Effective Sources Of Resistance Research Articles

Evaluation of Cultivated and Wild Allium Accessions for Resistance to Fusarium oxysporum f. sp. cepae

Fusarium basal rot (FBR) caused by Fusarium oxysporum f. sp. cepae (FOC) is a highly destructive soil borne disease incurring heavy damage in pre and post harvest onion and garlic crops worldwide. Only a few onion lines exhibit partial resistance against the pathogen and there is a need for identification of more effective resistance sources for use in breeding programmes. Selected sets of wild onion and garlic accession and seven related Allium species were screened for resistance to Fusarium basal rot using three FOC isolates. FOC infection revealed significant variation among the evaluated Allium species (at P = 0.001). A. sativum accession ‘CBT-As153’ showed high level of resistance to each isolate while A. cepa accession ‘CBT-Ac77’ exhibited intermediate resistance. Among related Allium species, A. fistulosum, A. roylei and A. schoenoprasum were highly resistant, A. tuberosum had mixed response while A. griffithianum was susceptible. Further, the root density of Allium species negatively correlated with disease incidence for different FOC isolates. Thus, the present study suggests that besides related Allium species, A. sativum ‘CBT-As153’ can be used as a potential donor of FBR resistance for genetic improvement of onion and garlic in India.

Host plant resistance towards the cabbage whitefly in Brassica oleracea and its wild relatives

The cabbage whitefly (Aleyrodes proletella) is a phloem-feeding insect that is a serious problem in Brassica oleracea crops like Brussels sprouts, kale and savoy cabbage. In order to develop whitefly-resistant varieties it is essential to identify effective sources of resistance. In this study, we screened a large collection of 432 accessions, including wild material and landraces of Brassica oleracea as well as crop wild relatives, to determine whitefly performance in a no-choice field experiment. Putatively resistant accessions were further tested under greenhouse conditions. Resistant accessions were identified among B. oleracea var. capitata (cabbage) landraces and in the species B. villosa, B. incana and B. montana. Whereas resistance in cabbage is only expressed in plants of at least 12 weeks old, some wild relatives were already starting to express resistance at 6 weeks. This could open up possibilities for breeding cabbages that are resistant at a young(er) plant age. Our research also shows again the importance of crop wild relatives for finding pest resistances.

Population study of Xanthomonas spp. from bean growing regions of Canada and response of bean cultivars to pathogen inoculation

Common bacterial blight caused by Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans pv. fuscans is a major disease of bean in Canada and elsewhere with limited disease management options. A population study was conducted to determine the diversity among strains isolated from bean plants, and to investigate whether different races exist in this population in Canada. A total of 49 strains, including reference strains from the USA and Mexico, were evaluated on seven bean accessions with different sources of resistance. A wide range of host responses was observed. Among the strains, 22 caused minute lesions on bean plants and were thus designated as non-pathogenic xanthomonads. Highest disease scores and the widest spread in score frequencies after inoculation with pathogenic strains were observed on the susceptible cultivar ‘CDC Pintium’ and the reportedly partially resistant cultivar ‘Weihing’, suggesting that the latter is not an effective source of resistance to the Canadian population. Low disease severities and score frequencies skewed towards resistance were observed on cultivars HR67, ICB-10, XR 235-1-1 and ‘OAC Rex’, whereas ‘OAC Seaforth’ showed intermediate reactions. Continuous distributions of disease severity scores of strains for each cultivar and detailed analyses of disease data revealed no evidence for a race structure in the population tested.

Pea Fusarium wilt races in western Algeria

The fungus Fusarium oxysporum f.sp. pisi (FOP), the pea wilt pathogen, causes appreciable yield losses under favourable environmental conditions in Algeria. Studies on the pathogen variability and distribution of races are essential to identify effective sources of resistance to this disease. In this study, a survey was conducted during the period 2007–2011 in four different agro-climatic zones. Pathogenic variability in 52 isolates of FOP, collected from different pea-growing areas of western Algeria, were evaluated using 7 genotypes as differential hosts. Results indicated that the disease was prevalent in all fields prospected and isolates obtained were assigned to Races 1, 2, 5 or 6 by their pathogenicity. It was found out that Races 1 and 2 were more common in all areas with 61.5 and 19.2%, respectively. This study is the first report of pea Fusarium wilt races distribution in Algeria.  

