Virulence analysis of wheat powdery mildew population (Erysiphe graminis f.sp.tritici) from the region of Slovakia and Hungary

In the year 1992 a total of 163 isolates of wheat powdery mildew were tested. The samples of mildew isolates were obtained by means of a mobile spore catching apparatus. The populations from 4 regions of Slovakia and 3 regions of Hungary were analyzed. The resistance due toPm5, Pm8 andMl-i genes at the observed locations has already been overcome. The resistance genesPm1, Pm2 and a gene combinationPm2+Pm6 ensure the protection only against a part of the patho-types of powdery mildew population. Virulence corresponding to thePm4b gene has been low so far. The regional patterns of pathogen virulence are in good agreement with the gene resistance spectrum by the cultivars grown regionally. Little differences in virulence among the populations from the regions of Slovakia and Hungary indicate that this part of Eastern Europe should be considered as an epidemiologic unit.     

A new powdery mildew resistance allele at the Pm4 wheat locus transferred from einkorn (Triticum monococcum)

Powdery mildew is one of the most destructive foliar diseases of wheat. A set of differential Blumeria graminis f.sp. tritici (Bgt) isolates was used to test the powdery mildew response of a Triticum monococcum-derived resistant hexaploid line, Tm27d2. Segregation analysis of 95 F_2:3 lines from a Chinese Spring/Tm27d2 cross revealed that the resistance of Tm27d2 is controlled by a single dominant gene. Using monosomic analysis and a molecular mapping approach, the resistance gene was localized to the terminal end of chromosome 2AL. The linkage map of chromosome 2AL consisted of nine simple sequence repeat markers and one sequence-tagged site (STS) marker (ResPm4) indicative for the Pm4 locus. According to the differential reactions of 19 wheat cultivars/lines with known powdery mildew resistance genes to 13 Bgt isolates, Tm27d2 carried a new resistance specificity. The complete association of the resistance allele with STS marker ResPm4 indicated that it represented a new allele at the Pm4 locus. This new allele was designated Pm4d. The two flanking markers Xgwm526 and Xbarc122 closely linked to Pm4d at genetic distances of 3.4 and 1.0 cM, respectively, are present in chromosome bin 2AL1-0.85-1.00.     

The mildew resistance of Hordeum bulbosum and its transference into H. vulgare genotypes

Mildew from susceptible genotypes (SI and S2) of Hordeum bulbosum was found to be avirulent on all H. vulgare genotypes tested, including such cultivars as Proctor with no known genes for resistance to mildew. The H. bulbosum genotype SI (2n =14) proved resistant to all isolates of mildew from H. vulgare. The mildew isolates used for these tests possessed most of the common virulence factors which attack the current ‘vulgare’ cultivars in Western Europe. Some H. bulbosum genotypes were resistant to the ‘bulbosum’ mildew. Attempts at combining resistance from both species are presented and discussed.     

Pathotypes of Blumeria graminis f. sp. tritici,the causal agent of wheat powdery mildew from some regions of Iran

Wheat powdery mildew is controlled mainly by race-specific resistance. To be effective, breeding wheat for resistance to powdery mildew requires knowledge of virulence diversity in local populations of the pathogen. Isolates of Blumeria graminis, collected in 2009 and 2010 from three areas of Iranian production, were analysed for virulence using a host differential series comprised of 16 known genes conferring resistance to powdery mildew. The results showed that high-virulence frequencies to genes Pm1, Pm2, Pm4a, Pm5, Pm6, Pm7, Pm8 and Pm9 were found over both years and across all three areas. Virulence frequencies for Pm3a and Pm3b were intermediate, while virulence frequencies for Pm3a, Pm3c, Pm4a and Pm2, 6 were low. Genes Pm1, 2, 9 and Pm2, 4b, 8 were highly resistant in all regions. Virulence to Pm8 increased to high levels, while virulence to Pm4a decreased across the area surveyed from 2009 to 2010.     

Germination of Erysiphe gratninis f.sp. hordei conidia on barley leaves

Germination of Erysiphe graminis f.sp. hordei conidia on leaves of several barley cultivars was studied in the laboratory. On both detached leaves and intact plants, within 48 h of inoculation a higher proportion of conidia had germinated on the basal and middle portions of the adaxial leaf surface than on the corresponding portions of the abaxial surface. Such differences between surfaces were not observed near the leaf tip. Similar results were obtained with all the cultivars and growth stages tested, and with five isolates of E. graminis, and are consistent with the observation that there is usually less powdery mildew on the abaxial than the adaxial surface of barley leaves. With most of the barley genotype/mildew isolate combinations tested, within 48 h of inoculation higher proportions of conidia germinated on seedlings and juvenile plants than on older plants. Inherited characteristics which affect spore germination on the leaf surface may be important factors in the development of adult-plant resistance of barley to powdery mildew, particularly in certain genotypes.     

