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.     

Discovery,characterization and exploitation of Mlo powdery mildew resistance in barley

Summary Mlo resistance to barley powdery mildew is a relatively new kind of resistance. It was originally described in a powdery mildew resistant barley mutant in 1942 and has been mutagen-induced repeatedly since then. About 1970 it was also recognized in barley landraces collected in Ethiopia in the 1930s. It is unique in that 1) Mlo resistance does not conform to the gene-for-gene system; 2)mlo genes originating from different mutational events map as non-complementing recessive alleles in one locus; 3) all alleles confer the same phenotype, though with small quantitative differences; 4) it is effective against all isolates of the pathogen; and 5) the resistance is caused by rapid formation of large cell wall appositions at the encounter sites preventing penetration by the fungus. Powdery mildew isolates with elevated Mlo aggressiveness have been produced on barley in the laboratory, but have not been found in nature. Mlo resistance is considered very durable. The exploitation of Mlo resistance has been hampered by pleiotropic effects of themlo genes, vix. necrotic leaf spotting and reduced grain yield, but they have been overcome by recent breeding work. During the 1980s Mlo-resistant spring barley varieties have become cultivated extensively in several European countries, in 1990 on about 700,000 ha.     

Diallel analysis of partial resistance to powdery mildew caused by Erysiphe graminis f. sp. hordei in spring barley (Hordeum vulgare L.)

Summary Six spring barley genotypes with growth stage dependent expression of partial resistance to powdery mildew were crossed following a half diallel scheme. Data for percentage infected leaf area at different growth stages from the F1 and F2 generation and the F2 offspring were analyzed. The absence of a contrast between parents and offspring indicated that dominance effects were not important at the young growth stages. Diallel analysis showed that general combining ability effects (GCA) were important at all growth stages. Although significant, specific combining ability effects were of minor importance. The cross between the most resistant parents with the largest, negative GCA resulted in the most resistant progeny.     

Detection of tolerance of barley cultivars to infection by powdery mildew (Erysiphe graminis f.sp. hordei)

Summary Barley genotypes representing a wide range of resistance expressions and origins, from major resistance genes in modern cultivars to field resistances in landraces, were assessed for tolerance to disease under glasshouse and field conditions. A few genotypes were picked out as showing less yield loss than would be expected from the level of mildew infection. Genotypes showing more than the expected yield loss were also found. The potential use of tolerance as a breeding character is discussed.     

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.     

Partial resistance to Erysiphe graminis hordei in old European barley varieties

Summary A collection of 133 genotypes consisting mainly of old European barleys and land varieties was assessed for level of partial resistance to barley mildew at the adult plant stage. Identification of any known resistance factors conferring hypersensitive reactions to the mildew races present at various frequencies in N.W. Europe was ensured through seedling tests with an appropriate AO mildew isolate.Thirty-nine varieties with relatively high levels of adult (partial) resistance and lacking known race-specific resistance genes were tested over a period of three years in field disease nurseries. At least thirteen varieties showed significantly less percentage leaf area infected than the control variety Proctor. These constitute a useful reserve of non-hypersensitive resistance, probably under the control of genes with mainly additive effects. In combination, these genes give enhanced levels of resistance, which could be of a more stable nature.     

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.     

Associations between three ml-o powdery mildew resistance genes and agronomic traits in barley

Summary A population of 198 chromosome-doubled haploid lines of spring barley was scored for segregation in locus ml-o (powdery mildew reaction) on chromosome 4 and in the linked loci s (rachilla hair length) and ddt (reaction to the insecticide DDT) on chromosome 7. They were also tested in a disease-free field trial for the agronomic traits: grain yield, thousand grain weight, lodging, and necrotic leaf spotting. The three mutagen-induced resistance genes ml-o5, ml-o6 (from Carlsberg II) and ml-10 (from Foma) showed no detectable differences with respect to effects on agronomic traits. They all conferred a four per cent reduction in grain yield caused mainly by lower thousand grain weight, and an increase in necrotic leaf spotting. The two original mutants of Carlsberg II had additional mutant genes affecting agronomic traits. Lines with gene S (long hair) had on average a three per cent higher thousand grain weight than those with s. The alleles in locus ddt showed no association with the agronomic traits. It is concluded i) that the associations between the three ml-o alleles and agronomic traits are caused by pleiotropy, ii) that ml-o resistant, high-yielding lines may be selected, and iii) that the association between gene s and thousand grain weight may be due to genetic linkage.Abbreviations DH-lines chromosome-doubled haploid lines     

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).     

