Single Resistance Gene Research Articles

PmLX66 and PmW14: New Alleles of Pm2 for Resistance to Powdery Mildew in the Chinese Winter Wheat Cultivars Liangxing 66 and Wennong 14.

Wheat powdery mildew (caused by Blumeria graminis f. sp. tritici) can be effectively managed by growing resistant cultivars. 'Liangxing 66' and 'Wennong 14' are the current winter wheat cultivars grown in northern China where powdery mildew is epidemic. Both cultivars have been demonstrated to carry single dominant genes for resistance to powdery mildew, tentatively designated PmLX66 and PmW14, on chromosome 5DS and share common linked markers with Pm2. Allelism tests were performed using a total of 15,657 plants of F2 segregating populations to determine the relationship between PmLX66, PmW14, and Pm2. All progeny from the crosses Liangxing 66 × 'Ulka/8*Chancellor' (Ulka/8*Cc), Wennong 14 × Ulka/8*Cc, and Liangxing 66 × Wennong 14 were resistant when tested with B. graminis f. sp. tritici isolate E20, indicating that PmLX66 and PmW14 are allelic to Pm2 and to each other. Liangxing 66 was resistant to 76.7% of the 60 B. graminis f. sp. tritici isolates from northern China, a slightly smaller proportion than Ulka/8*Cc (78.3%). However, Wennong 14 (85.0%) was more resistant against this set of B. graminis f. sp. tritici isolates than Ulka/8*Cc and Liangxing 66. Liangxing 66 and Wennong 14 differed from Ulka/8*Cc in respect to a number of B. graminis f. sp. tritici isolates. Based on these findings, PmLX66 and PmW14 are new alleles at the Pm2 locus.

Detection and location of Lr11 and other leaf rust resistance genes in the durably resistant wheat cultivar Buck Poncho

Three race-specific genes were identified in a cross between the durably leaf rust resistant wheat cultivar Buck Poncho (BP) and leaf rust susceptible cultivar Purple Straw (P). Two seedling resistance genes, temporarily named LrBP1 and LrBP2, and one adult plant resistance gene named LrBP3 were identified. Multi-pathotype tests on seedlings of BP and Thatcher near-isogenic lines carrying single resistance genes indicated that Lr10 and Lr11 were candidates for LrBP1 and LrBP2, respectively. The identification of LrBP1 as Lr10 was corroborated by markers previously reported to be linked to Lr10. LrBP2 was linked to 12 markers located on the short arm of chromosome 2D, the closest of which was SCAR32/35 at 0.3 cM. Lr11 was previously located in chromosome 2A, but monosomic analysis and telocentric mapping indicated that Lr11 was distally located in chromosome 2DS. The presence of the expected PCR fragment of SCAR32/35 in BP and wheat lines carrying Lr11, and results from an allelism test between a stock carrying LrBP2 with one carrying Lr11, indicated that these two genes are most likely the same. Thirteen markers located in chromosome 6A were loosely linked to LrBP3. Lines carrying the three resistance genes were evaluated for leaf rust response in the field. None of the gene combinations showed infection severities as low as BP, indicating that the three genes only partially explained the observed level of resistance in BP.

Resistance of ‘Zhongmai 155’ Wheat to Powdery Mildew: Effectiveness and Detection of the Resistance Gene

