Pathogenic variation among isolates of Rhynchosporium secalis from cultivated barley growing in Victoria,Australia

Summary Three hundred and nineteen Rhynchosporium secalis isolates from cultivated barley were divided into five groups on the basis of their virulence on 15 differential barley varieties. Pathogenic variation was also demonstrated for isolates from different scald lesions within the same crop and amongst different spores from the same lesions.     

Genes for resistance in barley to Finnish isolates of Rhynchosporium secalis

Summary Twenty Finnish isolates of Rhynchosporium secalis (Oud.) J.J. Davis, the causal agent of scald, were taken from infected barley (Hordeum vulgare L.) plants and inoculated on to seedlings of a differential series of barley containing a range of major genes for resistance to the fungus, as well as on to six Nordic 6-row spring barleys and three winter ryes (Secale cereale L.). These fungal isolates derived from four sites and three host varieties. Disease development was monitored on two leaves of seedlings in the greenhouse employing a standard scale, and on adult plants in the field by assessing the diseased area on the three uppermost leaves. A comparison was also made between the pathogenicity and virulence of ten Finnish and ten Canadian R. secalis isolates. The Finnish isolates varied in virulence, but with the exception of Algerian (CI 1179) seedlings and adult La Mesita (CI 7565) all seedlings and adult plants of the entire differential series were resistant to all isolates. Canadian isolates were, on average, less virulent than Finnish isolates. All the Nordic checks were susceptible to all Finnish and seven Canadian isolates, but differences in the degree of susceptibility were evident. Isolates of R. secalis from barley were non-pathogenic on rye, isolates from Elymus repens L. were non-pathogenic on barley and rye, and isolates from rye were only pathogenic on rye. Finnish R. secalis isolates contain no redundant pathogenic diversity. The differential series represents a useful, but as yet untapped, source of resistance to R. secalis for Finnish barley breeders.     

Race differentiation and search for sources of resistance to Rhynchosporium secalis in barley in Italy

Summary Barley in Italy has recently been seriously affected by Rhynchosporium secalis. The pathogenic variation of the fungus was studied and 17 races were differentiated on 13 barley cultivars carrying most of the currently known genes for resistance. RC 1, the most virulent and most frequent race, was virulent on 10 out of the 13 differentials and the remaining races proved to be less virulent variants of RC 1. Atlas (C.I. 4118), Atlas 46 (C.I. 7323) and Osiris (C.I. 1622) were the only three differentials resistant to all the analyzed single-spore isolates.Differential cultivars previously assumed to have identical resistance factors did not react in the same way to all the Italian races, thereby revealing either undisclosed differences in the genes described or the presence of additional unidentified ones.Our findings were compared with previous data about virulence of scald populations from different countries, on the basis of tests with common differentials: fundamental differences were found between the Italian population and those of other countries with regard to virulence patterns.The susceptible reactions to race RC 1 of most barley cultivars grown in Italy indicate the urgent need for resistance genes to be incorporated in the cultivated material. Seventy-one barley accessions, known as sources of resistance in different parts of the world, were screened for their behaviour to races RC 1 and RC 13. Twenty-two appeared resistant to both of them.     

Pathogenicity of the Rhynchosporium secalis population in the Western Cape province of South Africa

The virulence spectra of 50 Rhynchosporium secalis isolates from a population in the Western Cape province of South Africa were determined, and 21 races were detected when evaluated against 17 differential cultivars. The virulence spectrum of the R. secalis population shows considerable variation, and carries unnecessary virulence genes which is quite unexpected, since chiefly susceptible barley cultivars are grown in the south Western Cape. The two most prevalent races, namely races 4 and 7 had three and four virulence genes respectively. Both race 4 and 7 were virulent on the most susceptible cultivars, West China, Steudelli, C.I.8618 and C.I.2226. Considering the resistance genes reported for the cultivars Atlas 46, Turk, and C.I.3515 which showed no susceptible cultivar-pathogen interaction, it would appear that the Rh-Rh3-Rh4 complex is primarily involved in conferring resistance to the local R. secalis isolates This revised version was published online in July 2006 with corrections to the Cover Date.     

