Mapping spot blotch resistance genes in four barley populations

Bipolaris sorokiniana (teleomorph: Cochliobolus sativus) is the fungal pathogen responsible for spot blotch in barley (Hordeum vulgare L.) and occurs worldwide in warmer, humid growing conditions. Current Australian barley varieties are largely susceptible to this disease and attempts are being made to introduce sources of resistance from North America. In this study we have compared chromosomal locations of spot blotch resistance reactions in four North American two-rowed barley lines; the North Dakota lines ND11231-12 and ND11231-11 and the Canadian lines TR251 and WPG8412-9-2-1. Diversity arrays technology-based PCR, expressed sequence tag and SSR markers have been mapped across four populations derived from crosses between susceptible parental lines and these four resistant parents to determine the location of resistance loci. Quantitative trait loci (QTL) conferring resistance to spot blotch in adult plants (APR) were detected on chromosomes 3HS and 7HS. In contrast, seedling resistance (SLR) was controlled solely by a locus on chromosome 7HS. The phenotypic variance explained by the APR QTL on 3HS was between 16 and 25% and the phenotypic variance explained by the 7HS APR QTL was between 8 and 42% across the four populations. The SLR QTL on 7HS explained between 52 and 64% of the phenotypic variance. An examination of the pedigrees of these resistance sources supports the common identity of resistance in these lines and indicates that only a limited number of major resistance loci are available in current two-rowed germplasm.     

Mapping Rph20: a gene conferring adult plant resistance to Puccinia hordei in barley

A doubled haploid (DH) barley (Hordeum vulgare L.) population of 334 lines (ND24260?×?Flagship) genotyped with DArT markers was used to map genes for adult plant resistance (APR) to leaf rust (Puccinia hordei Otth) under field conditions in Australia and Uruguay. The Australian barley cultivar Flagship carries an APR gene (qRphFlag) derived from the cultivar Vada. Association analysis and composite interval mapping identified two genes conferring APR in this DH population. qRphFlag was mapped to the short arm of chromosome 5H (5HS), accounting for 64?C85% of the phenotypic variation across four field environments and 56% under controlled environmental conditions (CEC). A second quantitative trait locus (QTL) from ND24260 (qRphND) with smaller effect was mapped to chromosome 6HL. In the absence of qRphFlag, qRphND conferred only a low level of resistance. DH lines displaying the highest level of APR carried both genes. Sequence information for the critical DArT marker bPb-0837 (positioned at 21.2?cM on chromosome 5HS) was used to develop bPb-0837-PCR, a simple PCR-based marker for qRphFlag. The 245?bp fragment for bPb-0837-PCR was detected in a range of barley cultivars known to possess APR, which was consistent with previous tests of allelism, demonstrating that the qRphFlag resistant allele is common in leaf rust resistant cultivars derived from Vada and Emir. qRphFlag has been designated Rph20, the first gene conferring APR to P. hordei to be characterised in barley. The PCR marker will likely be effective in marker-assisted selection for Rph20.     

Conversion of an amplified fragment length polymorphism marker into a co-dominant marker in the mapping of the<Emphasis Type="Italic"> Rph15</Emphasis> gene conferring resistance to barley leaf rust,<Emphasis Type="Italic"> Puccinia hordei</Emphasis> Otth.

Leaf rust, caused by Puccinia hordei, is an important disease afflicting barley (Hordeum vulgare) in many production regions of the world. The leaf rust resistance gene Rph15 was identified in an accession of wild barley (Hordeum vulgare subsp. spontaneum) and is one of the most broadly effective resistance genes known. Using amplified fragment length polymorphism (AFLP) and simple sequence repeat markers, Rph15 was mapped to chromosome 2HS in an F₂ population derived from a cross between Bowman (Rph15), a Bowman backcross-derived line carrying Rph15, and the susceptible cultivar Bowman. AFLP marker P13M40 co-segregated with Rph15 in this mapping population and two others involving Bowman (Rph15) and cultivars Proctor and Nudinka. The dominant AFLP marker P13M40 was converted to a co-dominant PCR-based marker that may be useful in breeding programs employing marker-assisted selection. The allelic relationship between Rph15 and the gene Rph16, also mapping to chromosome 2HS, was studied. The lack of segregation in F₂ progeny derived from the two resistance sources indicates that Rph15 and Rph16 are alleles of the same locus.Communicated by F. Salamini     