ABSTRACTPowdery mildew, caused by the biotrophic parasitic fungus Blumeria graminis f. sp. tritici (Bgt), is a prevalent disease in many wheat (Triticum aestivum L.)–producing regions throughout the world. To reduce yield losses and avoid environmental pollution caused by fungicide application, growing resistant cultivars is the preferred means for managing wheat powdery mildew. ‘Zhongmai 155’, a commercial winter wheat cultivar released in 2012, was resistant to 92 of the 115 Bgt isolates originating from northern China. Using segregating F2 and F2:3 populations derived from Zhongmai 155 × ‘Han 4564’ cross, genetic analysis indicated that a single dominant resistance gene, tentatively designated PmZ155, was responsible for the resistance of Zhongmai 155 to the Bgt isolate E09. The gene was subsequently localized on chromosome 5DS on the basis of the results of molecular marker analysis. The flanking markers SCAR203 and Xcfd8 are linked to gene PmZ155 at genetic distances of 0.3 and 4.9 cM, respectively, and were physically mapped to the deletion bin 5DS‐1‐0‐0.63; therefore, PmZ155 is located in this region of chromosome 5DS and shares the same molecular markers as Pm2. However, Zhongmai 155 differed from Ulka/8*Cc (CI 14118), which carries gene Pm2 on chromosome 5DS, in its reactions to 26 of 111 commonly tested isolates, of which 18 isolates were avirulent on Zhongmai 155 but virulent on Ulka/8*Cc, while 8 isolates were virulent on Zhongmai 155 but avirulent on Ulka/8*Cc. Zhongmai 155 also differed from ‘Tabasco’ (carrying Pm48 on chromosome 5DS) in its reaction to 14 of 62 isolates. On the basis of its chromosome position, close linkage to the molecular markers Xcfd8 and SCAR203, but different responses to an array of Bgt isolates, it appears that PmZ155 is most likely an allele of locus Pm2.

The gene PmYB confers broad-spectrum powdery mildew resistance in the multi-allelic Pm2 chromosome region of the Chinese wheat cultivar YingBo 700

It is important to detect wheat broad-spectrum powdery mildew resistance (Pm) genes in commercial cultivars with superior agronomic performance. YingBo 700, a Chinese commercial cultivar registered and released in 2012, showed a broad spectrum of resistance to 48 of 49 tested Blumeria graminis f. sp tritici (Bgt) isolates from different regions of China. Genetic analysis of F2 populations from the crosses YingBo 700 × Mingxian 169 and YingBo 700 × Shimai 15 and F2:3 lines from YingBo 700 × Mingxian 169 demonstrated that the resistance was controlled by a single dominant resistance gene, which was temporarily designated PmYB. Using molecular markers and bulked segregant analysis of the F 2:3 lines from YingBo 700 × Mingxian 169, PmYB was mapped to the multi-allelic Pm2 region of chromosome arm 5DS co-segregated with the sequence-characterized amplified region (SCAR) marker SCAR112 and flanked by the simple sequence repeat (SSR) marker Cfd81 and SCAR marker SCAR203 with genetic distance of 0.9 and 1.9 cM, respectively. YingBo 700 showed a unique response pattern to different Bgt isolates compared with the lines with documented Pm2a, Pm2b, Pm48, PmLX66, PmW14, PmZ155 and PmX3986-2 on chromosome arm 5DS. Therefore, PmYB appears to be a novel allele in this multi-allelic region. In order to validate applicability of the closely linked markers for marker-assisted selection, the closely linked markers Cfd81, SCAR112, SCAR203 and Mag6176 were evaluated for applicability in diagnosing the resistance gene in 62 cultivars from different regions of China. The results will contribute to rapidly transfer of PmYB into more cultivars.

Collections of Puccinia triticina in Different Provinces of China Are Highly Related for Virulence and Molecular Genotype.

Collections of Puccinia triticina, the wheat leaf rust pathogen, were obtained from seven provinces in China from 2009 and 2010. Single uredinial isolates were derived and tested for virulence phenotype to 20 lines of Thatcher wheat that differ for single leaf rust resistance genes, and for molecular genotype with 23 simple sequence repeat (SSR) primers. Forty-eight virulence phenotypes were described among the 155 isolates tested for virulence. All but four isolates were virulent to Lr26, and no isolates with virulence to Lr18 or Lr24 were found. The three most common phenotypes, FCBQQ, PCGLN, and PCGLL, were found in five, five, and three provinces, respectively. Thirty-six SSR genotypes were found among the 100 isolates tested for molecular variation. Isolates with identical virulence phenotypes and SSR genotypes were found in more than one province. Analysis of variation showed no overall differentiation of SSR genotypes or virulence phenotypes based on province of origin. The SSR genotypes had high levels of linkage disequilibrium, high levels of observed heterozygosity, and significant correlation with the virulence phenotypes, all measures that indicated clonal reproduction. Bayesian cluster analysis and principle component plots indicated three groups of SSR genotypes that also varied significantly for virulence. The seven provinces are continuously adjacent to each other and likely form a single epidemiological zone for P. triticina.

