Stripe Rust Reaction Research Articles

Identification of genetic immunity to the warrior race and local population of stripe rust using conventional techniques in winter wheat genotypes

Wheat stripe rust caused by the Puccinia striiformis f. sp. tritici (Pst) is a major and devastating biotic stress inducing notable product losses in wheat (Triticum spp.). Possibility of the breakdown of important genes conferring resistance against to stripe rust and the threat of emergence of new aggressive races remain a concern in all wheat growing areas around the world. Therefore, breeding programs must continue screening of wide genetic diversity to improve new varieties with different gene combinations against race changes. Within this scope, 140 wheat genotypes obtained from various countries and programs were evaluated in terms of their reactions to stripe rust at both seedling and adult-plant stages in 2020 and 2021 growing seasons. The adult-plant stage evaluations of these genotypes were conducted under artificial inoculation of PstS7 (Warrior) race at Haymana location of Field Crops Central Research Institute in Ankara and under naturally occurring local stripe rust population at Menemen location of Aegean Agricultural Research Institute in Izmir. Whereas the seedling stage tests were carried out using the PstS7 race under controlled conditions at Pest and Disease Resistance Unit of the Field Crops Central Research Institute. More than 80% of the genotypes tested at both locations exhibited immune, resistant, and moderately resistant reactions at the adult-plant stage, while only 36.4% of the genotypes exhibited resistance reactions at the seedling stage. The coefficients of infection (CI) of the genotypes were calculated by using the stripe rust reaction and disease severity of each genotype. Following scales were used for classification of the genotypes based on the CI: 0 = Immune; 0.1-5.0 = Resistant; 5.01-20.0 = Moderately Resistant; 20.1-40.0 = Moderately Susceptible; 40.1-100 = Susceptible. In conclusion, 101 genotypes (72.1%) evaluated at both locations were found to be immune, resistant, and moderately resistant to both the PstS7 race and local stripe rust population. These genotypes have been proposed as resistance gene sources for cultivars to be developed.

Nebraska winter wheat unexpected flowering in Egypt: New improvement opportunities

AbstractThe University of Nebraska has bred some of the most winter hardy wheat (Triticum aestivum L.) cultivars in the United States. With the advent of a new race of stem rust in Africa, the breeding program identified a testing site in northern Egypt, a Mediterranean environment, for seedling disease resistance screening. Quite surprisingly, the Nebraska winter wheat lines flowered when grown in northern Egypt, indicating the lines were vernalized. To confirm this result, a panel of 112 Nebraska winter wheat genotypes and three commercially grown Egyptian spring wheat check cultivars were grown in three replicates in a randomized incomplete block design at two locations for 3 years (2016/2017–2018/2019) in Egypt. Seven traits were measured on all genotypes (the number of days to flowering; total chlorophyll content; plant height; leaf, stem, and stripe rust reaction; and grain yield). The winter wheat genotypes flowered and several of them were not significantly different in grain yield from the spring cultivars. A multi‐trait stability index identified 22 genotypes (20% selection intensity, the local checks, and 19 lines from the United States) for good performance. This study provides novel evidence that winter wheat can flower in northern Egypt, can be used for adult plant resistance screening, and provide a grain yield comparable to the commercially grown spring wheat cultivars. Our findings are expected to facilitate future efforts to hybridize winter‐by‐spring lines to develop new genotypes with unique combinations of genes to improve grain yield production, adaptation, disease resistance, and climate resiliency.

Fine mapping of a stripe rust resistance gene YrZM175 in bread wheat.