Association mapping of resistance to Puccinia hordei in Australian barley breeding germplasm

Key message

“To find stable resistance using association mapping tools, QTL with major and minor effects on leaf rust reactions were identified in barley breeding lines by assessing seedlings and adult plants.”

Abstract

Three hundred and sixty (360) elite barley (Hordeum vulgare L.) breeding lines from the Northern Region Barley Breeding Program in Australia were genotyped with 3,244 polymorphic diversity arrays technology markers and the results used to map quantitative trait loci (QTL) conferring a reaction to leaf rust (Puccinia hordei Otth). The F_3:5 (Stage 2) lines were derived or sourced from different geographic origins or hubs of international barley breeding ventures representing two breeding cycles (2009 and 2011 trials) and were evaluated across eight environments for infection type at both seedling and adult plant stages. Association mapping was performed using mean scores for disease reaction, accounting for family effects using the eigenvalues from a matrix of genotype correlations. In this study, 15 QTL were detected; 5 QTL co-located with catalogued leaf rust resistance genes (Rph1, Rph3/19, Rph8/14/15, Rph20, Rph21), 6 QTL aligned with previously reported genomic regions and 4 QTL (3 on chromosome 1H and 1 on 7H) were novel. The adult plant resistance gene Rph20 was identified across the majority of environments and pathotypes. The QTL detected in this study offer opportunities for breeding for more durable resistance to leaf rust through pyramiding multiple genomic regions via marker-assisted selection.     

Microarray analysis of the interaction between the aphid <Emphasis Type="Italic">Rhopalosiphum padi</Emphasis> and host plants reveals both differences and similarities between susceptible and partially resistant barley lines

The bird cherry-oat aphid (Rhopalosiphum padi L.) is an important pest on cereals causing plant growth reduction without specific leaf symptoms. Breeding of barley (Hordeum vulgare L.) for R. padi resistance shows that there are several resistance genes, reducing aphid growth. To identify candidate sequences for resistance-related genes, we performed microarray analysis of gene expression after aphid infestation in two susceptible and two partially resistant barley genotypes. One of the four lines is a descendant of two of the other genotypes. There were large differences in gene induction between the four lines, indicating substantial variation in response even between closely related genotypes. Genes induced in aphid-infested tissue were mainly related to defence, primary metabolism and signalling. Only 24 genes were induced in all lines, none of them related to oxidative stress or secondary metabolism. Few genes were down-regulated, with none being common to all four lines. There were differences in aphid-induced gene regulation between resistant and susceptible lines. Results from control plants without aphids also revealed differences in constitutive gene expression between the two types of lines. Candidate sequences for induced and constitutive resistance factors have been identified, among them a proteinase inhibitor, a serine/threonine kinase and several thionins. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetic mapping of a barley leaf rust resistance gene <Emphasis Type="Italic">Rph26</Emphasis> introgressed from <Emphasis Type="Italic">Hordeum bulbosum</Emphasis>

Key message

The quantitative barley leaf rust resistance gene, Rph26, was fine mapped within a H. bulbosum introgression on barley chromosome 1HL. This provides the tools for pyramiding with other resistance genes.