Rmg7, a New Gene for Resistance to Triticum Isolates of Pyricularia oryzae Identified in Tetraploid Wheat.

A single gene for resistance, designated Rmg7 (Resistance to Magnaporthe grisea 7), was identified in a tetraploid wheat accession, St24 (Triticum dicoccum, KU120), against Br48, a Triticum isolate of Pyricularia oryzae. Two other wheat accessions, St17 (T. dicoccum, KU112) and St25 (T. dicoccum, KU122), were also resistant against Br48 and showed a similar disease reaction pattern to St24. Crosses between these resistant accessions yielded no susceptible F2 seedlings, suggesting that St24, St17, and St25 carry the same resistance gene. Furthermore, a single avirulence gene corresponding to Rmg7 was detected in a segregation analysis of random F1 progenies between Br48 and MZ5-1-6, an Eleusine isolate virulent to St24 at a higher temperature. This avirulence gene was recognized not only by St24, but also by St17 and St25, thus supporting the preceding results indicating that all three accessions carry Rmg7. This resistance gene may have potential in future wheat breeding programs.

Characterisation of the late blight resistance in potato differential MaR9 reveals a qualitative resistance gene, R9a, residing in a cluster of Tm-2 (2) homologs on chromosome IX.

Key message The durable late blight resistance in potato plant Ma R9 is genetically characterized. A novel R -gene is mapped. The monogenic nature and map positions of R9 are negated and rectified. Late blight of potato (Solanum tuberosum), caused by Phytophthora infestans, can effectively be managed by genetic resistance. The MaR9 differential plant provides durable resistance to a broad spectrum of late blight strains. This resistance is brought about by at least seven genes derived from S. demissum including R1, Rpi-abpt1, R3a, R3b, R4, R8 and, so far uncharacterized resistance gene(s). Here we set out to genetically characterize this additional resistance in MaR9. Three BC1 populations derived from MaR9 were identified that segregated for IPO-C resistance but that lacked R8. One BC1 population showed a continuous scale of resistance phenotypes, suggesting that multiple quantitative resistance genes were segregating. In two other BC1 populations resistance and susceptibility were segregating in a 1:1 ratio, suggesting a single qualitative resistance gene (R9a). A chromosome IX PCR marker, 184-81, fully co-segregated with R9a. The map position of R9a on the distal end of the lower arm of chromosome IX was confirmed using PCR markers GP101 and Stm1021. Successively, cluster-directed profiling (CDP) was carried out, revealing six closely linked markers. CDPSw58, CDPSw59 and CDPSw510 flanked the R9a gene at the distal end (5.8 cM) and, as expected, were highly homologous to Sw-5. CDPTm22 flanked R9a on the proximal side (2.9 cM). CDPTm26 and CDPTm27 fully co-segregated with resistance and had high homology to Tm-2 2, showing that R9a resides in a cluster of NBS–LRR genes with homology to Tm-2 2. Besides R9a, additional resistance of quantitative nature is found in MaR9, which remains to be genetically characterized.Electronic supplementary materialThe online version of this article (doi:10.1007/s00122-015-2480-6) contains supplementary material, which is available to authorized users.

Virulence Characterization of Venturia inaequalis Reference Isolates on the Differential Set of Malus Hosts.