Distribution,Disease Level and Virulence Variation of Bremia lactucae on Lactuca sativa in the Czech Republic in the Period 1999–2011

We report the distribution and disease level of downy mildew on lettuce (Lactuca sativa) and virulence variation in populations of its causal agent (Bremia lactucae) in the Czech Republic during the period 1999–2011. Disease incidence was not high; among a total of 92 different localities surveyed, 43.4% of them were infected by lettuce downy mildew at least once during the whole period. However, among individual years, differences were found in disease incidence that ranged from 4.8% (2009) to 66.7% (2004). A total of 128 isolates of B. lactucae collected from infected leaf samples in 35 different localities during the surveying period were included in the virulence analysis. Virulence was examined on a set of 19 differential genotypes of Lactuca sativa and Lactuca serriola (EU‐A test set). Isolates exhibited quite a broad variation in virulence to individual Lactuca differential genotypes. Eighteen of 19 virulence factors (v‐factors) tested were present in the samples. The most frequently detected factors were v1–4, v5/8, v6, v7, v10–14, v16, v36 and v38; factor v17 was not found. The most pronounced temporal shift was recorded for factors v36 and v38 whose frequency increased during the studied period. V‐factors 15, 17, 18 and 37 were present in low frequencies in a pathogen population, and their corresponding gene (Dm15) or resistance factors (R17, R18 and R37) may have the best potential for resistance breeding in the Czech Republic. Broad diversity of v‐phenotypes (63 different ones) was identified during the study period. The numbers of v‐factors per v‐phenotype (resp. isolate) varied within a range of 5–15. Within the 128 analysed isolates, only 9 v‐phenotypes were recorded repeatedly (three or more times). Possible reasons of recorded virulence variation are discussed.     

Virulence Spectrum of Mycosphaerella graminicola Isolates on Wheat Genotypes Carrying Known Resistance Genes to Septoria tritici Blotch

The virulence spectrum of 23 monopycnidiospore isolates of Mycosphaerella graminicola was determined using wheat genotypes that carried different resistance genes (Stb1–Stb8 and Stb15). Disease severity was measured as the percentage of necrotic leaf area. The isolates used in the experiments were of diverse origin: eight from Poland, seven from Germany, and eight from other countries around the world. Analysis of variance revealed significant differences in the virulence of the isolates. Using multiple regression and Cook’s D statistic, 26 significant cultivar × isolate interactions were detected. The Israeli isolate IPO86036 showed the widest spectrum of specific reactions. It expressed specific virulence on at least four cultivars and specific avirulence on at least three. The other isolates showed specific interactions with 1–6 different cultivars. Despite the limited number of isolates that were tested, we recommend that a number of resistant lines, namely cultivars Veranopolis (Stb2), Cs/Synthetic 7D (Stb5), Arina (Stb15, Stb6 and partial resistance), and Liwilla (unknown resistance factors), could be incorporated into central European wheat breeding programmes that are aimed at developing resistance against septoria tritici blotch. In contrast, resistance gene Stb7, which is carried by cultivar Estanzuela Federal, was ineffective against most of the isolates that were used. These results on the virulence spectrum of M. graminicola isolates provide valuable information for effective wheat breeding programmes to develop resistance to the pathogen.     

Analysis of Virulence in Populations of Wheat Powdery Mildew in Europe

Random samples of spores of wheat powdery mildew were collected from the atmosphere by means of a jet spore sampler while driving through selected wheat growing areas in Europe in 1986. Their single colony progenies were tested in the laboratory forvirulence against wheat differential varieties with powdery mildew resistances Pm2, Pm4b, Pm8, Mli and the combinations Pm2+Pm6, Pm4b+Pm8 and Pm2+Pm4b+Pm8. There were regional differences in virulence frequencies. Virulence on Pm2, Pm2+Pm6 and, less expressed, on Pm4b was most frequent in England and decreased mainly to the east, whereas the reverse was true for Pm8. Virulence to Mli was generally high, and to the combined genotypes generally low. The results suggest that large parts of Europe are an epidemiological unit. The data are discussed with respect to selection caused by the varieties grown, as well as to the influence of wind distribution on the composition of the pathogen population.     