Identification of new sources of resistance to powdery mildew in oat

Powdery mildew caused by Blumeria graminis DC. f. sp. avenae Em. Marchal. is a deleterious foliar disease of common oat which occurs in many parts of the world. Resistance existing in oat cultivars is broken down by new races of this pathogen and because of this fact there is necessity to look for new and effective sources of resistance to powdery mildew in oat. To identify new potential resistance sources to powdery mildew, a total of 67 oat genotypes from five different species Avena sterilis, A. fatua, A. sativa, A. maroccana and A. murphyi and 20 powdery mildew isolates were investigated for disease reactions. Among tested oat accessions the highest level of resistance was associated with A. maroccana genotypes. Genotypes belonging to species of A. murphi and A. sterilis also showed a good level of resistance. Identified genotypes could be used in oat breeding programmes to improve levels of resistance to powdery mildew.

Resistance of winter wheat to Heterodera filipjevi in Turkey

Cereal cyst nematodes (CCNs) are plant parasites that significantly limit global cereal production. The most frequently reported pathogenic species are Heterodera avenae, H. filipjevi, and H. latipons. One of the most cost-effective, environmentally friendly, and easily adopted control measures is the use of genetic host resistance, which maintains nematode populations below the economic damage threshold level. Many effective sources of resistance to CCNs have been identified in cereals; however, their effectiveness and usefulness is dependent on the interaction of the specific putative resistant accession and the CCN pathotype found in a specific region. In this study, 719 wheat lines from the Facultative and Winter Wheat Observation Nurseries, representing a broad geographical spectrum of breeding lines and varieties from Europe, Central Asia, and the International Winter Wheat Improvement Program, were screened against H. filipjevi under controlled conditions. The results indicated that 114 and 90 genotypes were ranked resistant and moderately resistant, representing 15.8% and 12.5% of the screened genotypes, respectively. The frequency of resistant genotypes observed in the germplasms varied significantly among the different original countries and was the highest for genotypes that originated from Bulgaria (59.3%). From those phenotyped germplasms, a set of 289 lines was genotyped to understand if resistance sources are located at the same site or originate from different locations in the genome.

Characterization of Phytophthora resistance in soybean cultivars/lines bred in Henan province

Deployment of resistant soybean cultivars is the most effective and economical method of controlling Phytophthora root rot (PRR) incited by Phytophthora sojae, and characterization of Phytophthora resistance of the soybean cultivars greatly facilitates the effective utilization. The objective of this study was to characterize the resistance phenotype in 30 soybean cultivars/lines bred in Henan province and 4 ancestral cultivars which were inoculated with 26 P. sojae pathotypes. The 34 soybean cultivars/lines showed 34 different reaction types of resistance to 26 P. sojae pathotypes. The reaction types produced on the cultivars/lines were compared with those produced on the differential lines to postulate which Rps gene was present. The gene Rps5 and Rps3a or gene combination Rps3a+5 were postulated to be present in Zhoudou17 and Zheng77249, respectively. The other 32 cultivars/lines exhibited novel reaction types that were different from known single or two Rps gene combinations. The cluster analysis of the reaction types revealed 10 groups among the 34 soybean cultivars/lines, 17 differentials and the cultivar Williams at the similarity coefficient 0.6540. This study indicated that Phytophthora resistance was extremely diverse in this region. The cultivars/lines with broad spectrum resistance could provide effective sources of resistance for the control of PRR in the future.

Allele Characterization of Genes Required for rpg4-Mediated Wheat Stem Rust Resistance Identifies Rpg5 as the R Gene