A set of differential hosts has recently been identified for 17 apple scab resistance genes in an updated system for defining gene-for-gene (GfG) relationships in the Venturia inaequalis-Malus pathosystem. However, a set of reference isolates characterized for their complementary avirulence alleles is not yet available. In this paper, we report on improving the set of differential hosts for h(7) and propose the apple genotype LPG3-29 as carrying the single major resistance gene Rvi7. We characterized a reference set of 23 V. inaequalis isolates on 14 differential apple hosts carrying major resistance genes under controlled conditions. We identified isolates that were virulent on at least one of the following defined resistance gene hosts: h(1), h(2), h(3), h(4), h(5), h(6), h(7), h(8), h(9), h(10), and h(13). Sixteen different virulence patterns were observed. In general, the isolates carried one to three virulences, but some of them were more complex, with up to six virulences. This set of well-characterized isolates will be helpful for the identification of additional apple scab resistance genes in apple germplasm and the characterization of new GfG relationships to help improve our understanding of the host-pathogen interactions in the V. inaequalis-Malus pathosystem.

Eastern Filbert Blight Resistance in Hazelnut Accessions ‘Culplà’, ‘Crvenje’, and OSU 495.072

European hazelnut (Corylus avellana L.) is a significant crop in Oregon, where 99% of United States hazelnuts are produced. Eastern filbert blight (EFB) caused by Anisogramma anomala (Peck) E. Müller is an important disease that infects the trees, reduces yield, and causes premature death. Managing the disease through cultural methods and fungicide applications is laborious and expensive, and genetic host resistance is considered the most viable option for control. Genetic resistance from ‘Gasaway’ has been used to develop resistant cultivars including Yamhill and Jefferson, but concern about the durability of this single resistance gene stimulated a search for additional sources of resistance. This study used three recently identified sources of EFB resistance: ‘Culplà’ from Spain, ‘Crvenje’ from Serbia, and OSU 495.072 from southern Russia. RAPD markers linked to resistance from ‘Gasaway’ were absent in all three accessions. Disease response was noted in segregating progenies following greenhouse or structure inoculation, and the resistance loci were mapped using microsatellite markers. In only four of the nine progenies did segregation for disease response fit the ratio of 1 resistant:1 susceptible expected for a single locus, a heterozygous resistant parent, and a dominant allele for resistance. Three progenies showed an excess of resistant seedlings while two showed a deficiency of resistant seedlings. The reciprocal translocations reported in several leading hazelnut cultivars may be present in the parents of the studied progenies, and affecting the segregation ratios. Microsatellite marker A614, previously mapped to linkage group (LG) 6, was closely linked to resistance from all three sources. Maps were constructed for LG6 for each resistant parent using microsatellite markers. The three resistance loci mapped to the same region on LG6 where resistance from ‘Gasaway’ and OSU 408.040 are located. The resistance alleles in all five accessions may be the same, or more likely are a cluster of different resistance genes in the same region. Markers LG628, LG610, and LG696 will be useful to breed new hazelnut cultivars with resistance from Culplà, Crvenje, and OSU 495.072.

Effect of Marker Aided Pyramiding of Anthracnose and Pythium Root Rot Resistance Genes on Plant Agronomic Characters among Advanced Common Bean Genotypes