Genetic Characterization and Pathogenicity of Rhizoctonia solani Associated with Common Bean Web Blight in the Main Bean Growing area of Argentina

Common bean web blight (WB), caused by the fungus Rhizoctonia solani (teleomorph Thanatephorus cucumeris), is among the endemic fungal diseases of major impact in north‐western Argentina (NWA). This study aimed to analyse the genetic and pathogenic diversity of R. solani in Salta, NWA, where 97 isolates were recovered from commercial bean cultivars and wild beans showing WB symptoms in a major bean production area. The isolates were characterized on the basis of specific primers, rDNA‐ITS sequences and morphological characteristics. All the isolates were identified as R. solani AG 2‐2WB, and they exhibited considerable intragroup variation. The phylogenetic tree generated with the ITS sequences confirmed the isolates identification. Aggressiveness of the isolates towards bean seedlings was assessed in the greenhouse. A great variability in virulence was observed among the isolates analysed. On the basis of the disease reaction on foliar tissues, the isolates were grouped into three virulence categories as follows: weakly virulent (30%), moderately virulent (38%) and highly virulent (32%). However, no correlation between virulence and geographical origin was detected. The information generated in this study provides initial data on the population variability of the WB pathogen in north‐western Argentina and represents a valuable contribution to regional breeding programmes aimed to obtain cultivars with durable resistance.     

Identification and mapping of two powdery mildew resistance genes in <Emphasis Type="Italic">Triticum boeoticum</Emphasis> L.

Powdery mildew (PM) caused by Blumeria graminis f. sp. tritici (Bgt), is one of the important foliar diseases of wheat that can cause serious yield losses. Breeding for cultivars with diverse resources of resistance is the most promising approach for combating this disease. The diploid A genome progenitor species of wheat are an important resource for new variability for disease resistance genes. An accession of Triticum boeoticum (A^bA^b) showed resistance against a number of Bgt isolates, when tested using detached leaf segments. Inheritance studies in a recombinant inbred line population (RIL), developed from crosses of PM resistant T. boeoticum acc. pau5088 with a PM susceptible T. monococcum acc. pau14087, indicated the presence of two powdery mildew resistance genes in T. boeoticum acc. pau5088. Analysis of powdery mildew infection and molecular marker data of the RIL population revealed that both powdery mildew resistance genes are located on the long arm of chromosome 7A. Mapping was conducted using an integrated linkage map of 7A consisting of SSR, RFLP, STS, and DArT markers. These powdery mildew resistance genes are tentatively designated as PmTb7A.1 and PmTb7A.2. The PmTb7A.2 is closely linked to STS markers MAG2185 and MAG1759 derived from RFLP probes which are linked to powdery mildew resistance gene Pm1. This indicated that PmTb7A.2 might be allelic to Pm1. The PmTb7A.1, flanked by a DArT marker wPt4553 and an SSR marker Xcfa2019 in a 4.3 cM interval, maps proximal to PmT7A.2. PmTb7A.1 is putatively a new powdery mildew resistance gene. The powdery mildew resistance genes from T. boeoticum are currently being transferred to cultivated wheat background through marker-assisted backcrossing, using T. durum as bridging species.     

Resistance to oat powdery mildew in Britain and Europe: a review

The evolution of virulence in UK oat powdery mildew (Blumeria graminis f.sp. avenae) populations is presented along with comparative information on the deployment of resistant cultivars. Virulence frequencies have followed classical gene‐for‐gene principles, and there are no effective resistance genes currently deployed in cultivars grown in the UK. The incidence of powdery mildew in continental Europe and pathogen variation is reviewed as well as other strategies for the control of this disease. New resistant sources have been identified and are being used in breeding programmes throughout Europe.     