A highly virulent form of the wheat stem rust pathogen Puccinia graminis f. sp. tritici race TTKSK is virulent on both wheat and barley, presenting a major threat to world food security. The recessive and temperature-sensitive rpg4 gene is the only effective source of resistance identified in barley (Hordeum vulgare) against P. graminis f. sp. tritici race TTKSK. Efforts to position clone rpg4 localized resistance to a small interval on barley chromosome 5HL, tightly linked to the rye stem rust (P. graminis f. sp. secalis) resistance (R) gene Rpg5. High-resolution genetic analysis and post-transcriptional gene silencing of the genes at the rpg4/Rpg5 locus determined that three tightly linked genes (Rpg5, HvRga1, and HvAdf3) are required together for rpg4-mediated wheat stem rust resistance. Alleles of the three genes were analyzed from a diverse set of 14 domesticated barley lines (H. vulgare) and 8 wild barley accessions (H. vulgare subsp. spontaneum) to characterize diversity that may determine incompatibility (resistance). The analysis determined that HvAdf3 and HvRga1 code for predicted functional proteins that do not appear to contain polymorphisms determining the compatible (susceptible) interactions with the wheat stem rust pathogen and were expressed at the transcriptional level from both resistant and susceptible barley lines. The HvAdf3 alleles shared 100% amino acid identity among all 22 genotypes examined. The P. graminis f. sp. tritici race QCCJ-susceptible barley lines with HvRga1 alleles containing the limited amino acid substitutions unique to the susceptible varieties also contained predicted nonfunctional rpg5 alleles. Thus, susceptibility in these lines is likely due to the nonfunctional RPG5 proteins. The Rpg5 allele analysis determined that 9 of the 13 P. graminis f. sp. tritici race QCCJ-susceptible barley lines contain alleles that either code for predicted truncated proteins as the result of a single nucleotide substitution, resulting in a stop codon at amino acid 161, a single cytosine indel causing a frame shift, and a stop codon at amino acid 217, or an indel that deleted the entire STPK domain. The three P. graminis f. sp. tritici race QCCJ-susceptible lines (Swiss landraces Hv489, Hv492, and Hv611) and the wild barley accession WBDC160 contain rpg5 alleles predicted to encode full-length proteins containing a nonsynonomous nucleotide substitution that results in the amino acid substitution E1287A. This amino acid substitution present in the uncharacterized C-terminal domain is not found in any resistant line and may be important to elicit the resistance reaction. These data suggest that rpg4-mediated resistance against many wheat stem rust pathogen races, including P. graminis f. sp. tritici race TTKSK, rely on the Rpg5 R gene; thus, rpg4- and Rpg5-mediated resistance rely on a common R gene and should not be considered completely distinct. The data also determined that Rpg5 gene-specific molecular markers could be used to detect rpg4-mediated wheat stem rust resistance for marker-assisted selection.

Genetic mapping of rust resistance genes in confection sunflower line HA-R6 and oilseed line RHA 397

Few widely effective resistance sources to sunflower rust, incited by Puccinia helianthi Schwein., have been identified in confection sunflower (Helianthus annuus L.). The USDA inbred line HA-R6 is one of the few confection sunflower lines resistant to rust. A previous allelism test indicated that rust resistance genes in HA-R6 and RHA 397, an oilseed-type restorer line, are either allelic or closely linked; however, neither have been characterized nor molecularly mapped. The objectives of this study are (1) to locate the rust resistance genes in HA-R6 and RHA 397 on a molecular map, (2) to develop closely linked molecular markers for rust resistance diagnostics, and (3) to determine the resistance spectrum of two lines when compared with other rust-resistant lines. Two populations of 140 F2:3 families each from the crosses of HA 89, as susceptible parent, with HA-R6 and RHA 397 were inoculated with race 336 of P. helianthi in the greenhouse. The resistance genes (R-genes) in HA-R6 and RHA 397 were molecularly mapped to the lower end of linkage group 13, which encompasses a large R-gene cluster, and were designated as R 13a and R 13b, respectively. In the initial maps, SSR (simple sequence repeat) and InDel (insertion and deletion) markers revealed 2.8 and 8.2cM flanking regions for R 13a and R 13b, respectively, linked with a common marker set of four co-segregating markers, ORS191, ORS316, ORS581, and ZVG61, in the distal side and one marker ORS464 in the proximal side. To identify new markers closer to the genes, sunflower RGC (resistance gene candidate) markers linked to the downy mildew R-gene Pl 8 and located at the same region as R 13a and R 13b were selected to screen the two F2 populations. The RGC markers RGC15/16 and a newly developed marker SUN14 designed from a BAC contig anchored by RGC251 further narrowed down the region flanking R 13a and R 13b to 1.1 and 0.1cM, respectively. Both R 13a and R 13b are highly effective against all rust races tested so far. Our newly developed molecular markers will facilitate breeding efforts to pyramid the R 13 genes with other rust R-genes and accelerate the development of rust-resistant sunflower hybrids in both confection and oilseed sunflowers.