Abstract

A novel quantitative resistance gene, Rph26, effective against barley leaf rust (Puccinia hordei) was introgressed from Hordeum bulbosum into the barley (Hordeum vulgare) cultivar ‘Emir’. The effect of Rph26 was to reduce the observed symptoms of leaf rust infection (uredinium number and infection type). In addition, this resistance also increased the fungal latency period and reduced the fungal biomass within infected leaves. The resulting introgression line 200A12, containing Rph26, was backcrossed to its barley parental cultivar ‘Emir’ to create an F₂ population focused on detecting interspecific recombination within the introgressed segment. A total of 1368 individuals from this F₂ population were genotyped with flanking markers at either end of the 1HL introgression, resulting in the identification of 19 genotypes, which had undergone interspecific recombination within the original introgression. F₃ seeds that were homozygous for the introgressions of reduced size were selected from each F₂ recombinant and were used for subsequent genotyping and phenotyping. Rph26 was genetically mapped to the proximal end of the introgressed segment located at the distal end of chromosome 1HL. Molecular markers closely linked to Rph26 were identified and will enable this disease resistance gene to be combined with other sources of quantitative resistance to maximize the effectiveness and durability of leaf rust resistance in barley breeding. Heterozygous genotypes containing a single copy of Rph26 had an intermediate phenotype when compared with the homozygous resistant and susceptible genotypes, indicating an incompletely dominant inheritance.

     

Molecular mapping of a new gene in wild barley conferring complete resistance to leaf rust (Puccinia hordei Otth)

 A dominant gene conferring resistance to all known races of Puccinia hordei Otth was identified in two accessions of Hordeum vulgare ssp. spontaneum. Using restriction fragment length polymorphism (RFLP) markers the gene was mapped on chromosome 2HS in doubled-haploid populations derived from crosses of both accessions to the susceptible cultivar L94. Until now, complete leaf rust resistance was not known to be conditioned by genetic factors on this barley chromosome. Therefore, the designation Rph16 is proposed for the gene described in this study. A series of sequence tagged site (STS) and cleaved amplified polymorphic sequence (CAPS) markers were generated by conversion of RFLP probes which originate from the chromosomal region carrying the resistance gene. Two PCR-based markers were shown to co-segregate with the Rph16 gene in both populations thus providing the basis for marker-assisted selection. Received: 20 May 1998 / Accepted: 9 June 1998     

Gene family encoding basic pathogenesis-related 1 proteins in barley

A genomic (prb1) and two cDNA clones (PRb1-2 and PRb1-3) corresponding to two new barley basic PR-1 proteins (prb1-2 and prb1-3) were isolated from Hordeum vulgare. Genomic analysis of DNA suggests that the barley genome contains at least 6 members corresponding to the gene family encoding PR-1 proteins. Expression of these genes was induced in primary leaf tissues of the H. vulgare cv. Psaknon 4^* (F14) Man. carrying Mlp resistance gene (cv. Mlp) and the near-isogenic susceptible cultivar (cv. mlp) after inoculation with Erysiphe graminis f.sp. hordei.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">Rph14</Emphasis> in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

An incompletely dominant gene conferring resistance to Puccinia hordei, Rph14, identified previously in an accession of Hordeum vulgare, confers resistance to all known pathotypes of P. hordei in Australia. Knowledge of the chromosomal location of Rph14 and the identification of DNA markers closely linked to it will facilitate combining it with other important leaf rust resistance genes to achieve long lasting resistance. The inheritance of Rph14 was confirmed using 146 and 106 F₃ lines derived from the crosses ‘Baudin’/‘PI 584760’ (Rph14) and ‘Ricardo’/‘PI 584760’ (Rph14), respectively. Bulk segregant analysis on DNA from the parental genotypes and resistant and susceptible DNA bulks using DArT markers located Rph14 to the short arm of chromosome 2H. DArT marker bPb-1664 was identified as having the closest genetic association with Rph14. PCR based marker analysis identified a single SSR marker, Bmag692, linked closely to Rph14 at a map distance of 2.1 and 3.8 cm in the ‘Baudin’/‘PI 584760’and ‘Ricardo’/‘PI 584760’ populations, respectively.     