Field resistance of spring barley cultivars to powdery mildew in Lithuania

During vegetative period 2004–2005 powdery mildew (Erysiphe graminis DC. f. sp. hordei Em. Marchal) field resistance of spring barley cultivars was investigated at the Lithuanian Institute of Agriculture. The spring barley genotypes tested were Lithuania-registered cultivars, cultivars from genetic resources collection, and the new cultivars used for initial breeding. In total, 23 resistance genes were present in the 84 cultivars studied. Among mono-genes only mlo and 1-B-53 showed very high resistance. Slight powdery mildew necroses (up to 3 scores) formed on cultivars possessing these genes. The maximal powdery mildew (PM) severity reached a score of 8.5 and the area under disease progress curve (AUDPC) a value of 1216.8. The cultivars ‘Primus’, ‘Astoria’, ‘Power’, ‘Harrington’ and ‘Scarlett’ were the most resistant among the non mlo cultivars. Severity of PM on ‘Primus’ reached a score of 3.5 (3.0 of PM necrosis) in average, the other cultivars were diseased from 4.5 (3.0) to 5.0 (2.0). The AUDPC values for these cultivars except ‘Scarlett’ were the lowest (85.0–145.3) among the other cultivars. The highest contrast in development of the other leaf diseases was between highly resistant and susceptible to PM cultivar groups. The fast development of PM depressed development of the other diseases 4.7 times.     

Comparative studies on the activities of chitinase, β-1,3-glucanase, peroxidase and phenylalanine ammonia lyase in the leaves of triticale and wheat infected with Stagonospora nodorum

The activities of chitinase, β-1,3-glucanase, peroxidase and phenylalanine ammonia lyase, constitutive and induced by Stagonospora nodorum were examined in the 10 – 14 day old seedlings of three triticale and two wheat cultivars under controlled environmental conditions and in flag leaves of two triticale cultivars in the field. Two S. nodorum isolates of different virulence were used. Both the constitutive and induced activities in triticale and wheat depended on genotype and in triticale the effect of growth conditions was also evidenced. The constitutive activities of chitinase, β-1,3-glucanase and peroxidase were several fold lower in flag triticale leaves in plants from the field than in the seedlings, growing under controlled conditions, but induction in the infected flag leaves was significantly more pronounced. In triticale genotypic differences in the response to infection were revealed only upon inoculation by S. nodorum isolate of higher virulence. The enzymatic activities increased several fold during successive days after the infection except for phenylalanine ammonia lyase. Induction of this enzyme was only transient and the activity decreased 48 or 96 h after infection when the activities of other enzymes were rising. In flag leaves in the field this activity was differentiated only after infection with more a virulent strain. A tendency appeared in triticale seedlings for association of the resistance to the pathogen with lower enzymatic constitutive activities. This relationship became more evident in triticale infected by S. nodorum and may imply that although the investigated enzymes are certainly involved in general, non-specific defense mechanism, they do not decide on the resistance to pathogen at least in the early stages of infection and cooperate with other factors in the complex pathogen-plant interaction. One can also assume that the enzymatic activities are associated with severity of infection rather than resistance to pathogen.     

<Emphasis Type="Italic">Pm37</Emphasis>, a new broadly effective powdery mildew resistance gene from <Emphasis Type="Italic">Triticum </Emphasis><Emphasis Type="Italic">timopheevii</Emphasis>

Powdery mildew is an important foliar disease in wheat, especially in areas with a cool or maritime climate. A dominant powdery mildew resistance gene transferred to the hexaploid germplasm line NC99BGTAG11 from T. timopheevii subsp. armeniacum was mapped distally on the long arm of chromosome 7A. Differential reactions were observed between the resistance gene in NC99BGTAG11 and the alleles of the Pm1 locus that is also located on chromosome arm 7AL. Observed segregation in F_2:3 lines from the cross NC99BGTAG11 × Axminster (Pm1a) demonstrate that germplasm line NC99BGTAG11 carries a novel powdery mildew resistance gene, which is now designated as Pm37. This new gene is highly effective against all powdery mildew isolates tested so far. Analyses of the population with molecular markers indicate that Pm37 is located 16 cM proximal to the Pm1 complex. Simple sequence repeat (SSR) markers Xgwm332 and Xwmc790 were located 0.5 cM proximal and distal, respectively, to Pm37. In order to identify new markers in the region, wheat expressed sequence tags (ESTs) located in the distal 10% of 7AL that were orthologous to sequences from chromosome 6 of rice were targeted. The two new EST-derived STS markers were located distal to Pm37 and one marker was closely linked to the Pm1a region. These new markers can be used in marker-assisted selection schemes to develop wheat cultivars with pyramids of powdery mildew resistance genes, including combinations of Pm37 in coupling linkage with alleles of the Pm1 locus.     