Pathotype diversity among Algerian isolates of Pyrenophora teres f. teres

Barley Net Blotch caused by Pyrenophora teres f. teres (Ptt) is one of the most important foliar diseases in north western Algeria. The disease could cause appreciable yield losses under favorable environmental conditions. Studies on pathogen variability and distribution of virulence in Ptt are essential to identify effective sources of resistance to net blotch of barley. In this study, pathogenic variability in 48 isolates of Ptt , collected from different barley-growing areas of north-western Algeria were evaluated using 22 barley genotypes as differential hosts. Trials carried out under controlled conditions confirmed the large variation in the virulence of this fungus . The genotypes exhibited variability in response ranging from complete resistance to high susceptibility. This is the first report of pathogenic variability in Ptt in north western Algeria. The 48 isolates tested were further differentiated into 12 pathotypes according to their reaction on the host. Eight differential cultivars were resistant to all isolates tested, but others were susceptible to one or more of the isolates. The variability in the pathogen and the resistance identified in some genotypes used in this study are being investigated further to develop superior, adapted germplasm for use in barley breeding programs in north western Algeria.

Pathogenicity Spectra and Screening for Resistance in Barley Against Tunisian Pyrenophora teres f. teres.

This work aimed to determine patterns of pathogenicity in Pyrenophora teres f. teres and to identify potentially effective resistance sources that could be used as breeding material to control net blotch in Tunisia. Extensive pathogenic variability was detected in 85 isolates of P. teres causing net blotch of barley in Tunisia. Based on unweighted pair-group method with arithmetic averaging clustering and mean disease rating scores, three distinct virulence groups were identified. The isolates were classified into 23 pathotypes. Pathogenic variability within the groups was higher than that between the groups, a finding that can guide a rational choice of isolates for screening lines as part of a breeding program. Conversely, studying the relationship between geographic and pathotypic structure allowed us to detect a significant isolation by distance pattern, suggesting a regular and gradual dispersal of the pathogen over this spatial scale. Using specific resistance properties of individual barley genotypes as virulence markers, all the differential barley genotypes were shown to be distinct, and no single source of resistance was totally effective against all isolates.

Mapping of QTL for Fusarium head blight resistance and morphological and developmental traits in three backcross populations derived from Triticum dicoccum × Triticum durum

Breeding for resistance to Fusarium head blight (FHB) in durum wheat continues to be hindered by the lack of effective resistance sources. Only limited information is available on resistance QTL for FHB in tetraploid wheat. In this study, resistance to FHB of a Triticum dicoccum line in the background of three Austrian T. durum cultivars was genetically characterized. Three populations of BC1F4-derived RILs were developed from crosses between the resistant donor line T. dicoccum-161 and the Austrian T. durum recipient varieties DS-131621, Floradur and Helidur. About 130 BC1F4-derived lines per population were evaluated for FHB response using artificial spray inoculation in four field experiments during two seasons. Lines were genetically fingerprinted using SSR and AFLP markers. Genomic regions on chromosomes 3B, 4B, 6A, 6B and 7B were significantly associated with FHB severity. FHB resistance QTL on 6B and 7B were identified in two populations and a resistance QTL on 4B appeared in three populations. The alleles that enhanced FHB resistance were derived from the T. dicoccum parent, except for the QTL on chromosome 3B. All QTL except the QTL on 6A mapped to genomic regions where QTL for FHB have previously been reported in hexaploid wheat. QTL on 3B and 6B coincided with Fhb1 and Fhb2, respectively. This implies that tetraploid and hexaploid wheat share common genomic regions associated with FHB resistance. QTL for FHB resistance on 4B co-located with a major QTL for plant height and mapped at the position of the Rht-B1 gene, while QTL on 7B overlapped with QTL for flowering time.Electronic supplementary materialThe online version of this article (doi:10.1007/s00122-012-1951-2) contains supplementary material, which is available to authorized users.

Virulence in Oat Crown Rust (Puccinia coronata f. sp. avenae) in the United States from 2006 through 2009.