One of the factors that accounts for the low yields in common bean is the simultaneous occurrence of diseases on the common bean crop. Bean root rots and anthracnose are the most important common bean diseases that simultaneously occur on the bean crop in Uganda. Moreover, Colletotrichum lindemuthianum, the pathogen that causes bean anthracnose, possesses a high genetic variability which makes it easily break down single gene resistance. Pyramiding resistance genes for both diseases in commercial varieties would ensure reduction of yield losses resulting from the two diseases. However, the effect of marker assisted gene pyramiding on plant agronomic characters is not well understood. Three-way crosses were made to pyramid three anthracnose and one Pythium root rot resistance genes in four susceptible market class varieties. Sequence characterized amplified regions (SCAR) markers were used to facilitate the pyramiding scheme. Correlation analysis and Path coefficient analysis were used to assess the association between number of pyramided genes and different plant agronomic characters. Number of pyramided genes was negatively correlated with number of pods per plant (-0.32), number of seeds per plant (-0.25), number of seeds per pod (-0.18), pod length (-0.17), days to 50% flowering (-0.09) and 100-seed weight (-0.02). Path coefficient analysis showed that number of pyramided genes, plant height, days to 50% flowering, number of seeds per pod and number of pods per plant had negative direct effects on seed weight per plant. Number of seeds per plant had the highest positive direct effects (0.98) followed by 100-seed weight (0.28) while days to maturity had the least positive direct effect (0.03) on seed weight per plant. Only number of seeds per plant had its correlation coefficient (0.94) almost equal to the direct path coefficient (0.97). Number of pyramided genes had significant (P < 0.05) negative indirect effect on seed weight per plant only through number of seeds per plant (-0.25). Therefore, pyramiding higher numbers of resistance genes may cause a grain yield reduction via number of seeds per plant. Therefore, it is important for breeders to simultaneously select for number of pyramided genes with number of seeds per plant and other highly associated traits.

Geographic and genetic identification of RppS, a novel locus conferring broad resistance to southern corn rust disease in China

Southern corn rust, a devastating fungal disease of maize, can cause significant loss of grain yield during intermittent outbreak. Growing resistant varieties is the most cost-effective way to manage southern rust in field corn. However, the prevalence of incomplete resistance owing to rapid evolution of pathogen warrants the necessities of discovering new resistant resources. To this end, the disease occurrence of 15 maize inbred lines in response to southern rust was examined in fields located in tropical, subtropical and temperate regions respectively. Overall, the character of resistance or susceptibility was rather steady among locations for most of inbred lines although infrequent variations existed. A tropical inbred line, SCML205, displayed fully resistant response in all the locations tested. By contrast, Jing24, W1636, Zheng58, W541 and G10-2426 exhibited completely susceptible response. A 3:1 segregation ratio of resistance to susceptibility was identified in the progeny of F2 population generated from the cross of SCML205 × Jing24, demonstrating SCML205 harbors a single dominant resistant gene. Allelism test was performed to confirm the resistance conferred by a novel locus, which was designated as RppS. Genetic mapping were conducted to locate RppS to the distal arm of chromosome 10S, a region close to other resistant genes known so far. Together, our survey of the resistance to southern rust in numerous inbred lines leads to the identification of a novel resistant gene, which would be tremendously valuable for plant breeding.

Inheritance of resistance to bud fly infestation in linseed (Linum usitatissimumL.)

Genetic analysis was carried out to determine the mode of inheritance of resistance to bud fly which is a key pest of linseed during flowering stage. Parents, F1, F2, BC1 and BC2 populations of three crosses involving two resistant, IC 15888 and JRF4 and two susceptible parents, viz., Neelum and GS-41 were the experimental materials. Reaction to bud fly in F1s of all the three crosses indicated that resistance was dominant over susceptibility. The F2 and back cross segregants were categorized in to resistant and susceptible classes based on per cent bud infestation. The genetic analysis in F2 generation revealed that two dominant complementary genes in IC 15888 control the bud fly resistance, while JRF 4 carried a single dominant gene for resistance. The F2 results were confirmed by genetic analysis of back cross populations. It can be categorically stated that the knowledge of number and nature of resistance genes in IC 15888 and JRF4 will be valuable for development of bud fly resistant varieties in linseed.

The genetic basis of resistance to barley grass yellow rust (Puccinia striiformis f. sp. pseudo-hordei) in Australian barley cultivars.