Quantitative Differences in Powdery Mildew Resistance Among Spring Barley Cultivars

Quantitative powdery mildew resistance in compatible host-pathogen-combinations was measured by the number of pastules/cm² leaf area. Spring barley cultivar ‘Proctor’ was significantly less infected than ‘Golden Promise”. Using these two cultivars (having no effective major resistance gene) as controls, MO- and AR-resistant cultivars were inoculated with virulent mildew isolates. ‘Mona”, ‘Grit’ and ‘Nudinka’ had a higher or, at least, the same level of quantitative resistance as ‘Proctor”. None of the remaining cultivars showed the high susceptibility expressed by ‘Golden Promise”. Ranking of host genotypes was nearly constant while that of mildew isolates varied considerably. Only a small portion of the observed variance was due to interaction between host cultivars and pathogen isolates. ‘Triesdorfer Diva’ gave a resistant infection type after inoculation with different AR-virulent isolates, indicating that this cultivar has major resistance other than that conditioned by gene Ml-a12.     

Genetic Structure and Aggressiveness of Rhizoctonia solani AG1‐IA,the Cause of Sheath Blight of Rice in Southern China

One hundred and eighty isolates of Rhizoctonia solani AG1‐IA, the causal agent of rice sheath blight, were obtained from six locations in southern China. The genetic structure of R. solani isolates was investigated using random amplified polymorphic DNA (RAPD) markers, and a considerable genetic variation among R. solani isolates was observed. Most of the genetic diversity was distributed within populations, rather than among them. The distribution pattern of the genetic variation of R. solani appears to be the result of high gene flow (Nm) and low‐genetic differentiation among populations. The aggressiveness of R. solani was visually assessed by rice seedlings of five different cultivars in the glasshouse. All isolates tested were found to induce significantly different levels of disease severity, reflecting considerable variation in aggressiveness. The isolates were divided into highly virulent, moderately virulent and weakly virulent groups, and the moderately virulent isolates were dominant in R. solani population. No significant correlation was observed among the genetic similarity, pathogenic aggressiveness and geographical origins of the isolates. Information obtained from this study may be useful for breeding for improved resistance to sheath blight.     

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.     

Cold-hardening of winter triticale (<Emphasis Type="Italic">x Triticosecale</Emphasis> Wittm.) results in increased resistance to pink snow mould <Emphasis Type="Italic">Microdochium nivale</Emphasis> (Fr., Samuels &; Hallett) and genotype-dependent chlorophyll fluorescence modulations

The resistance of triticale (x Triticosecale Wittm.) to infection of snow mould Microdochium nivale (Fr., Samuels & Hallett) was examined under different temperature pre-treatment regimes. The results of laboratory “cold chamber” resistance tests correlated with the breeders’ report from field experiments. Studied genotypes differed substantially in their resistance to infection. Two cultivars: ‘Magnat’ (susceptible) and ‘Hewo’ (relatively resistant) were further studied as a plant model to test the role of pre-hardening and cold-hardening induction of resistance expression. Both model cultivars were susceptible to M. nivale infection without cold pre-treatment and gained genotype-depended level of resistance after 4 weeks treatment at 4°C, moreover the resistance grew gradually. Simultaneously to the resistance tests, the measurements of chlorophyll fluorescence parameters were taken. The results showed that higher vitality index Rfd of cold-hardened triticale seedlings correlated with increased pink snow mould resistance while differences in other parameters of fluorescence were not distinctly significant. Establishment of Rfd in 4 weeks hardened triticale seedlings could be used for a large scale screening of breeding material in order to select potentially resistant genotypes. Such analyses have not been reported for triticale before.     

Infection of cereals and grasses by isolates of Polymyxa graminis (Plasmodiophorales)

The host range of isolates of Polymyxa was tested in mono-fungal sand cultures. Fourteen isolates of P. graminis, obtained from barley, wheat, oats or Poa annua and from several different countries, all infected barley and all but one infected wheat. Rye was also a good host, whereas oats (nine cultivars), Lolium multiflorum and Poa pratensis became only slightly infected. Wheat cultivars differed in susceptibility, with Galahad much more resistant than Avalon. Several common weed and pasture grasses were not infected by the two isolates tested. A range of wild Hordeum spp. were mostly susceptible to P. graminis and/or barley mild mosaic virus, which it transmits. An isolate of P. betae, used for comparison, caused slight infection on oats but not on other cereals. The variation within and between species of Polymyxa needs more detailed investigation.