The use of race-specific seedling genes for resistance is the primary means of controlling crown rust of oat (Puccinia coronata f. sp. avenae) in the United States. To better utilize those resistance genes, knowledge of the occurrence and frequency of corresponding virulence in the population of P. coronata f. sp. avenae in the United States is essential. In total, 571 single-pustule isolates of oat crown rust were collected from cultivated and wild oat (Avena sativa and A. fatua, respectively) in the major oat production areas of the United States from 2006 through 2009. They were tested for virulence on seedlings of 31 differential oat lines in the greenhouse. In all, 201 races were found among the 357 isolates from the spring oat region of the north-central United States, and 140 races were found among 214 isolates from the southern winter oat region. The crown rust populations from the winter and spring oat regions were clearly differentiated from one another, differing in the frequency of virulence for 24 of the 31 differentials. Some virulence associations previously reported in the U.S. oat crown rust population were also found in both regions in this survey, even when the dataset was clone corrected. Associations between virulence to the Pc genes were predominately positive in both regions but both positive and negative associations occurred more frequently in the winter oat region, where sexual reproduction does not occur. Some of the virulence diversity in the oat crown rust population in the United States can be related to the deployment of resistance genes in commercial oat cultivars and virulence associations existing in the oat crown rust population. When data from a previous report covering 2001 through 2005 is combined with data reported in this article, the mean virulence of the U.S. populations of crown rust continued to increase from 2001 to 2009. Virulence to Pc38, Pc39, Pc45, Pc48, Pc52, Pc55, Pc56, Pc57, Pc59, Pc62, Pc63, Pc64, Pc68, and Pc96 significantly increased in one or both regions during this time period. No significant declines in virulence frequency were found in either region. Genes for crown rust resistance derived from A. sterilis appear to be as rapidly defeated as has happened to Pc genes from A. sativa. There is an urgent need to find additional sources of effective resistance to P. coronata f. sp. avenae and introgress it into adapted oat cultivars.

Comparative virulence of Pyrenophora teres f. teres from Syria and Tunisia and screening for resistance sources in barley: implications for breeding

The aim of this study is to investigate the pathogenic diversity and virulence groups among Pyrenophora teres f. teres isolates, sampled from Syria and Tunisia, and to identify the most effective source of resistance in barley that could be used in breeding programmes to control net blotch in both countries. One hundred and four isolates of P. teres f. teres were collected from barley in different agroecological zones of Tunisia and Syria. Their virulence was evaluated using 14 barley genotypes as differential hosts. The UPGMA clustering identified high pathogenic variability; the isolates were clustered onto 20 pathotypes that were sheltered under three virulence groups, with high, intermediate and low disease scores. According to susceptibility/resistance frequencies and mean disease ratings, CI05401 cultivar ranked as the best differential when inoculated with the Syrian isolates. However, CI09214 cultivar was classified as the best effective source of resistance in Tunisia. All P. teres f. teres isolates were differentially pathogenic. CI09214 and CI05401 cultivars were released as the most effective sources of resistance in Syria and Tunisia. National and international barley breeding programmes that seek to develop resistance against P. teres f. teres in barley should strongly benefit from this study. This resistance cannot be achieved without the proper knowledge of the pathogen virulence spectrum and the sources of host resistance.

Registration of Seven Winter Wheat Germplasm Lines Carrying the Wsm1 Gene for Wheat Streak Mosaic Virus Resistance

Seven winter wheat (Triticum aestivum L.) germplasm lines carrying the Wsm1 gene conferring resistance to Wheat streak mosaic virus (WSMV)—Alliance‐Wsm1 (Reg. No. GP‐858, PI 653710), Arrowsmith‐Wsm1 (Reg. No. GP‐859, PI 653711), Goodstreak‐Wsm1 (Reg. No. GP‐860, PI 653712), Harry‐Wsm1 (Reg. No. GP‐861, PI 653713), Millennium‐Wsm1 (Reg. No. GP‐862, PI 653714), Wahoo‐Wsm1 (Reg. No. GP‐863, PI 653715), and Wesley‐Wsm1 (Reg. No. GP‐864, PI 653716)—were codeveloped by Washington State University, Pullman, WA; the University of Nebraska, Lincoln, NE; and the USDA‐ARS. These seven different winter wheat cultivars were selected to provide more sources of effective resistance to WSMV in winter wheat cultivars of Nebraska and adjoining states. Resistance to WSMV is conferred by the Wsm1 gene, which was translocated from Thinopyrum intermedium (Host) Barkworth & D.R. Dewey [Agropyron intermedium (Horst.) Beauv.] into wheat. The STSJ15 marker was used to select for the gene in the backcross progeny until the BC4F1 generation. In BC4F2 generation, screening for disease resistance was done using the Sidney 81 isolate of WSMV, along with the recurrent parents. Lines showing high levels of resistance to WSMV were further selected for seed increase and field evaluation. These lines may serve as a winter wheat source of WSMV resistance and may be used for gene pyramiding and for studying the effect of the Wsm1 gene in different backgrounds.