Resistance to Rhynchosporium secalis in the winter barley cultivar Vulcan

The six-rowed winter barley cultivar Vulcan was found to be highly resistant to Rhynchosporium secalis in the field, although this resistance was not apparent in routine screening tests of barley genotypes carried out at the seedling stage. More precise seedling tests, using graded inoculum levels to derive dose-response curves, showed that cv. Vulcan had an ED₅₀ which was 4.8 times that of the susceptible, two-rowed cv. Maris Otter, and 4.1 times that of cv. Maris Puma. The inheritance of resistance was investigated at the adult plant stage in a cross with cv. Maris Otter. Although a clear-cut segregation of the F₃ progeny into resistant, segregating and susceptible phenotypes was not observed, the genetic variance of the F₃ family means was compatible with the hypothesis of monogenic control of resistance. Resistance was inherited independently of the factor for six-rowed head type.     

Accumulation of genes for susceptibility to rust fungi for which barley is nearly a nonhost results in two barley lines with extreme multiple susceptibility

Nonhost resistance is the most common type of resistance in plants. Understanding the factors that make plants susceptible or resistant may help to achieve durably effective resistance in crop plants. Screening of 109 barley (Hordeum vulgare L.) accessions in the seedling stage indicated that barley is a complete nonhost to most of the heterologous rust fungi studied, while it showed an intermediate status with respect to Puccinia triticina, P. hordei-murini, P. hordei-secalini, P. graminis f. sp. lolii and P. coronata ff. spp. avenae and holci. Accessions that were susceptible to a heterologous rust in the seedling stage were much more or completely resistant at adult plant stage. Differential interaction between barley accessions and heterologous rust fungi was found, suggesting the existence of rust-species-specific resistance. In particular, many landrace accessions from Ethiopia and Asia, and naked-seeded accessions, tended to be susceptible to several heterologous rusts, suggesting that some resistance genes in barley are effective against more than one heterologous rust fungal species. Some barley accessions had race-specific resistance against P. hordei-murini. We accumulated genes for susceptibility to P. triticina and P. hordei-murini in two genotypes called SusPtrit and SusPmur, respectively. In the seedling stage, these accessions were as susceptible as the host species to the target rusts. They also showed unusual susceptibility to other heterologous rusts. These two lines are a valuable asset to further experimental work on the genetics of resistance to heterologous rust fungi.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00425-004-1319-1Abbreviations ff. spp Formae speciales - RIL Recombinant inbred line - DC Double cross - DC-S Progeny produced by selfing of double-cross plants     

Identification of additional sources of resistance to Puccinia striiformis f. sp. hordei (PSH) in a collection of barley genotypes adapted to the high input condition

A total of 336 barley genotypes consisting of released cultivars, advanced lines, differentials and local landraces from the ICARDA barley breeding programme were screened for seedling and adult‐plant resistances to barley stripe rust pathogen (Puccinia striiformis f. sp. hordei [PSH]). Seedling resistance tests were undertaken at Shimla, India by inoculating 336 barley genotypes with five prevalent PSH races [Q (5S0), 24 (0S0‐1), 57 (0S0), M (1S0) and G (4S0)] in India. Barley genotypes were also evaluated at the adult‐plant stage for stripe rust resistance at Durgapura (Rajasthan, India) in 2013 and 2014, and at Karnal (Haryana, India) in 2014 under artificial PSH infection in fields, using a mixture of the five races. Twelve barley genotypes (ARAMIR/COSSACK, Astrix, C8806, C9430, CLE 202, Gold, Gull, Isaria, Lechtaler, Piroline, Stirling, and Trumpf) were resistant to all five PSH races at the seedling and adult‐plant stages. Two of these genotypes, Astrix and Trumpf, were part of international differentials and reveal that five races were avirulent to genes Rps4 (yr4), rpsAst, rpsTr1 and rpsTr2. These genes were highly effective against PSH races prevalent in India. The virulence/avirulence formula reported in this study helped to determine the effectiveness of PSH resistance genes against Indian races. Forty‐five genotypes showed adult‐stage plant resistance (APR) in the field. The identified PSH resistant genotypes may possess novel resistance genes and might serve as potential donors of PSH resistance at seedling and APR in the future. Further research is needed to determine the nature of resistance genes through allelic studies and mapping of these genes.     