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.     

Evaluation of sources of partial resistance to mildew in barley using enzyme-linked immunosorbent assay and other assessment methods

Summary Twenty-nine spring barley genotypes ranging from primitive landraces to modern cultivars were characterised for partial resistance to powdery mildew on detached leaves and in the field. Detached seedling and adult plant leaves were produced under spore-free glasshouse conditions and were assessed for infection frequency (IF) and then total mildew biomass using enzyme-linked immunosorbent assay (ELISA), and thereby biomass per colony (BMC), following controlled infection. Field plants were assessed visually for infection and for total mildew biomass using ELISA. Several genotypes were identified as having below average IF and BMC on both seedling and adult detached leaves and field scores of leaf area infected with mildew. IF at the seedling stage was highly correlated with IF at adult plant stage and with mildew biomass in the field. Adult plant IF was highly correlated with visual field scores. BMC data were often negatively correlated with other assessments.     

Resistance to powdery mildew in selections from Tunisian landraces of barley

Two-hundred and thirty-two accessions of barley landraces collected from Tunisia were screened for resistance to powdery mildew. A number of race-specific genes were detected using the detached leaf technique. Among the 232 accessions tested, 169 were susceptible to powdery mildew, 20 were resistant and 43 showed differential reactions to the three isolates of powdery mildew used. An attempt was made to determine the number of genes, the types of gene, the types of gene action and the gene loci in 20 resistant accessions. Three types of cross were made: (1) the accessions were crossed to the susceptible variety ‘Pallas’, (2) the accessions were crossed with ‘Pallas’ isolines, and (2) accessions with identical powdery mildew reaction patterns were intercrossed. Three isolates of Erysiphe graminis f. sp. hordei were used: Bzm-1, KM 18-75, R13C. A number of different resistance genes were detected among the 19 resistant accessions. Surprisingly, segregation indicating single genes only were detected with the isolates used. Some of these genes could be associated with loci already known. In 19 cases a dominant and in one a recessive mode of inheritance was detected. The recessive gene was not located at the mlo locus. This investigation represents the first systematic study of race-specific genes for powdery mildew resistance in Tunisian landraces. The newly identified sources of resistance may be used in many strategies of breeding for disease resistance.     

Pleiotropic effects on the ml-o mildew resistance gene in barley in different genetical backgrounds

Summary The effects on quantitative characters of the ml-o5 allele has been investigated by means of doubled haploid lines produced by the Hordeum bulbosum method. In one series of experiments the genetic background in each cross was found significantly to modify the negative pleiotropic effects. Doubled haploids were also produced from a F1 cross and tested in the field during two years, one with strong and one with weak mildew infections. In the first year only an insignificant reduction in grain yield was detected in the mlo lines, in the second it was significantly reduced with 5.9%. Thousand grain weights were, however, higher in the resistant lines. In both series of experiments the possibility to extract very high yielding mlo genotypes was demonstrated. The nature of the genetic background effect is discussed.     

EcoTILLING for the identification of allelic variation in the powdery mildew resistance genes mlo and Mla of barley

In this investigation the successful implementation of a CEL I‐based mutation detection technique for the discovery and detection of DNA polymorphism in the genes mlo and Mla of barley is described. The technique is called EcoTILLING, which is a high‐throughput method to detect and discover new point mutations and small insertions/deletions in DNA. That the method not only reveals polymorphism between different alleles but can also be used as a powerful genetic marker is demonstrated. The genes mlo and Mla are involved in the defence of barley against the fungal pathogen powdery mildew. The powdery mildew resistance gene mlo is a single copy gene, whereas multiple alleles exist at the Mla locus. With EcoTILLING it was possible to identify point mutations and deletions in each of the 11 mlo mutants tested. For Mla 25 natural barley variants were tested, and although the identification was complex due to the presence of highly similar paralogues of Mla, most of the recently identified alleles from Hordeum vulgare ssp. spontaneum were identified. This method offers the possibility to combine different mlo alleles with different Mla alleles from wild barley to obtain cultivars with more durable 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.     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. II. Field evaluation

