Interaction between powdery mildew and barley with mlo5 mildew resistance

Powdery mildew infection of barley with the mlo5 barley powdery mildew resistance gene was examined, using near-isogenic barley lines, with and without mlo5 resistance, and two near-isogenic powdery mildew isolates, HL3/5 and GE3 with high (virulent) or low (avirulent) penetration efficiency on the resistant barley line. In all isolate–host combinations (except GE3 on the resistant barley line), frequency of haustorium formation increased significantly from zero at 11 h after inoculation to a maximum by 13 h, and there was no subsequent increase up to 24 h. In the susceptible barley line, 27% of appressoria from both isolates formed haustoria. Although this was significantly higher than the frequency of haustorium formation (18%) of HL3/5 on the resistant barley line, HL3/5 was much more successful than GE3 (frequency of haustorium formation less than 1%). The fact that HL3/5 did not possess a generally higher ability to penetrate successfully to form haustoria on the susceptible barley line, indicates that HL3/5 did not overcome the mlo5 resistance by being generally more vigorous. In the resistant barley line, papillae were larger than in the susceptible line; however, both isolates were associated with papillae of the same diameter at the time of penetration. We suggest that the mlo5 resistant barley line confers two different forms of resistance: isolate-specific and isolate-nonspecific.     

Mining wild barley for powdery mildew resistance

Powdery mildew, caused by Blumeria graminis f. sp. hordei (Bgh), is a worldwide disease problem on barley (Hordeum vulgare) with potentially severe impact on yield. Historically, resistance genes have been identified chiefly from cultivated lines and landraces; however, wild barley (H. vulgare subsp. spontaneum) accessions have proven to be extraordinarily rich sources of powdery mildew resistance. This study describes the characterization of a collection of 316 wild barley accessions, known as the Wild Barley Diversity Collection (WBDC), for resistance to powdery mildew and the genetic location of powdery mildew resistance loci. The WBDC was phenotyped for reaction to 40 different Bgh isolates at the seedling stage and genotyped with 10 508 molecular markers. Accessions resistant to all 40 isolates of Bgh were not found; however, three accessions (WBDC 053, 085 and 089) exhibited resistance to 38 of the isolates. Gene postulation analyses revealed that many accessions, while resistant, contained none of the 12 genes present in the Pallas near‐isogenic lines Mla1, Mla3, Mla6, Mla7, Mla9, Mla12, Mla13, Mlk1, MlLa, Mlg, Mlat and Ml(Ru2), suggesting that the accessions carry novel genes or gene combinations. A genome‐wide association study of powdery mildew resistance in the WBDC identified 21 significant marker‐trait associations that resolved into 15 quantitative trait loci. Seven of these loci have not been previously associated with powdery mildew resistance. Taken together, these results demonstrate that the WBDC is a rich source of powdery mildew resistance, and provide genetic tools for incorporating the resistance into barley breeding programmes.     

Effects of barley genotype on induced resistance to powdery mildew

Infection of barley by an avirulent isolate of Erysiphe graminis f.sp. hordei reduces the development of powdery mildew disease by a virulent isolate, inoculated subsequently. This phenomenon, known as induced resistance, was studied in three sets of near-isogenic lines of barley, each of which included varieties with four different race-specific resistance genes. The extent of induced resistance, expressed as a reduction in the number of colonies formed, differed in lines carrying different race-specific resistance genes, being most effective in lines with Mla7 , followed by Mth , then Mla6 and Mla13. These differences may be related to the extent of the hypersensitive response to infection by avirulent spores. A further effect of induced resistance in reducing sporulation, taking the number of colonies into account, was least effective in Mth lines, with the other lines ranking as they did when induced resistance was expressed as colony numbers. By contrast, the genetic backgrounds of the three near-isogenic sets had similar effects on the fractional reduction of colony number by induced resistance. The effect of induced resistance on sporulation was strongest in the most resistant background, and weakest in the most susceptible.     