Inheritance and molecular mapping of a gene conferring seedling resistance against Puccinia hordei in the barley cultivar Ricardo

Genetic studies were undertaken to determine the inheritance and genomic location of uncharacterised seedling resistance to leaf rust, caused by Puccinia hordei, in the barley cultivar Ricardo. The resistance was shown to be conferred by a single dominant gene, which was tentatively designated RphRic. Bulk segregant analysis (BSA) and genetic mapping of an F₃ mapping population using multiplex-ready SSR genotyping and Illumina GoldenGate SNP assay located RphRic in chromosome 4H. Given that this is the first gene for leaf rust resistance mapped on chromosome 4H, it was designated Rph21. The presence of an additional gene, Rph2, in Ricardo, was confirmed by the test of allelism. The seedling gene Rph21 has shown effectiveness against all Australian pathotypes of P. hordei tested since at least 1992 and hence represents a new and useful source of resistance to this pathogen.     

Investigating successive Australian barley breeding populations for stable resistance to leaf rust

Key message

Genome-wide association studies of barley breeding populations identified candidate minor genes for pairing with the adult plant resistance gene Rph20 to provide stable leaf rust resistance across environments.

Abstract

Stable resistance to barley leaf rust (BLR, caused by Puccinia hordei) was evaluated across environments in barley breeding populations (BPs). To identify genomic regions that can be combined with Rph20 to improve adult plant resistance (APR), two BPs genotyped with the Diversity Arrays Technology genotyping-by-sequencing platform (DArT-seq) were examined for reaction to BLR at both seedling and adult growth stages in Australian environments. An integrated consensus map comprising both first- and second-generation DArT platforms was used to integrate QTL information across two additional BPs, providing a total of four interrelated BPs and 15 phenotypic data sets. This enabled identification of key loci underpinning BLR resistance. The APR gene Rph20 was the only active resistance region consistently detected across BPs. Of the QTL identified, RphQ27 on chromosome 6HL was considered the best candidate for pairing with Rph20. RphQ27 did not align or share proximity with known genes and was detected in three of the four BPs. The combination of RphQ27 and Rph20 was of low frequency in the breeding material; however, strong resistance responses were observed for the lines carrying this pairing. This suggests that the candidate minor gene RphQ27 can interact additively with Rph20 to provide stable resistance to BLR across diverse environments.

     

Lr68: a new gene conferring slow rusting resistance to leaf rust in wheat

The common wheat cultivar Parula possesses a high level of slow rusting, adult plant resistance (APR) to all three rust diseases of wheat. Previous mapping studies using an Avocet-YrA/Parula recombinant inbred line (RIL) population showed that APR to leaf rust (Puccinia triticina) in Parula is governed by at least three independent slow rusting resistance genes: Lr34 on 7DS, Lr46 on 1BL, and a previously unknown gene on 7BL. The use of field rust reaction and flanking markers identified two F₆ RILs, Arula1 and Arula2, from the above population that lacked Lr34 and Lr46 but carried the leaf rust resistance gene in 7BL, hereby designated Lr68. Arula1 and Arula2 were crossed with Apav, a highly susceptible line from the cross Avocet-YrA/Pavon 76, and 396 F₄-derived F₅ RILs were developed for mapping Lr68. The RILs were phenotyped for leaf rust resistance for over 2 years in Ciudad Obregon, Mexico, with a mixture of P. triticina races MBJ/SP and MCJ/SP. Close genetic linkages with several DNA markers on 7BL were established using 367 RILs; Psy1-1 and gwm146 flanked Lr68 and were estimated at 0.5 and 0.6 cM, respectively. The relationship between Lr68 and the race-specific seedling resistance gene Lr14b, located in the same region and present in Parula, Arula1 and Arula2, was investigated by evaluating the RILs with Lr14b-avirulent P. triticina race TCT/QB in the greenhouse. Although Lr14b and Lr68 homozygous recombinants in repulsion were not identified in RILs, γ-irradiation-induced deletion stocks that lacked Lr68 but possessed Lr14b showed that Lr68 and Lr14b are different loci. Flanking DNA markers that are tightly linked to Lr68 in a wide array of genotypes can be utilized for selection of APR to leaf rust.     