Resistance to Puccinia striiformis in 18 barleys was conferred by one or more genes. In two genotypes, resistance mapped to chromosomes 5HL and 7HL (seedling), and 5HS (adult plant). Twenty barley genotypes were assessed for resistance to a variant of P. striiformis [barley grass yellow rust (BGYR)] that is adapted to wild Hordeum sp. (barley grass) and is known to be virulent on several Australian barley cultivars. With the exception of Biosaline-19, all of the genotypes tested were resistant to BGYR. Genetic analyses of 16 Australian and three exotic barley lines indicated that each carried at least a single gene for resistance. Seedling resistance genes identified in the doubled haploid population developed from a cross between Franklin and Yerong were mapped to the long arm of chromosomes 5H and 7H, respectively. These genes were given the temporary designations of Rpsp-hFranklin and Rpsp-hYerong. Three QTL were detected in the same population when tested at the adult plant stage, two of them being in a similar position to Rpsp-hFranklin and Rpsp-hYerong and the third one was mapped to 5HS. Allelism tests between genotypes that exhibited seedling infection type responses to BGYR that were similar to Franklin and Yerong revealed that resistance in most were genetically independent of Rpsp-hFranklin and Rpsp-hYerong.

Genetic analysis of leaf rust resistance in six durum wheat genotypes.

Leaf rust, caused by Puccinia triticina, is one of the main fungal diseases limiting durum wheat production. This study aimed to characterize previously undescribed genes for leaf rust resistance in durum wheat. Six different resistant durum genotypes were crossed to two susceptible International Maize and Wheat Improvement Center (CIMMYT) lines and the resulting F1, F2, and F3 progenies were evaluated for leaf rust reactions in the field and under greenhouse conditions. In addition, allelism tests were conducted. The results of the study indicated that most genotypes carried single effective dominant or recessive seedling resistance genes; the only exception to this was genotype Gaza, which carried one adult plant and one seedling resistance gene. In addition, it was concluded that the resistance genes identified in the current study were neither allelic to LrCamayo or Lr61, nor were they related to Lr3 or Lr14a, the genes that already are either ineffective or are considered to be vulnerable for breeding purposes. A complicated allelic or linkage relationship between the identified genes is discussed. The results of the study will be useful for breeding for durable resistance by creating polygenic complexes.

Chromosomal location and comparative genomics analysis of powdery mildew resistance gene Pm51 in a putative wheat-Thinopyrum ponticum introgression line.

Powdery mildew (PM) is a very destructive disease of wheat (Triticum aestivum L.). Wheat-Thinopyrum ponticum introgression line CH7086 was shown to possess powdery mildew resistance possibly originating from Th. ponticum. Genomic in situ hybridization and molecular characterization of the alien introgression failed to identify alien chromatin. To study the genetics of resistance, CH7086 was crossed with susceptible genotypes. Segregation in F2 populations and F2:3 lines tested with Chinese Bgt race E09 under controlled conditions indicated that CH7086 carries a single dominant gene for powdery mildew resistance. Fourteen SSR and EST-PCR markers linked with the locus were identified. The genetic distances between the locus and the two flanking markers were 1.5 and 3.2 cM, respectively. Based on the locations of the markers by nullisomic-tetrasomic and deletion lines of ‘Chinese Spring’, the resistance gene was located in deletion bin 2BL-0.89-1.00. Conserved orthologous marker analysis indicated that the genomic region flanking the resistance gene has a high level of collinearity to that of rice chromosome 4 and Brachypodium chromosome 5. Both resistance specificities and tests of allelism suggested the resistance gene in CH7086 was different from previously reported powdery mildew resistance genes on 2BL, and the gene was provisionally designated PmCH86. Molecular analysis of PmCH86 compared with other genes for resistance to Bgt in the 2BL-0.89-1.00 region suggested that PmCH86 may be a new PM resistance gene, and it was therefore designated as Pm51. The closely linked flanking markers could be useful in exploiting this putative wheat-Thinopyrum translocation line for rapid transfer of Pm51 to wheat breeding programs.

