.     

Spectrum of resistance conferred by ML-O powdery mildew resistance genes in barley

Summary Ten barley mutants and five Ethiopian barley lines representing 11 independently arisen powdery mildew resistance genes in the ml-o locus were tested at the seedling stage to cultures of the powdery mildew fungus from Europe, Israel, USA. Canada, and Japan. They were resistant with infection type 0/(4) in all tests. They were also resistant to field populations of the pathogen when scored in disease nurseries at more than 78 locations in 29 countries in Europe, the Near East, North and South America. New Zealand, and Japan. This indicates that the 11 genes confer the same, world-wide spectrum of powdery mildew resistance. They have no effect on several other barley diseases such as stripe rust and leaf rust.Part of the research reported here was carried out under IAEA Research Agreement No 1043 and Research Contract No 139-74-1 BIO DK with the European Atomic Energy Community.     

Field assessment of partial resistance to powdery mildew in spring barley

Summary Partial resistance to powdery mildew in spring barley was evaluated in three plot types: large isolation plots, in 1.4 m² plots in chessboard design with guard plots of spring wheat and in single rows. Percentage leaf area covered by powdery mildew was scored four to six times during the season and partial resistance was characterized by the area under the disease progress curve. Varietal differences were revealed in al three plot designs, differences between the most resistant and susceptible genotypes being of a factor five. Differences between varieties decreased with decreasing plot size. The relationship between single scores of amount of powdery mildew on the upper four leaves and the area under the disease progress curve was high in all plot designs during the first two to three weeks after heading, allowing selection for the trait by one or two scorings. Differential ranking of varieties between different plot designs was observed, and is assumed to be due to increasing plot interference with reduced plot size and reduced distance between plots. A reliable selection for partial resistance could be made in large isolation plots and in 1.4 m² plots, but hardly in single rows.     

The introduction into bread wheat of a major gene for resistance to powdery mildew from wild emmer wheat

Summary A new source of resistance to wheat powdery mildew caused by Erysiphe graminis has been transferred to hexaploid bread wheat, Triticum aestivum, from the wild tetraploid wheat, Triticum dicoccoides. The donor was crossed to bread wheat and the pentaploid progeny was then self-pollinated. Plants having a near stable hexaploid chromosome complement were selected in the F₃ progeny and topcrossing and backcrossing of these to a second wheat cultivar to improve the phenotype was undertaken. Monosomic analysis of early backcross lines showed the transferred gene to be located on chromosome 4A. The gene has been designated Pm16.     

The oat line Pc54 as a source of resistance to crown rust,stem rust and powdery mildew in Europe

Summary The oat line Pc54 was found to be resistant to powdery mildew under both field and glasshouse conditions. The ratio of resistant to susceptible F₂ and F₂ progeny of a cross between a selection from the Pc54 line (Cc7422) and a susceptible cultivar (Selma) showed that, in addition to carrying the crown rust resistance gene Pc54 and the pg15 gene for stem rust resistance, the mildew resistance of the Pc54 line was conditioned by a single incompletely dominant gene along with additional factors which modified the expression of resistance. Previous results, that there was no linkage between genes Pc54 and Pg15, were confirmed. In addition, there was no evidence of linkage between the mildew resistance gene and gene Pc54. Evaluation of selections from within the Pc54 line showed that the expression of both stem rust and mildew resistance was modified by, or linked to, plant height. The effectiveness of genes Pc54 and Pg15, as measured by virulence frequencies, in central and eastern Europe is described.     

Two unnecessary powdery mildew resistance genes in a synthetic rye population are neutral on fitness

A synthetic winter rye population was produced with two race-specific powdery mildew resistance genes, one dominant (Rm1) and the other (rm2) recessive, each at a frequency of about 0.50. The population was advanced by open-pollination in an isolated plot under mildew-free conditions for eight years. Samples of generations Syn-0 through Syn-7 were inoculated in the laboratory with two mildew isolates, one avirulent to either resistance gene, the other virulent to Rm1 and avirulent to rm2, to discriminate resistant and susceptible phenotypes. From the proportions of resistant plants, frequencies of Rm1 and rm2 were calculated and the fitness of carriers of resistance alleles was estimated in relation to carriers of susceptibility alleles at the two loci using continuous models and linear regression analyses. Frequencies of the two resistance genes oscillated only weakly over the eight generations. Coefficients of selection against Rm1-and rm2rm2 genotypes were –0.04 and –0.02, respectively, and not significantly different from zero. Thus the two resistance genes were selectively neutral. It is concluded that pyramiding of major powdery mildew resistance genes in rye varieties should not reduce their yield potential in the absence of mildew.     