PCR markers for selection of adult plant leaf rust resistance in barley (Hordeum vulgare L.)

Adult plant resistance (APR) is considered potentially more durable for controlling barley leaf rust than seedling Rph (Resistance to Puccinia hordei) genes. A major gene for adult plant resistance to barley leaf rust has been mapped to the telomere region of chromosome 5HS. PCR-based molecular markers were developed for saturation of this region based on previously mapped simple sequence repeat, restriction fragment length polymorphism and Diversity Arrays Technology markers. In addition, defence gene homologue (DGH) and wheat expressed sequence tags mapped in specific bins were used to develop new PCR markers. Seventeen PCR-based markers were mapped to the short arm of chromosome 5H in 292 doubled haploid lines from a cross of Pompadour × Stirling, in which seven markers were mapped within 5 cM of the APR gene. The closest linked marker was about 0.7 cM from the APR gene. The wheat deletion bin map together with defence gene homologues was demonstrated to be an efficient tool for development of new molecular markers associated with the disease resistance gene. Four DGH markers were associated with the APR gene. The new molecular markers are a useful tool for marker-assisted selection of the APR gene and provided a better understanding of the molecular mechanism for leaf rust resistance.     

Differential expression of putative cell death regulator genes in near-isogenic,resistant and susceptible barley lines during interaction with the powdery mildew fungus

We analysed pathogenesis-related expression of genes, that are assumed to be involved in ubiquitous plant defence mechanisms like the oxidative burst, the hypersensitive cell death reaction (HR) and formation of localized cell wall appositions (papillae). We carried out comparative northern blot and RT-PCR studies with near-isogenic barley (Hordeum vulgareL. cv. Pallas) lines (NILs) resistant or susceptible to the powdery mildew fungus race A6 (Blumeria graminis f.sp. hordei, BghA6). The NILs carrying one of the R-genes Mla12, Mlg or the mlo mutant allele mlo5 arrest fungal development by cell wall appositions (mlo5) or a HR (Mla12) or both (Mlg). Expression of an aspartate protease gene, an ascorbate peroxidase gene and a newly identified cysteine protease gene was up-regulated after inoculation with BghA6, whereas the constitutive expression-level of a BAS gene, that encodes an alkyl hydroperoxide reductase, was reduced. Expression of a newly identified barley homologue of a mammalian cell death regulator, Bax inhibitor 1, was enhanced after powdery mildew inoculation. An oxalate oxidase-like protein was stronger expressed in NILS expressing penetration resistance. A so far unknown gene that putatively encodes the large subunit of a superoxide generating NADPH oxidases was constitutively expressed in barley leaves and its expression pattern did not change after inoculation. A newly identified barley Rac1 homologue was expressed constitutively, such as the functionally linked NADPH oxidase gene. Gene expression patterns are discussed with regard to defence mechanisms and signal transduction.     