Construction of genetic linkage map and graphical genotyping of pseudo-backcrossed F2 (BC’2) progeny to introduce a CTV resistance from Poncirus trifoliata (L.) Raf. into Citrus by introgression breeding

Most citrus tristeza virus (CTV) strains infect almost all Citrus species and have caused serious damage on citrus production in many countries. Poncirus trifoliata (trifoliate orange) has a single dominant resistance gene effective against a broad range of CTV strains. P. trifoliata is sexually compatible with Citrus species, and introgression breeding programs have been initiated to introduce the CTV resistance into Citrus. “Kankitsu Chukanbohon Nou 8 Gou” (Nou-8) was developed by crossing Citrus sp. “Kiyomi” with an F1 hybrid derived from Citrus hassaku × P. trifoliata, but it still had undesirable eating traits. In aim to progress marker-assisted introgression breeding for CTV resistance efficiently, we selected 189 DNA markers which could be applied to genotyping for Citrus and P. trifoliata and constructed a genetic linkage map of Nou-8. The map included 189 DNA markers on 10 linkage groups and extended about 722.8 cM in total length. CTV resistance locus was located at 2.4 cM position in linkage group 02_(1), with the same location found when progeny were scored for resistance using an ELISA assay and for DNA marker for CTV resistant (CTV103). Graphical genotypes of Nou-8 and its progeny suggested that (1) 36 % of the chromosome in the BC’2 population were inherited from Nou-8 without any recombination sites, (2) Nou-8 had been already completely substituted by Citrus genotype in two linkage groups, and (3) four of 93 progeny of Nou-8 had recombination sites in the flanking region of the CTV resistance gene locus.

Combination of seedling and adult plant resistance to leaf scald for stable resistance in barley

Rhynchosporium secalis can overcome a single resistance gene of barley in a relatively short period of time. Novel genes and quantitative trait locis (QTLs) are therefore vital to control scald in barley. A population of 220 double haploid lines was developed from a cross of Vlamingh and WABAR2147, where Vlamingh showed adult plant resistance (APR) and WABAR2147 showed seedling resistance to a group of isolates. The population was tested for APR to scald under natural infection in two consecutive seasons in addition to a seedling screen with three isolates. One single gene was mapped to chromosome 6H based on the seedling test, and two QTLs (QSc.VlWa.4H and QSc.VlWa.6H) were mapped to chromosomes 4H and 6H based on APR. Epistatic interaction was observed between the two QTLs, and environment/QTL interaction was only observed for QSc.VlWa.6H which co-segregated with the seedling resistance gene and contributed to basal resistance against scald during whole growth stages. QSc.VlWa.4H explained 42.5 and 57.8 % of the phenotypic variation in the two independent trials when the effect of QSc.VlWa.6H was excluded from the analysis. We developed a high-density consensus genetic map with 7,876 molecular makers and anchored 43 QTLs and 7 genes for scald resistance from different mapping populations. No known QTLs or genes were reported in a similar position to QSc.VlWa.4H, and it was the first major QTL for APR of scald on chromosome 4HS in barley. Combination of the two QTLs achieved better and stable scald resistance across four different environments.

CURRENT STATUS OF EASTERN FILBERT BLIGHT RESISTANCE SOURCES AND MAPPING

Eastern filbert blight (EFB), caused by the pyrenomycete Anisogramma anomala (Peck) E. MA¼ller, is a devastating disease of European hazelnut (Corylus avellana L.) in the USA’s Pacific Northwest. Complete resistance from ‘Gasaway’, controlled by a dominant allele at a single locus, has been extensively used in the hazelnut breeding program at Oregon State University (OSU) since its discovery in 1975. Most advanced selections and the cultivars ‘Santiam’, ‘Yamhill’ and ‘Jefferson’ carry resistance from ‘Gasaway’. Concern over the breakdown of a single resistance gene prompted a continuing search for new sources of resistance. Greenhouse inoculation studies identified several sources of qualitative resistance, including accessions of C. avellana, other Corylus species and interspecific hybrids. The challenges of breeding for resistance to a pathogen absent in Corvallis, state quarantine regulations to slow its spread, its two-year life cycle, and difficulties in determining disease response stimulated a search for molecular markers linked to resistance. We have used the robust RAPD markers 152-800 and 268-580 in marker-assisted selection for ‘Gasaway’ resistance for more than a decade. Molecular markers have also been identified linked to resistance from ‘Ratoli’, OSU 759.010 and OSU 408.040. Using simple sequence repeat (SSR) markers as anchor loci, we assigned resistance from ‘Gasaway’, ‘Ratoli’, OSU 759.010, OSU 408.040 and ‘Culpla’ to linkage groups 6, 7, 2, 6 and 6, respectively. In this paper, we summarize the current status of the EFB resistance sources being used in the OSU hazelnut breeding program. For resistance loci on different linkage groups, we intend to use pyramiding with an expectation that the combined resistance will be more durable. Molecular markers linked to each source of resistance will facilitate pyramiding.