Genotypic diversity in barley powdery mildew populations in northern France

Diversity in populations of Erysiphe graminis ( Blumeria graminis ) f.sp. hordei was studied with virulence and molecular markers. Isolates were sampled in two locations in northern France from a winter barley cultivar (Plaisant) and a spring cultivar (Caruso). Only a few pathotypes (determined by virulence markers) were common. The rest of the population was diverse. Diversity within common pathotypes, estimated by five RAPD and two SCAR markers, was generally high, except for one pathotype, which was frequent on Plaisant. This pathotype carried only one virulence, Va22, out of the 11 virulences tested. It appeared as a clonal lineage, which had occurred previously, at least in 1992, in northern France, demonstrating survival of asexual lineages in populations that often reproduce sexually.     

Characterization and chromosomal location of powdery mildew resistance genes from wild barley PI282605

Journal of Plant Diseases and Protection - The objective of this work was to find the identity of three resistance genes against powdery mildew by mapping in an F2 population derived from a cross...     

Seasonal changes in pathotype complexity in French populations of barley powdery mildew

Airborne conidia of Erysiphe graminis f.sp. hordei were sampled in three regions and a single locality in the northern part of France for 2 years. Sampling was carried out in early spring, in late spring and in autumn, in order to separate the effects of winter barley cultivars, carrying few specific resistance alleles, and of spring barley cultivars, carrying diverse resistance alleles, on the structure of the pathogen population. Although complex pathotypes with three to 10 virulences were selected by spring cultivars, simple pathotypes, including a pathotype with the single unnecessary virulence allele Va22 , which formed a clear majority of the samples, remained dominant in early spring, when winter but not spring cultivars were growing. In early spring, simple pathotypes were more prevalent in the north, where the winter cultivars represented 90% of the barley acreage, than in the east, where winter cultivars represented 65%. In the west, the frequency of simple pathotypes was limited compared to the north, possibly because of the resistance allele Mlg in winter cultivars. The high frequency of simple pathotypes in early spring could be explained by a differential adaptation between simple and complex pathotypes or by delayed epidemics on spring cultivars compared to winter cultivars.     

Factors affecting build-up of fungicide resistance in powdery mildew in spring barley

In replicate-plot field experiments done in the UK, at one site in Avon for 3 years and another in Warwickshire for 2 years, application of ethirimol or triadimenol sprays or seed treatments was followed by decreases in sensitivity of mildew samples to the particular fungicide applied. Application of ethirimol-triadimenol or tridemorph-triadimenol mixtures caused smaller or no decreases in sensitivity. Differences between isolates in responses to ethirimol and to triadimenol treatment were usually negatively correlated. Early-season inoculum differed in fungicide sensitivity between sites. At one site sensitivity shifted markedly from one season to another. No clear interactions between cultivar, mildew pathotype and shifts in fungicide response could be discerned. There were no major differences in resistance build-up between seed or spray treatments.     

Factors affecting build-up of fungicide resistance in powdery mildew in spring barley

In replicate-plot field experiments done in the UK, at one site in Avon for 3 years and another in Warwickshire for 2 years, application of ethirimol or triadimenol sprays or seed treatments was followed by decreases in sensitivity of mildew samples to the particular fungicide applied. Application of ethirimol-triadimenol or tridemorph-triadimenol mixtures caused smaller or no decreases in sensitivity. Differences between isolates in responses to ethirimol and to triadimenol treatment were usually negatively correlated. Early-season inoculum differed in fungicide sensitivity between sites. At one site sensitivity shifted markedly from one season to another. No clear interactions between cultivar, mildew pathotype and shifts in fungicide response could be discerned. There were no major differences in resistance build-up between seed or spray treatments.In veldexperimenten met herhalingen uitgevoerd in Engeland, gedurende een periode van 3 jaar in Avon en gedurende een periode van 2 jaar in Warwickshire, werd na behandeling van zomergerst met ethirimol of triadimenol een verminderde gevoeligheid van meeldauw voor deze fungiciden waargenomen.Behandelingen met mengsels van ethirimol-triadimenol of tridemorf-triadimenol gaven weinig tot geen verminderde gevoeligheid. Verminderde gevoeligheid van isolaten voor ethirimol was meestal gecorreleerd met een verhoogde gevoeligheid voor triadimenol en omgekeerd. Vroeg in het seizoen werd in de meeldauwpopulatie op de twee proefvelden een verschil in gevoeligheid voor de fungiciden waargenomen. Op één proefveld trad van het ene op het andere seizoen een aanzienlijke verandering in de gevoeligheid voor de fungiciden op. Er was geen duidelijke correlatie tussen de waargenomen verminderde gevoeligheid voor de fungiciden en de gebruikte cultivars of voorkomende fysio's. Verminderde gevoeligheid voor de fungiciden werd zowel bij zaadbehandeling als bij het bespuiten van planten waargenomen.     