Pathotypes and NIP1 gene sequences of Finnish Rhynchosporium secalis isolates from barley, couch grass and rye

Rhynchosporium secalis (Oud.) J.J. Davis, causal agent of scald of barley (Hordeum vulgare L.), was isolated from barley, couch grass (Elymus repens L.) and rye (Secale cereale L.). Isolates were used to inoculate seedlings of a differential barley series containing several sources of major gene resistance to the disease. The series included Atlas46, (resistance gene Rrs1) and the isogenic line Atlas, which lacks the gene (rrs1). The necrosis-inducing peptide NIP1 has been suggested to be the product of the avirulence gene AvrRrs1 (NIP1) that with the barley resistance gene Rrs1 determines the incompatibility of the pathogen-host interaction. All R. secalis isolates were virulent only on the susceptible barley cultivars Arve and Chan, irrespective of spore concentration or original host species. There were no indications of redundant virulence among the R. secalis isolates. The NIP1 gene was sequenced from each isolate and there was no correlation with source of the isolate or sequence modification and virulence. Four isolates, from barley and couch grass, were characterised by a basic Type I NIP1sequence. The recorded NIP1 sequence changes are consistent withR. secalis populations not receiving selection through deployment of the Rrs1 gene in commercial barley cultivars in Finland. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance to Rhynchosporium secalis in six Nordic barley genotypes

Six six-row Nordic spring barley genotypes (Hordeum vulgare L.) were assessed in the field in Finland (1994 and 1995) for resistance to Rhynchosporium secalis (Oud.) J.J. Davis, the causal pathogen of scald, in artificially inoculated plots. The barleys were known not to contain major genes for resistance to scald and the purpose of these experiments was to identify quantitative differences in resistance to scald which might be exploitable in a breeding programme. Disease development was monitored, grain yield and yield components were recorded, and these data were compared with measurements taken from plants in plots kept free of disease. Data, averaged over both years, for disease development on the uppermost three leaves — areas under disease progress curves, terminal severity and apparent infection rates — indicated that ‘Verner’, ‘Pohto’ and ‘Pokko’ were symptomatically significantly more resistant to scald than ‘Arve’, ‘Loviisa’ and Jo 1599. Grain yields, thousand-grain weights, test weights and proportions of plump grains were all significantly reduced in plots inoculated with scald compared with those kept free of disease; ‘Verner’ appeared to be the most useful genotype for use in crossing programmes to improve scald resistance in Finnish barleys.     

Genetic analysis of resistance to barley scald (Rhynchosporium secalis) in the Ethiopian line `Abyssinian' (CI668)

A doubled haploid barley (Hordeum vulgare L.) population from a cross between the cultivar `Ingrid' and the Ethiopian landrace `Abyssinian' was mapped by AFLP, RFLP, SSR and STS markers and tested for resistance to isolates`4004', `2', `16-6', `17', `22' and `WRS 1872' of Rhynchosporium secalis (Oudem.) J.J. Davis, the causal agent of leaf scald. Resistance tests were conducted on parents, DH-lines, a near-isogenic line of `Abyssinian' (NIL) into `Ingrid', and an F₂ population descended from the same F₁ plants as the DHs. The DH population segregated for at least two major R. secalis resistance QTL. All isolates tested identified a major QTL on chromosome 3 (3H) associated with R. secalis resistance, in a 4 cM support interval between the co-segregating markers Bmac0209/Falc666 and MWG680. The QTL was linked with the markers Falc666 (2.3 cM), YLM/ylp (0.3 cM), MWG680 (1.7 cM), cttaca2 (2.5 cM) and agtc17 (9.8 cM). The second QTL was located on chromosome 1 (7H).However, this QTL was only detected by one isolate and was located in an interval of 16 cM in the distal part of the chromosome. At this QTL the allele for improved scald resistance originated from the parent `Ingrid'. There were a number of minor QTL on chromosomes 2 (2H), 4 (4H) and 6 (6H) that were not repeatable either across replications or analysis methods. The importance of checking QTL-models by cross-validation is stressed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Field screening of simulated segregating barley populations for resistance to scald

Summary Simulated segregating barley populations were screened for resistance to scald (Rhynchosporium secalis) in the field at commercial seeding rates. A reduction in infection on the susceptible component occurred with increasing proportions of resistant genotypes. Similar trends were seen in space planted experiments but the use of susceptible buffer rows counteracted the effect, enhanced the infection in susceptible plants and greatly improved discrimination between resistant and susceptible. These results have been applied to the routine testing of F₂ populations in the barley breeding programme.     