Fontes de resistência em acessos de Solanum (secção Lycopersicon) a Verticillium dahliae raças 1 e 2

A murcha-de-verticílio (Verticillium dahliae) é uma das doenças mais severas do tomateiro (Solanum lycopersicum L.). Duas raças fisiológicas de V. dahliae foram caracterizadas infectando o tomateiro e ambas estão presentes no Brasil. A estratégia de controle mais efetiva para isolados da raça 1 tem sido a utilização de cultivares resistentes em associação com práticas culturais. Entretanto, não existem cultivares comerciais de tomate resistentes à raça 2 do patógeno. Neste contexto, torna-se importante a identificação de novas fontes de resistência que sejam efetivas contra as duas raças de V. dahliae. Buscou-se fontes de resistência a isolados de V. dahliae (raças 1 e 2). Cem acessos de uma coleção de germoplasma contendo espécies cultivadas e selvagens de Solanum (secção Lycopersicon) foram inicialmente inoculados pelo método de imersão de raízes com um isolado de V. dahliae raça 1 (5 mL de uma suspensão de 10(6) conídios/mL) em condições de casa de vegetação. A avaliação foi feita aos 30 dias após a inoculação, usando uma escala de notas variando de 1 (planta sadia) a 5 (planta morta). Foram considerados resistentes os acessos que apresentaram nota média entre 1 e 2. Um grupo composto por 38 acessos classificados como resistentes neste primeiro ensaio foi reavaliado para resistência a outros quatro isolados de V. dahliae pertencentes às raças 1 e 2. As cultivares Ponderosa e Floradade foram utilizadas como controles. O ensaio foi conduzido em casa de vegetação em delineamento experimental de blocos ao acaso em arranjo fatorial 5 x 40, com três repetições (vasos com quatro plantas). A avaliação foi feita baseada nos parâmetros epidemiológicos: período de incubação e índice de doença. Foram identificados acessos com resistência raça-específica e também com resistência a ambas as raças. Estes acessos podem ser indicados para futuros programas de melhoramento genético visando incorporar resistência ampla à doença.

Virulence in Hessian Fly (Diptera: Cecidomyiidae) Field Collections From the Southeastern United States to 21 Resistance Genes in Wheat

Genetic resistance in wheat, Triticum aestivum L., is the most efficacious method for control of Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). However, because of the appearance of new genotypes (biotypes) in response to deployment of resistance, field collections of Hessian fly need to be evaluated on a regular basis to provide breeders and producers information on the efficacy of resistance (R) genes with respect to the genotype composition of Hessian fly in regional areas. We report here on the efficacy of 21 R genes in wheat to field collections of Hessian fly from the southeastern United States. Results documented that of the 21 R genes evaluated only five would provide effective protection of wheat from Hessian fly in the southeastern United States. These genes were H12, H18, H24, H25, and H26. Although not all of the 33 identified R genes were evaluated in the current study, these results indicate that identified genetic resistance to protect wheat from Hessian attack in the southeastern United States is a limited resource. Historically, R genes for Hessian fly resistance in wheat have been deployed as single gene releases. Although this strategy has been successful in the past, we recommend that in the future deployment of combinations of highly effective previously undeployed genes, such as H24 and H26, be considered. Our study also highlights the need to identify new and effective sources of resistance in wheat to Hessian fly if genetic resistance is to continue as a viable option for protection of wheat in the southeastern United States.