Cellular characteristics of temporary partial breakdown of mlo- resistance in barley to powdery mildew

When water-stress is relieved, powdery mildew (Erysiphe graminis f.sp. hordei) infection increases on both Mlo -susceptible and mlo -resistant spring barley cultivars. The breakdown is temporary and is determined by the genetic background rather than the specific resistance allele. The relief of water-stress time-point for maximum mildew infection frequency on mlo -resistant barley is approximately 7 h post-inoculation. The degree of breakdown varies with epidermal cell type; increased infection frequency is greatest on short followed by long and adjacent epidermal cell types, rather than on the stomatal subsidiary cells with which occasional colonies are typically associated. Infection frequency on the short cells of mlo -resistant cv. Atem increases from less than 1% under a non-stressed watering regime to more than 10% after a relief of water-stress at 7 h post-inoculation. Following haustorium formation, colonies develop and reach sporulation within 7 days post-inoculation. In host epidermal cells, the extent of cross-linked protein at interaction sites is reduced under conditions of water-stress or stress-relief. Cross-linked protein is reduced in terms of the frequency of occurrence at primary germ tube interaction sites 7 h post-inoculation and the deposition size at appressorial germ tube interaction sites 24 h post-inoculation. This indicates a delayed or reduced defence response during the recovery period. These data demonstrate a differential cellular response to powdery mildew in barley genotypes prone to resistance expression breakdown following relief of water-stress.     

Adult plant resistance to powdery mildew (Erysiphe graminis)in three barley cultivars

The mechanism(s) of adult plant resistance (a.p.r.) to Erysiphe graminis f. sp. hordei were examined in two spring barley cultivars, Athos and Porthos, which possess similar identified mildew resistance genes and seedling resistance, but differential field resistance. These cultivars were compared with Golden Promise, a universally susceptible cultivar. Differential a.p.r. to a compatible race was detected as a reduction in the number and sporulation of colonies on later-formed Athos leaves. Golden Promise exhibited some a.p.r., but much less than either Athos or Porthos. Differential a.p.r. was expressed at several infection stages, viz. appressorial formation, penetration and sporulation. Epidermal hypersensitivity occurred infrequently in all leaf position/cultivar interactions. Papillae were associated with >90% of all penetration failures. The incidence of papillae in successful penetrations at 96 h was 20% on later-formed leaves of Athos compared to c. 60% on all other leaves.     

The effect of humidity on the expression of partial resistance to powdery mildew in barley

Seedling leaves of four cultivars of barley, three of which express partial resistance, were inoculated with powdery mildew ( Erysiphe graminis f.sp. hordei ) and enclosed in an apparatus which enabled accurate control of humidity, air speed and temperature. Infection frequency was always greater at high humidity (>95%) than at low humidity (40–50%). Cultivar 7204 (Ethiopian) showed a greater reduction in infection frequency at both 12 and 16°C, and more reduction in biomass per colony at 16°C under low humidity than the other cultivars. The potential for exploiting such differences in barley breeding is discussed.     