Inheritance of resistance to powdery mildew (Erysiphe graminis f. sp. hordei) in eleven primitive barley varieties

The inheritance of the powdery mildew resistance of eleven primitive barley varieties was investigated. crosses with varieties with identified resistance genes revealed that at least three different genes are present in a group of six genotypes from Greece (Hor736, Hor847, Hor878, Hor1159, Hor1379 and Hor1873) and one from Turkey (Hor1188). One dominant gene is common to these seve genotypes and in the Greek accessions a second gene is present. The expression of this second gene was respectively dominant in the crosses with Hor736 and Hor1159, recessive in the crosses with Hor878 and Hor1873, and depended on the employed powdery mildew isolate in the crosses with Hor847 and Hor1379. The detected genes differ from the resistance genes of the crossing partners, viz. Ml-al2, Ml-(Ab), Ml-(CP) and Ml-(1402). The accessions from China (Hor824 and Hor4021) each possess two resistance genes which differ from Ml-a12, Ml-(La) and Ml-(CP). The major gene is common to both accessions and is either identical, allelic or closely linked to a gene in the variety Nigrate. The Columbian accession Hor1894 possesses one resistance gene linked to a resistance gene in Nigrate but this gene differs from the Ml-a locus. The variety Palestine (Hor3997) possesses two resistance genes of which one is allelic or closely linked to Ml-(at).     

Resistance to powdery mildew in selections from Moroccan barley landraces

Seventy-five barley landraces from Morocco were tested for resistance to powdery mildew and a number of different resistance genes were detected. Thirty-five isolates of Blumeria graminis f. sp. hordei and the Pallas isoline differential set were used. Isolates used in the experiment had virulences corresponding to all major resistance genes used in Europe. Forty-four of the tested landraces showed resistant reactions. From each of these landraces, one to five resistant plants were selected and 92 single plant lines were created. Six lines selected from 3landraces were assumed to carry the mlo gene but they were discarded after microscopic investigation. Seventeen lines were tested in the seedling stage with 17isolates and another 69 lines were tested with 23 differential isolates. These lines showed 71 reaction spectra to isolates of powdery mildew. Eight lines (9%), 255-3-3, 282-3-4, 286-1-1, 294-2-3,294-2-4, 295-1-2, 308-1-2 and 327-2-1, selected from 7 landraces showed resistance to all isolates. Seventy-eight lines (90%) showed a resistant infection type 2with more than 50% of the isolates used. In most of the selected lines (86%) unknown genes, alone or in combination with known specific resistance genes, were detected. Four different resistance alleles (Mlat, Mla6, Mla14 and Mla1) were postulated to be present in the tested lines. The most common was Mlat, which was postulated in 35 (41%) lines. The use of newly identified sources of powdery mildew resistance in barley breeding is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differences in partial resistance of barley to powdery mildew (Erysiphe graminis DC F.SP. hordei Marchal) after chemomutagenesis. II. Reaction of mutants to pathotypes

Summary Seedlings of 45 barley mutants with partial resistance in the field and their parental cultivars Asse, Bomi, and Vada were exposed to six local and two foreign races of powdery mildew, in climatically controlled cabinets. Infectability, pustule size, infection grade, and infection type were estimated. No mutant did demonstrate strong race-specific reactions although some race-specific interactions of moderate grade could be detected. The results of infection of mutants with single races confirmed the quantitative character of their change in resistance determined in the earlier field assessments. The 20 investigated mutants with increased resistance expressed a lower level of disease with each of the used races. Out of the 10 mutants with higher susceptibility in the field, 4 mutants again exhibited higher degrees of infection over all the different races, while the remaining 6 mutants were not distinguishable from the original cultivar under the given growing conditions. Out of the tested 14 developmentally resistant mutants, only the 5 genotypes with different seedling reaction could be analyzed for race-specificity in this study. In all the above cases as the result of one gene mutation, quantitative shifts in level of infection were recorded over all the 8 races.Qualitative estimates of infection type were supported by data on the frequency of chlorotic or necrotic lesions and green islands on the infected leaves. But quantitative methods of assessment, e.g. infection frequency and pustule size, were more effective in studying partial resistance of the mutants. Infection grade, estimated visually 14 days after inoculation, was in agreement with the quantitative parameters accurately measured 7 days after inoculation. Therefore, its careful use can be recommended to speed the screening by narrowing down the materials. Differences in virulence level of the races were observed and their influence on race-specificity studies was discussed.     

Characterisation and origin of rust and powdery mildew resistance genes in VPM1 wheat

Summary The expression of rust resistances conferred by closely linked genes derived from VPM1 varied with environmental conditions and with genetic backgrounds. Under low light and low temperature conditions seedlings carrying Yr17 showed susceptible responses. Stem rust and leaf rust resistance genes Sr38 and Lr37 tended to confer more resistance at 17±2° C than at normal temperatures above > 20° C. These studies supported the hypothesis that Yr17, Lr37 and Sr38 were derived from Aegilops ventricosa, whereas Pm4b was probably derived from T. persicum. Studies on certain addition lines and parental stocks indicated that wheat cytoplasm may enhance the expression of Sr38.     

The evaluation of adult plant resistance to powdery mildew (Erysiphe graminis f.sp. avenae) in transgressive lines of oats

Summary Previously identified segregant lines of oats with levels of adult plant resistance to powdery mildew (Erysiphe graminis f.sp. avenae) better than the resistant parent maintained this high level of resistance in field nurseries over two growing seasons. This enhancement was not expressed on inoculated detached leaves under laboratory conditions where no differences between the more resistant parent cv. Maldwyn and the most resistant segregants was detected. Reduced resistance was not detected in tests where leaf segments of the transgressive lines were inoculated with trained isolates. Problems associated with selecting for adult plant resistance under both field and laboratory conditions are discussed.