Evaluating resistance of barley breeding lines to scald in field plots

Summary A 0–4 scoring system to quantify scald (Rhynchosporium secalis) infection is suggested. Scores 1, 2, 3 and 4 allocated to represent 1/4, 1/2, 3/4 and 4/4 of the crop canopy scalded are easy to comprehend and intermediate scores e.g. 0.5, 1.5, 2.5 and 3.5 give it the breadth of a quantitative scale. Scores on a large number of lines showed a high degree of repeatability and were found to be highly correlated with the log transformed values of the actual leaf area damage. Although it was suggested that predictions of leaf area damage at scores 3–4 should be applied with caution, broad generalization of the scores in discriminating the amount of disease were shown to be soundly based and offered plant breeders a tool to standardize the evaluation of scald resistance in field plots on a large scale with this quick and reliable scoring system.     

Inheritance of resistance to Rhynchosporium secalis in spring barley (Hordeum vulgare L.)

The inheritance of durable resistance of selected spring barley varieties to Rhynchosporium secalis was investigated. Data from the F₂ generation of a 4 × 4 diallel, without reciprocals and the F₄ generation of three crosses selected out of this diallel, suggest that resistance in this sample of varieties tested is complex in inheritance. Significant additive effects were detected indicating that the resistance level of barley cultivars may be improved by the hybridisation of suitable varieties. However, the genes conferring resistance seem to be concealed by the expression of one completely dominant resistance gene in our set of varieties. These results are partly in conflict with previous results on the inheritance of resistance to R. secalis in the breeder's line ‘11258/2286₁₃A’ indicating that the effectiveness of this resistance gene may be greatly influenced by the genetic background of the current population of R. secalis.     

The genetic basis of scald resistance in western Canadian barley cultivars

Summary The genetic basis of resistance to scald (Rhynchosporium secalis) within barley breeding populations is poorly understood. The design of effective genetically based resistance strategies is predicated on knowledge of the identity of the resistance genes carried by potential parents. The resistance exhibited by a broad selection of western Canadian barley lines was investigated by evaluating their reactions to five R. secalis isolates. Results were compared to the resistance exhibited by previously characterized lines. This comparison, combined with pedigree analysis indicated that there are two different resistance genes present inwwestern Canadian cultivars. These genes were shown to be independent through analysis of a segregating population derived from a cross between Falcon and CDC Silky. This evidence, along with observed linkage of the gene in CDC Silky with an allele specific amplicon developed for a Rhynchosporium secalis resistance locus on chromosome 3, provides evidence that the gene in Falcon is the Rh2 gene derived from Atlas, and the gene (s) in CDC Silky is located within the Rh/Rh3/Rh4 cluster and is similar to the Rh gene in Hudson.     

Development of SCAR markers linked to a scald resistance gene derived from wild barley

The F₂ progeny of a third backcross(BC₃) line, BC line 240, derived from a Turkish accession of wild barley (Hordeum vulgare ssp. spontaneum),segregated for resistance to scald (Rhynchosporium secalis) in a manner indicating the presence of a single dominant resistance gene. Two SCAR marker slinked to this resistance were developed from AFLP markers. Screens of disomic and ditelosomic wheat-barley addition lines with the SCAR markers demonstrated that the scald resistance gene is located in the centromeric region of barley chromosome 3H,a region previously reported to contain a major scald resistance locus, Rrs1. Markers that flank the Rrs1 locus were used to screen the wild barley-derivedBC₃F₂ population. These markers also flank the wild barley-derived scald resistance, indicating that it maps to the same locus as Rrs1; it may beallelic, or a separate gene within a complex locus. However, BC line 240 does not respond to treatment with the Rhynchosporium secalis avirulence factorNIP1 in the same way as the Rrs1-carrying cultivar Atlas46. This suggests that the scald resistance gene derived from wild barley confers a different specificity of response to theRrs1 allele in Atlas46.In order to increase the durability of scald resistance in the field, we suggest that at least two scald resistances should be combined into barley cultivars before release. The scald resistance gene described here will be of value in the Australian environment, and the several markers linked to it will facilitate pyramiding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of resistance genes against scald (Rhynchosporium secalis (Oudem.) J.J. Davis) in barley (Hordeum vulgare L.) lines from central Norway, by means of genetic markers and pathotype tests