Ontogenic resistance to powdery mildew in grape berries

ABSTRACT Berries of Vitis vinifera are reported to be susceptible to infection by Uncinula necator until soluble solids levels (brix) reach 8%, and established colonies are reported to sporulate until brix reach 15%. However, our analysis of disease progress on fruit of selected V. vinifera cultivars indicated that severity became asymptotic several weeks earlier in fruit development. When mildew-free fruit clusters of V. vinifera 'Chardonnay', 'Riesling', 'Gewürztraminer', and 'Pinot Noir' were inoculated at stages ranging from prebloom to 6 weeks postbloom, only fruit inoculated within 2 weeks of bloom developed severe powdery mildew. Substantial ontogenic resistance to infection was expressed in fruit nearly 6 weeks before fruit brix reached 8% and over 2 months before they reached 15%. Rachises of 'Chardonnay' and 'Riesling' fruit clusters developed severe powdery mildew when inoculated at bloom, and disease increased steadily over the next 60 days. The rachis of fruit clusters inoculated 31 days after bloom developed only trace levels of powdery mildew. Berry weight of all four cultivars at harvest was reduced when fruit clusters were inoculated at bloom or 16 days postbloom, primarily by splitting, rotting, and dehydration of mildewed berries, but the weight of later-inoculated berries was not reduced. Inoculation of berries just as ontogenic resistance increased markedly, approximately 3 to 4 weeks postbloom, resulted in the development of inconspicuous, diffuse, non-sporulating mildew colonies on berries, sometimes associated with a network of necrotic epidermal cells. Rather than a protracted and relatively static period of berry susceptibility lasting 3 months, fruit of V. vinifera appear to acquire ontogenic resistance rapidly after fruit set. A refocusing of disease management on this critical period of high fruit susceptibility should greatly improve the efficacy of fungicides directed against powdery mildew.     

A transient expression system to assay putative antifungal genes on powdery mildew infected barley leaves

A transient expression system has been established to assay the regulation and impact of putative antifungal proteins at the single cell level in barley (Hordeum vulgareL.) epidermal leaf cells infected with powdery mildew (Erysiphe graminisf.sp.hordei).The system combines biolistic transformation with direct comparison of the fungal attack in transfected and wild type cells. Many epidermal cells showed detectable expression of marker genes including a novel improved version of green fluorescent protein (GFP). The marker was detectable for at least 10 days during which individual host cells supported an unaffected asexual life-cycle of the pathogen.     

Spatial aggregation of pathotypes of barley powdery mildew

Aggregation in the distribution of pathotypes of Erysiphe graminis f.sp. hordei , the barley powdery mildew pathogen, was investigated in field plots of 'Golden Promise', 'Proctor' and 'Tyra'. 'Golden Promise' and 'Proctor' have no effective mildew resistance alleles, whereas 'Tyra' has Mla1 , which was only effective against a proportion of the mildew population. Isolates of mildew were sampled according to a grid sampling scheme and their virulence spectra ascertained in order to group them according to pathotype. The populations were very diverse, and evidence for aggregation (quantified using join counts) was found only in the 'Tyra' plots, at distances of up to 1m. This aggregation was reduced in a subsequent sample. The results are consistent with a model in which mildew epidemics are started by a large number of initial infections, which then form diffuse, overlapping aggregations of clones. These aggregations then become more diffuse, so that the amount of aggregation reduces with time. The greater amount of aggregation seen in the 'Tyra' plots might have been caused by there being less initial inoculum with virulence towards that cultivar.     

Utility of defeated resistance genes to powdery mildew,Erysiphe graminis f. sp. hordei,in spring barley variety mixtures