Pathotypes and Genetic Diversity of Blumeria graminis f. sp. hordei in the Winter Barley Regions in China

Barley powdery mildew caused by Blumeria graminis f. sp. hordei ( Bgh) is one of the most destructive foliar diseases of barley in the winter barley region of China. The evaluation and assessment of the virulence and diversity of Bgh populations help to determine effective sources of resistance to the pathogen. 515 isolates were collected from seven populations of Bgh on cultivated barley in seven geographically distant locations in 2006. Their virulence was determined by inoculation onto 26 differential host lines. All of the isolates belonged to 58 pathotypes and 13 of which included 81% of these isolates. The most abundant pathotype was pathotype 0047 (18.3%), the second most abundant was pathotype 0045 (11.8%) and the third most abundant was pathotype 0057 (7.8%). Most of virulent genes investigated in this study showed similar frequencies in the seven different areas. These indicate that the seven locations may be in a uniform epidemiological region and barley cultivars in these areas may have similar genetic background. Diversities within these populations and distances between these populations measured by KOIND package were different. Correlations were not found between the genetic distance and the geographical distances between different locations. This suggested that long distance spread and local epidemics existed in the major winter barley growing regions in China.

Comparison of the Effects in Field Tests of Soybean Cyst Nematode Resistance Genes from Different Resistance Sources

The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) continues to be the most damaging soybean [Glycine max (L.) Merr.] pathogen in the United States. Many quantitative trait loci (QTL) that confer SCN resistance have been mapped. These QTL include rhg1, which was mapped in several sources of SCN resistance, and two resistance QTL mapped from Glycine soja Siebold & Zucc. The objective of this study was to test the effects of different QTL on SCN reproduction in the field, as well as their associated effects on agronomic traits. Three populations of near isogenic lines (NILs) that segregated for major SCN resistance QTL were developed and planted in field plots in 2006 and 2007. Plant introduction (PI) 88788, PI 437654, and G. soja PI 468916 were the sources of the SCN resistance QTL. Soybean cyst nematode field population densities were estimated from soil samples taken shortly after emergence [initial population density (Pi)] and again close to maturity [final population density (Pf)]. A lower SCN reproductive factor (Rf = Pi/Pf) was found on NILs with the SCN resistance alleles from PI 88788 and PI 468916 (p < 0.05) than those with the susceptibility alleles. The rhg1 allele from PI 88788 was also associated with greater yields in multiple environments than those lines with the susceptibility allele. The results indicate that the rhg1 allele from PI 88788 remains as an effective source of resistance to SCN, and with further research, the SCN resistance QTL alleles from G. soja may serve as a novel source of SCN resistance.

Development of a PCR Assay and Marker‐Assisted Transfer of Leaf Rust and Stripe Rust Resistance Genes Lr57 and Yr40 into Hard Red Winter Wheats

ABSTRACTThe wheat (Triticum aestivum L.)‐Aegilops geniculata introgression T5DL·5DS‐5MgS(0.95), with stripe rust resistance gene Yr40 and leaf rust resistance gene Lr57, is an effective source of resistance against most isolates of the rust pathogen in Kansas and India. Rust resistance genes Lr57 and Yr40 were transferred to the hard red winter wheat (HRWW) cultivars Jagger and Overley by standard backcrossing and marker‐assisted selection (MAS). PCR‐based cleaved amplified polymorphic sequence (CAPS) markers were developed and used to select backcross F1 plants with rust resistance. Diagnostic polymorphism detected by CAPS marker XLr57/Yr40‐MAS‐CAPS16 was consistent between the resistant source and a set of wheat cultivars selected by the U.S. Wheat Coordinated Agricultural Project. Homozygous BC3F2 plants with rust resistance genes were selected using the Restriction Fragment Length Polymorphisms (RFLP)‐based diagnostic DNA marker polymorphism detected by the wheat grain softness protein (GSP). Deletion of Puroindoline a (Pina‐Mg1) and Puroindoline b (Pinb‐Mg1) gene sequences in the chromosome segment of 5MgS substituting for 5DS of wheat suggests that the backcross derivatives with Lr57/Yr40 can be used to develop hard textured wheats. The HRWW germplasm with Lr57 and Yr40 will provide breeders in the Southern Great Plains with adapted lines having an additional source of resistance that can be used to achieve durability of rust resistance.