Rhynchosporium secalis is a serious pathogen of barley (Hordeum vulgare L.) in central Norway. A breeding effort was initiated in 1977 to introduce resistance from different sources into adapted genotypes, and the first cultivar from the program was recently released. However, little is known about the resistance genes introgressed in this cultivar or in advanced breeding lines. An effort was made to address this issue through a set of isolates and available molecular markers. Fourteen breeding lines and their resistance donors were investigated by evaluating their reactions to 11 R. secalis isolates. Bulked segregant analysis was used to identify molecular markers linked to resistance genes in 12 of the breeding lines. The isolates were found to be of less discriminating value than the markers. Useful information has been obtained as to the nature of several of the resistance genes introgressed. Eight of the 12 breeding lines contained introgressed genes that were located at the `complex Rh' locus on chromosome 3H and hence may not easily be pyramided into the same genotype. Previous information about the nature of the resistance in `Jet' is questioned. Neither of the resistance genes Rh or Rh2 seems to have been incorporated into Norwegian breeding material.     

The transfer of a gene conferring resistance to scald (Rhynchosporium secalis) from Hordeum bulbosum into H. vulgare chromosome 4HS

Scald is a serious foliar disease that infects barley (Hordeum vulgare L.) causing reduced yields and adversely affecting quality. A means to combat the disease is to breed cultivars that possess genetic resistance. However, all known resistance alleles have so far originated from within the primary genepool of barley. This reliance on H. vulgare and H. vulgare ssp. spontaneum as resistance sources may encourage virulent forms of the pathogen to become established. To broaden the genetic base of cultivated barley and provide novel resistances to many diseases we have used a species from the secondary genepool of barley, H. bulbosum, in a resistance‐breeding programme. In this study we describe the development and trialling of a scald‐resistant recombinant line derived from a hybrid between H. vulgare and H. bulbosum. The scald resistance is simply inherited and located on the short arm of barley chromosome 4 (4HS).     

Genes for scald resistance from wild barley (Hordeum vulgare ssp spontaneum) and their linkage to isozyme markers

Summary Accessions of Hordeum vulgare ssp. spontaneum, the wild progenitor of barley, collected in Israel (70), Iran (15) and Turkey (6) were screened for seedling response to four isolates of Rhynchosporium secalis, the pathogen causing leaf scald in barley. Resistance was very common in the collection (77%) particularly among accessions from the more mesic sites (90%). The genetics of this resistance were investigated in fifteen backcross (BC₃) lines that contained an isozyme variant from H.v. ssp. spontaneum in a H.v. ssp. vulgare (cv. Clipper) background and were resistant to scald. Segregation in the BC₃F₂ families conformed with a single dominant resistance gene in 9 of the 15 lines. Scald resistance and the isozyme marker were closely linked in three of the BC₃-lines, loosely linked in four and unlinked in the remaining eight. Scald resistance genes were identified on barley chromosomes 1, 3, 4 and 6. Crosses between several of the scald resistant BC-lines together with the linkage data indicated that at least five genetically independent resistances are available for combining together for deployment in barley. The linkage of scald resistance in several BC₃-lines to the isozyme locus Acp2 is of special interest as this locus is highly polymorphic in wild barley.     