In 1980 and 1981 experiments have been performed to study the utility of defeated resistance genes to powdery mildew in spring barley variety mixtures. For this purpose the epidemic development of powdery mildew in pure stands and mixtures of four spring barley varieties was monitored. In three varieties the resistance was overcome several years ago. One variety is still resistant. Changes in frequencies of corresponding virulence genes were also studied.It was found that the variety mixtures slowed down the mildew epidemic only at the beginning of the season. Some reduction was lost later on since the pure stands reached their saturation level of infection at an earlier date than the more healthy mixtures. The level of reduction of infection depended on the composition of the mixtures and the number of components. Two-way mixtures appeared to be very risky due to the high frequency of corresponding combinations of virulence genes in the pathogen. Furthermore, two-way mixtures appeared to be unable to reduce the infection rate sufficiently when the infection pressure was high.In 1981 the frequencies of combinations of corresponding virulence genes increased very much in the Diva-Mazurka mixture but declined in the pure stands of both varieties.On the basis of these results it is concluded that the utility of defeated resistance genes involved in this study is very limited.The low number and the irregular distribution of the Dutch spring barley varieties over the different resistance groups prohibit the use of variety mixtures in the Netherlands.Samenvatting In 1980 en 1981 is onderzoek verricht over de bruikbaarheid van doorbroken resistentiegenen tegen meeldauw,Erysiphe graminis f. sp.hordei, in zomergerstrassenmengsels. Daartoe werd de epidemieontwikkeling in monocultures en mengsels van vier zomergerstrassen bestudeerd. Van drie rassen is de resistentie reeds verscheidene jaren doorbroken. Eén ras is nog resistent. De frequenties van compatibele meeldauwvirulentiegenen werden eveneens bepaald.Het bleek dat in vergelijking met de monocultures in de rassenmengsels de meeldauwepidemie alleen in het begin van het seizoen werd afgeremd. Later in het seizoen ging weer enige reductie in aantasting van de mengsels verloren doordat in de monocultures de aantasting eerder het verzadigingspunt bereikte dan in de minder aangetaste mengsels. De mate waarin de aantasting gereduceerd werd was afhankelijk van de aard en het aantal componenten in het mengsel. Twee-componentenmengsels bleken vrij riskant te zijn vanwege de aanwezigheid van combinaties van corresponderende virulentiegenen in de meeldauwpopulatie. Verder bleken tweecomponentenmengsels niet in staat de infectiesnelheid afdoende af te remmen wanneer de infectiedruk hoog werd.De frequentie van combinaties van corresponderende virulentiegenen nam sterk toe in het Diva-Mazurka-mengsel doch nam af in de monocultures van deze beide rassen.Gezien de verkregen resultaten lijkt de bruikbaarheid van de in dit onderzoek betrokken doorbroken resistentiegenen erg beperkt. Het geringe aantal zomergerstrassen op de rassenlijst en de ongelijke verdeling over de te onderscheiden resistentiegroepen belemmeren de toepassing van rassenmengsels in de Nederlandse akkerbouw.     

Movement of barley powdery mildew within field plots

Movement of barley powdery mildew (caused by Erysiphe graminis f.sp. hordei ) within fields was investigated by sowing the barley cultivars Tyra and Jupiter side by side in two field plots, and trapping spores along transects within the plots. The trapped spores were tested for virulence on the two cultivars. The epidemic on Tyra developed quickly, and a gradient in the proportion of spores with virulence on Tyra was detected in the Jupiter half-plots. In the Jupiter half-plots, the epidemic was much less severe; and no mildew could be found in one plot. Movement of spores from one half of the plot to the other usually declined steeply in the first 4 m from the boundary, and was not detectable beyond 12 m. There were exceptions where the gradient was much shallower, and these were consistent with differences in wind direction.     

Potassium phosphate induces systemic protection in barley to powdery mildew infection

In laboratory tests, treatment of the first leaves of barley (Hordeum vulgare L cv Golden Promise) with potassium phosphate led to significant reduction in infection of the second leaves with the powdery mildew fungus Blumeria graminis f sp hordei Marchal, with a 25 mM treatment giving 89% reduction in infection. Although the optimal interval between phosphate treatment of the first leaves and mildew inoculation of the second leaves was 2 days, significant protection was still obtained if the interval was increased to 12 days. Protection against powdery mildew infection was not as effective when the potassium phosphate was applied as a seed treatment or root drench. Phosphate treatment of the first leaves led to significant increases in activities of phenylalanine ammonia lyase (PAL), peroxidase and lipoxygenase in second leaves. Enzyme activities, especially PAL and peroxidase, were increased further when second leaves of phosphate-treated plants were inoculated with powdery mildew. Phosphate treatment of the first leaves did not adversely affect plant growth and, in a field trial, 25 mM potassium phosphate provided 70% control of mildew and gave a small increase in grain yield.