Molecular mapping of genes Yr64 and Yr65 for stripe rust resistance in hexaploid derivatives of durum wheat accessions PI 331260 and PI 480016

This manuscript reports two new genes ( Yr64 and Yr65 ) for effective resistance to stripe rust and usefulness of their flanking SSR markers for marker-assisted selection. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases of wheat worldwide and resistance is the best control strategy. Durum wheat accessions PI 331260 and PI 480016 were resistant to all tested Pst races. To transfer the resistance genes to common wheat and map them to wheat chromosomes, both accessions were crossed with the stripe rust-susceptible spring wheat 'Avocet S'. Resistant F3 plants with 42 chromosomes were selected cytologically and by rust phenotype. A single dominant gene for resistance was identified in segregating F4 lines from each cross. F6 populations for each cross were developed from single F5 plants and used for genetic mapping. Different genes from PI 331260 and PI 480016 were mapped to different loci in chromosome 1BS using simple sequence repeat markers. The gene from PI 331260 was flanked by Xgwm413 and Xgdm33 in bin 1BS9-0.84-1.06 at genetic distances of 3.5 and 2.0 cM; and the gene from PI 480016 was flanked by Xgwm18 and Xgwm11 in chromosome bin C-1BS10-0.50 at 1.2 and 2.1 cM, respectively. Chromosomal locations and race and allelism tests indicated that the two genes are different from previously reported stripe rust resistance genes, and therefore are named as Yr64 from PI 331260 and Yr65 from PI 480016. These genes and their flanking markers, and selected common wheat lines with the genes should be valuable for diversifying resistance genes used in breeding wheat cultivars with stripe rust resistance.

One gene versus two: a regional study on the efficacy of single gene versus pyramided resistance for soybean aphid management.

The soybean aphid (Aphis glycines Matsumura) is a threat to soybean production in the Midwestern United States. Varieties containing the Rag1 soybean aphid resistance gene have been released with limited success in reducing aphid populations. Furthermore, virulent biotypes occur within North America and challenge the durability of single-gene resistance. Pyramiding resistance genes has the potential to improve aphid population suppression and increase resistance gene durability. Our goal was to determine if a pyramid could provide improved aphid population suppression across awide range of environments. We conducted a small-plot field experiment across seven states and three years. We compared soybean near-isolines for the Rag1 or Rag2 gene, and a pyramid line containing both genes for their ability to decrease aphid pressure and protect yield compared with a susceptible line. These lines were evaluated both with and without a neonicitinoid seed treatment. All aphid-resistant lines significantly decreased aphid pressure at all locations but one. The pyramid line experienced lower aphid pressure than both single-gene lines at eight of 23 location-years. Soybean aphids significantly reduced soybean yield for the susceptible line by 14% and for both single-gene lines by 5%; however, no significant yield decrease was observed for the pyramid line. The neonicitinoid seed treatment reduced plant exposure to aphids across all soybean lines, but did not provide significant yield protection for any of the lines. These results demonstrate that pyramiding resistance genes can provide sufficient and consistent yield protection from soybean aphid in North America.