Exploitable genetic variation in a composite bulk population of barley

Summary Advanced agricultures are characterized by a great increase in the adoptation of uniform crop cultivars associated with a drastic reduction in locally adapted variation, the plant breeders should also examine means of creating and conserving genetic resources. Composite cross breeding is a technique that creates and preserves genetic variation in an exploitable form. The merits and problems of this method were discussed. using data obtained from barley Composite Cross XXI.     

The assessment of variation in two populations of Hordeum bulbosum L. for improving success rates in a doubled haploid barley programme

Summary Improvements in the success of doubled haploid production have been achieved partly by screening new accessions of Hordeum bulbosum. In order to assess levels of variation within stocks of this species, rates of embryo differentiation from the cross between H. vulgare × H. bulbosum were recorded for selections derived from two different stocks of H. bulbosum. There was little difference within stocks for this character despite variation in the morphology and banding patterns of two enzyme systems of one of the stocks. It is proposed that to obtain further increases in success rates a few selections from many accessions of H. bulbosum should be screened rather than many selections from a few stocks.     

Plant regeneration from embryo-callus culture in barley

Summary Plants were regenerated from callus cultures initiated from immature embryos of barley, Hordeum vulgare L. Immature embryos from seven diverse genotypes were cultured on modified Murashige and Skoog (MS) medium supplemented with 1.5 mg 2,4-D and 6.5 mg IAA/l. Of the 249 embryos cultured, 30% initiated callus within 8 days. Subculture of callus for 80 to 100 days on half-MS medium supplemented with 0.5 mg/l 2,4-D and 1.0 mg/l zeatin resulted in organogenesis. Culture of organogenic calli for 30 days on half-MS medium without growth regulators produced plants which originated mostly via multiple shoot formation. Callusing response of the tested genotypes ranged from zero to 44%; however, only 23% of the calli were regenerative. Regenerated plants included variants for chlorophyll deficiency, plant height, stem thickness, spike shape, pollen fertility, seed set and ploidy.     

Diversity among Finnish net blotch isolates and resistance in barley

Summary Seedlings of a differential barley (Hordeum vulgare L.) series (21 genotypes) and of six check genotypes were used in the greenhouse to assess variation in virulence among 20 single-spore isolates of the net blotch pathogen. Pyrenophora teres Drechs. f.teres Smedeg., collected from various sites in Finland. The experiment was run twice and symptom expression was recorded on the first three leaves. Analysis of second leaf symptom scores from Run 1 indicated differences in virulence between isolates, all of which were pathogenic, and differential resistance among the barleys. The virulence of P. teres isolates appeared to be conditioned by the host barley from which the isolate derived; the average virulence of isolates collected from a susceptible host was greater than that of isolates collected from a less susceptible host. Results from Run 2 were similar regarding resistance within the barleys, but variation in virulence among the P. teres isolates was not consistent with that from Run 1. CI 9819 caries duplicate genes for resistance to P. teres (Rpt1b and Rpt1c), and CI 7548 possesses Rpt3d. Both genotypes were highly resistant to all isolates. The Rpt1a gene of Tifang (CI4407) conferred resistance to all isolates in Run 2, but only to about half of the isolates in Run 1. The checks, including two of the symptomatically most resistant Nordic barley genotypes, were universally susceptible during these stringent tests. No selective pressure has been placed on Finnish isolates of P. teres through previous deployment of major resistance genes, and it is speculated that any variation in virulence among isolates is likely to be due to a combination of evolutionary forces including, natural selection, random genetic drift and gene flow.     

Genotypic variation in polyphenol content of barley grain

The polyphenol content in pearl barley, which is highly correlated to a browning reaction after heat treatment, was investigated using 1,347 cultivated barley varieties (H. vulgare) and two wild accessions (H. vulgare subsp. spontaneum) collected from different areas of the world. The polyphenol content in the cultivated barley shows a wide variation ranging from 0.19 to 0.75 mg/g with a nearly normal frequency distribution. The polyphenol content in the hulless varieties from Japan and Korea was low. On the other hand, the polyphenol content in wild barley was about two times higher than the average value recorded in cultivated barley. Based on HPLC analysis, five lowest-polyphenol content local varieties do not represent proanthocyanidin-free mutants. This revised version was published online in August 2006 with corrections to the Cover Date.