Interspecific hybrids of Zinnia elegans Jacq. and Z. angustifolia HBK: embryology,morphology and powdery mildew resistance

Summary Reciprocal crosses between 13 cultivars of Zinnia elegans Jacq. and one source of Z. angustifolia HBK resulted in interspecific hybrids. Florets examined 14 days after pollination showed that all 13 cross combinations of Z. angustifolia x Z. elegans yielded embryos in 47.2% of the florets pollinated. Six cross combinations of Z. elegans x Z. angustifolia produced embryos in 4.6% of the total Z. elegans florets pollinated. Interspecific crosses resulted in seed-propagated hybrids when Z. angustifolia was the maternal parent. Hybrids were obtained from reciprocal crosses between the two species when embryos were excised and cultured. Morphologically, most hybrids resembled Z. angustifolia but were sterile. Colchicine treatments of selected sterile hybrids resulted in shoots producing fertile inflorescences. Preliminary tests showed that hybrids were resistant to powdery mildew (Erysiphe cichoracearum DC ex Mecat).Scientific Article No. A 3108, Contribution No. 6173 of the Maryland Agricultural Experiment Station, Department of Horticulture.     

Genetics of powdery mildew resistance in pea

The genetics of powdery mildew (Erysiphe pisi) resistance in pea (Pisum sativum) was studied using five isolates of E. pisi in a controlled environment. Resistance was controlled by a single recessive gene in all the pea cvs. included in the study. The same recessive gene controlled resistance to all the five isolates of E. pisi. Tests for allelism showed that resistance in cvs. HPPC-63, HPPC-95, DPP-26, DPP-54, Mexique-4, SVP-950, Wisconsin-7104 and P-6588 is conferred by the same recessive gene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Differences in resistance of wild Lactuca species to natural infection of lettuce powdery mildew (Erysiphe cichoracearum)

Summary The natural infection of Erysiphe cichoracearum on 29 accessions of five wild Lactuca species (L. serriola, L. saligna. L. virosa, L. aculeata and L. dentata) and on one hybrid of L. serriola x L. sativa has been investigated for two and three years, respectively. No infection was observed on L. serriola (PI 255665), L. saligna (LSA/92/1 and LSA/92/2), L. virosa (LVIR/26 and LVIR/57/1) and L. dentata (PI 234204). The level of susceptibility of the control cultivars of L. sativa was found to be moderate (Fila) to fairly high (Hilde).     

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.     

Identification and inheritance of a new gene for powdery mildew resistance in mungbean (Vigna radiata L. Wilczek)

Powdery mildew (PM) is one of the important foliar diseases of mungbean. Resistance sources have been identified in India and the inheritance studies showed that complete resistance (RO) was controlled by two dominant genes, Pm1 , Pm2 . The breakdown of complete resistance (RO) into moderate resistance (R2) by race-2 (Akola) has been reported. It is assumed that the change in resistance reaction is due to a mutation in the pathogen. The present investigation was carried out with a view to screen germplasm, cultivars and mutants for identification of complete resistance (RO) sources against race-2 and to study their inheritance. 'Mulmarada', a local mungbean cultivar from Maharashtra state of India was identified as a complete resistance (RO) source for race-2. The inheritance of Mulmarada's resistance (RO) was studied. The F₁ and the segregation in F₂ and F₃ showed that the complete resistance (RO) in 'Mulmarada' is controlled by a single dominant gene, which is different from the earlier identified Pm1 and Pm2 resistance genes. Mulmarada's resistance gene is designated as Pm3 for PM resistance.     

Genetics and histology of powdery mildew resistance in apple

Summary Eight scab-resistant cultivars and selections along with eight commercial apple cultivars were evaluated for powdery mildew resistance in greenhouse and nursery tests. Dayton, Liberty, Delicious and Tolman Sweet were rated moderately resistant to infection in both greenhouse and nursery tests. Segregation of seedlings among 14 progenies for mildew reaction indicated that mildew resistance is polygenically controlled in this material with additive gene effects. Recovery of mildew resistant seedlings from crosses involving a scab-resistant parent(s) suggested that this material can be useful in developing scab- and mildew-resistant apple cultivars. Histological investigations were conducted to describe mildew symptoms of infected leaves.     

Identification of growth stage dependent expression of partial resistance of barley to powdery mildew

Summary Genotypic differences for growth stage dependent expression of partial resistance to barley powdery mildew have been identified on the basis of two components of resistance in the glasshouse and the pattern of epidemic development in the field.Differences for infection frequency and proportion of sporulating colonies were highly significant between the 16 genotypes investigated at four stages of plant development in the glasshouse. Both resistance components were significantly correlated mutually (r=0.73 to 0.86) and with the infection level on leaves developed at comparable growth stages in the field (r=0.52 to 0.73). The infection level of seedlings in the glasshouse was significantly correlated with the infection level of the leaves of the first until the third node in the field (r=0.70 to 0.73). Adult plant resistance was predominantly expressed at the uppermost leaf. A clear genotype × growth stage interaction was apparent for infection level; some genotypes showed partial resistance predominantly at the seedling stage and others predominantly at the adult plant stage.Abbreviations IF Infection Frequency, number of colonies per cm² leaf area, glasshouse experiment - IL Infection Level, number of colonies per cm² leaf area, field experiment - PSC Proportion Sporulating Colonies from total number of colonies, glasshouse experiment     

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.     

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.     

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.     

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.     

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.     

Shading of plants facilitates selection for powdery mildew resistance in Squash

Summary Powdery mildew development was assessed on squash (Cucurbita pepo) plants of a susceptible cultivar, a resistant accession, their F₁, and their F₂ in an early summer planting in the field, covered or not covered with a shading net. Three reaction types were observed: susceptible, powdery mildew on stems and on both, the upper and lower leaf surfaces, as in the susceptible parent; resistant, no powdery mildew on leaves or stems, as in the resistant parent; and partially resistant, powdery mildew on upper leaf surfaces only, as in the F₁. Disease presence on the stem was associated with susceptibility. Shading hastened the appearance of powdery mildew and increased the severity of infection on partially resistant and susceptible plants, facilitating identification of resistant individuals in the F₂ population.Contribution No. 1613-E from the Agricultural Research Organization, Bet Dagan, Israel     

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.     

Qualitative resistance to powdery mildew in hybrid sweet peas

Lathyrus odoratus L. × Lathyrus belinensis L. hybrids were produced using L. belinensis as the pollen parent, with fertile seed produced by the L. odoratus parent. The F1 hybrid plants were completely self-sterile, but produced viable seeds when backcrossed to L. odoratus. The plants produced by backcrossing resembled L. odoratus, the flower colour being purple/magenta, and were self-fertile. Both hybrid plants and those produced by back crossing to L. odoratus were resistant to Erysiphe pisi DC that causes powdery mildew in sweet peas. Continued backcrossing resulted in hybrid plants, that closely resembled the L. odoratus parent, but segregated for complete resistance/susceptibility to E. pisi,with a ratio of 2.46:1 resistant to sensitive plants. This suggests the presence of a single dominant gene that confers resistance. When resistant and sensitive plants were inoculated with E. pisi and their leaf surfaces examined,using a Scanning Electron Microscope, it was found that although spores germinated on the leaves of both resistant and sensitive plants, spores present on resistant plants collapsed shortly after germination. Possible reasons for this are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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).     

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.     

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.     

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.     

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.