Ryd4 Hb : a novel resistance gene introgressed from Hordeum bulbosum into barley and conferring complete and dominant resistance to the barley yellow dwarf virus

Barley yellow dwarf virus (BYDV) causes high yield losses in most of the major cereal crops worldwide. A source of very effective resistance was detected within the tetraploid wild species of Hordeum bulbosum. Interspecific crosses between a resistant H. bulbosum accession and H. vulgare cv. ‘Igri’ were performed to transfer this resistance into cultivated barley. Backcrosses to H. vulgare resulted in offspring which carried a single subterminal introgression of H. bulbosum chromatin on barley chromosome 3HL and proved to be fully resistant to BYDV-PAV, as inferred by ELISA values of zero or close to zero and lack of BYDV symptoms. Genetic analysis indicated a dominant inheritance of the BYDV-PAV resistance factor, which we propose to denote Ryd4 ^Hb . The identity and effect of Ryd4 ^Hb are discussed in relation to other known genes for BYDV resistance or tolerance, as well as the relevance of this gene for resistance breeding in barley.     

Interaction between Rhynchosporium secalis and Pyrenophora teres in the Field and Identification of Genotypes with Double Resistance in a Doubled-haploid Barley Population

Net blotch [Pyrenophora teres (Died.) Drechsl.] and scald [Rhynchosporium secalis (Oudem.) J.J. Davis] are the two most important foliar diseases of barley (Hordeum vulgare L.) in Tunisia. The use of cultivars with double resistance is the most effective method in controlling both diseases. A doubled‐haploid barley population derived from Tunisian cultivars was evaluated to both net blotch and scald during two growing seasons in the field. Mass disease index (MDI), area under the disease progress curve (AUDPC) and apparent infection type (r) were used to assess disease reaction. MDI of net blotch and scald reached up to 65% and 90% respectively. Least significant difference (LSD) test and comparison of the reaction of the doubled haploid (DH) lines to the overall population mean value were efficient in identifying lines with double resistance to both diseases. From the 59 DH lines screened, lines 21, 33, 37, 46 and 47 showed the best level of adult plant resistance to both diseases and may be used in a breeding program for diseases resistance. Interactions between R. secalis and P. teres were investigated at the level of the whole plant under variable epidemic conditions. Under low epidemic conditions, net blotch and scald developments were usually independent, but positively associated for tolerant lines for both diseases. Under high epidemic conditions, competition effects were obtained for susceptible and resistant genotypes. This competition seems to be an exploitation competition that is associated with decreasing resource availability as it occurs only with high levels of infestation or/and when susceptible lines are infected. This study demonstrates the variability of net blotch and scald interaction with epiphytotic conditions and group of resistance.     

Effect of yellow rust on yield components of barley cultivars with race-specific and slow rusting resistance to yellow rust

Yellow rust caused by Puccinia striiformis f. sp. hordei is an important disease of barley (Hordeum vulgare L.) in some parts of the world. We compared the effectiveness of different types of resistance in field plots at Ardabil Agricultural Research Station (Iran) during 2010–2011. Yield components along with slow rusting parameters including final rust severity (FRS), apparent infection rate (r), relative area under disease progress curve (rAUDPC) and coefficient of infection (CI) were evaluated for 25 barley cultivars. In all, two barley cultivars with race-specific resistance, 19 cultivars with different levels of slow rusting resistance and four susceptible cultivars were included in two experiments with and without fungicide protection under high disease pressure. Barley cultivars with slow rusting resistance displayed a range of severity responses indicating phenotypic diversity. Mean thousand kernels weight (TKW) losses for susceptible, race-specific and slow rusting genotypes were 31, 3 and 12%, respectively, and mean kernels per spike (KPS) losses for susceptible, race-specific and slow rusting genotypes were 19, 0.2 and 8%, respectively. Correlation coefficient of mean TKW and KPS losses with epidemiological parameters; rAUDPC, r, CI and FRS were highly significant. Slow rusting cultivars with low values of different parameters as well as genotypes with low yield component losses despite moderate disease levels were identified. Such genotypes can be used for breeding barely genotypes with high levels of resistance and negligible yield losses.