Summary Eight lines from the cross between Vada and Cebada Capa with long to very long latent periods and four barley cultivars representing the known range of partial resistance to barley leaf rust, caused by Puccinia hordei, were evaluated in the field for partial resistance and in the greenhouse for the latent period (LP) in the young flag leaf.Each of the 12 entries was sown (15-4-1983) on a plot of 1.0 m². There were four replicates. To reduce interplot interference the plots were separated from each other by 4.0 m of spring rye. The number of urediosori per tiller was evaluated at 27-6, 4-7, 12-7 and ten days after heading. The LP was measured on 10 to 15 plants per entry in 1982 and on 10 plants in 1983.The levels of partial resistance varied greatly. The difference in number of sori per tiller between the most susceptible cultivar, Akka, and the most resistant cultivar, Vada, was about 50 times. Between Akka and the most resistant line this was approximately 5000 times. The LP's varied similarly. Vada had a LP 64% longer than that of Akka, the LP of line 26-6-11 was 15% longer. The range of partial resistance has been extended more than twofold.The correlation coefficients between LP and the level of barley leaf rust, expressed in transformed scale units, varied from -0.99 for the first sampling date to -0.97 for the third sampling date. Sampling the same development stage, ten days after heading, did not improve the r-value (r=–0.98). The LP evaluated in the young flag leaf is shown to be a very reliable criterion for partial resistance in the barley-Puccinia hordei pathosystem.Earliness tends to be associated with susceptibility. The correlation of days to heading with LP was 0.63, and with the level of barley leaf rust in the field 0.64.     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. I. Selection for increased latent periods

Summary The barley cultivar Cebaba Capa was crossed to the cultivar L94, which is assumed to carry no genes for increased latent periods, and Vada, which is assumed to carry five to six minor genes for a longer latent period (LP). In the F2 selection was carried out for short and long LP's in the young flag leaves to Puccinia hordei in both crosses. In the F3, F4 and F5 the selection for short as well as for long LP continued by selecting the extreme plants in the extreme lines, a typical pedigree selection approach.The LP's are given relative to those of L94, set at 100 and of Vada, set at 185. From the cross with L94 homogeneous lines were obtained with relative LP's of 100 and of 220. From the cross with Vada the extreme lines had LP's of 135 and around or even beyond 300.Cebaba Capa is thought to carry four to six minor genes with an average gene effect slightly larger than those of the five to six minor genes in Vada. From the four to six minor genes one or two may be identical to or closely linked with minor genes of Vada, the others appeared to be different. In the lines with LP's of close to 300 or even more the number of minor genes accumulated is thought to be in the order of eight or nine. These gene number estimates are based on independent assortment. If linkage occurs the number of genes involved may be larger.Because of the high correlation between LP in the young flag leaf and the partial resistance in the field the selected lines are assumed to have a partial resistance to barley leaf rust far beyond that of Vada, which represents almost the highest level of partial resistance in European cultivars.     

New molecular markers linked to qualitative and quantitative powdery mildew and scald resistance genes in barley for dry areas

A partial genetic linkage map was constructed on 71 doubled-haploid lines derived from a cross between the barley lines Tadmor and WI2291 with 181 molecular markers. The segregating population was used to detect markers linked to the gene Mlg conferring resistance to powdery mildew (Erysiphe graminis f. sp. hordei) and to genes for quantitative resistance to scald (Rhynchosporium secalis). The gene Mlg on chromosome 4H was flanked by two AFLP markers at a distance of 2.0 and 2.4 cM, respectively. QTLs for resistance to scald were detected on chromosomes 2H and 3H. This association of molecular markers with qualitative and quantitative disease resistance loci represents a valuable starting-point for marker-assisted selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of adult plant resistance to powdery mildew in pea (Pisum sativum L.)

Summary An analysis of adult plant resistance of powdery mildew in 15 F₁, F₂ and F₃ populations of pea derived from crossing 15 diverse and susceptible lines with one resistant line revealed that resistance to powdery mildew is controlled by duplicate recessive genes. The genes were designated as er₁ and er₂.Disease reaction showed independent segregation with three known markers in the resistant parent, namely, af (afila, chromosome 1), st (stipule reduced, chromosome 3) and tl (clavicula, chromosome 7).Contribution form the Department of Genetics and Plant Breeding Banaras Hindu University, Varanasi-221005, India.