A stripe rust resistance gene YrZM175 in Chinese wheat cultivar Zhongmai 175 was mapped to a genomic interval of 636.4 kb on chromosome arm 2AL, and a candidate gene was predicted. Stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), is a worldwide wheat disease that causes large losses in production. Fine mapping and cloning of resistance genes are important for accurate marker-assisted breeding. Here, we report the fine mapping and candidate gene analysis of stripe rust resistance gene YrZM175 in a Chinese wheat cultivar Zhongmai 175. Fifteen F1, 7,325 F2 plants and 117 F2:3 lines derived from cross Avocet S/Zhongmai 175 were inoculated with PST race CYR32 at the seedling stage in a greenhouse, and F2:3 lines were also evaluated for stripe rust reaction in the field using mixed PST races. Bulked segregant RNA-seq (BSR-seq) analyses revealed 13 SNPs in the region 762.50-768.52Mb on chromosome arm 2AL. By genome mining, we identified SNPs and InDels between the parents and contrasting bulks and mapped YrZM175 to a 0.72-cM, 636.4-kb interval spanned by YrZM175-InD1 and YrZM175-InD2 (763,452,916-764,089,317bp) including two putative disease resistance genes based on IWGSC RefSeq v1.0. Collinearity analysis indicated similar target genomic intervals in Chinese Spring, Aegilops tauschii (2D: 647.7-650.5Mb), Triticum urartu (2A: 750.7-752.3Mb), Triticum dicoccoides (2A: 771.0-774.5Mb), Triticum turgidum (2B: 784.7-788.2Mb), and Triticum aestivum cv. Aikang 58 (2A: 776.3-778.9Mb) and Jagger (2A: 789.3-791.7Mb). Through collinearity analysis, sequence alignments of resistant and susceptible parents and gene expression level analysis, we predicted TRITD2Bv1G264480 from Triticum turgidum to be a candidate gene for map-based cloning of YrZM175. A gene-specific marker for TRITD2Bv1G264480 co-segregated with the resistance gene. Molecular marker analysis and stripe rust response data revealed that YrZM175 was different from genes Yr1, Yr17, Yr32, and YrJ22 located on chromosome 2A. Fine mapping of YrZM175 lays a solid foundation for functional gene analysis and marker-assisted selection for improved stripe rust resistance in wheat.

Identification of effective sources for spot blotch and stripe rust in barley

Stripe rust and spot blotch are major biotic constraints in yield maximization in barley. Stripe rust is caused by fungus, Puccinia striiformis f. sp. hordei and wide spread in northern hills, foot hills and in the states of Punjab, Haryana and Rajasthan. Spot blotch ( Bipolaris sorokiniana ) is one of the major diseases in north eastern Indo-Gangetic plains having warm and humid climate. The resistant sources for stripe rust are available but the genetic resources for spot blotch are indeed meagre. Therefore, the present investigation was carried out to identify combined sources for stripe rust and spot blotch in barley. The experiments for stripe rust were conducted at Durgapura and Karnal during 2015-16 to 2017-18. While, spot blotch evaluation was carried out for two years at Faizabad and Karnal centres during 2015-16 and 2016-17. The indigenous genotypes namely DWRB127, DWRB128, DWRB136, DWRB137, DWRB143, RD2786 and Dolma showed resistance for stripe rust. Based upon stripe rust and spot blotch reactions the genotypes namely, DWRB136, Notch-2, Orosus and Bella Union were found promising. Similarly, the cross combination viz., Lbiran/Una80//Lignee640/3/Bbsc/4/Chamico and P.sto/3/Libran/Una80//Lignee640/4/Bllu/5/ Petunia1 were also considered promising for barley stripe rust and spot blotch. Therefore, these identified genotypes are suggested to be incorporated in future barley breeding programmes for gainfully utilization. DOI.10.25174/2582-2675/2020/88973

Identification of QTL for adult plant resistance to stripe rust in bread wheat line C33

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a serious disease in bread wheat (Triticum aestivum L.). Identification and use of adult plant resistance (APR) resources are important for stripe rust resistance breeding. Bread wheat line C33 is an exotic germplasm that has shown stable APR to stripe rust for more than 10 years in Sichuan Province of China. Here, 183 recombinant inbred lines (RILs) derived from the cross between C33 and a susceptible line X440 were genotyped with diversity arrays technology (DArT) markers to identify resistance quantitative trait locus (QTL). Field trials were conducted in five years at Chengdu and Xindu of Sichuan Province, using maximum disease severity (MDS) as stripe rust reaction phenotypes. A total of four quantitative trait loci (QTLs) were detected, respectively designed as QYr.saas-3AS, QYr.saas-5AL, QYr.saas-5BL, and QYr.saas-7DS, explaining 4.14–15.21% of the phenotypic variances. QYr.saas-5BL and QYr.saas-7DS were contributed by C33. However, the level for stripe rust resistance contributed by them was not strong as C33, suggesting the presence of other unidentified QTLs in C33. QYr.saas-7DS corresponded to Yr18 and QYr.saas-5BL remains to be formally named. The RIL lines carrying combinations QYr.saas-5AL, QYr.saas-5BL, and QYr.saas-7DS showed comparability resistance with C33. The present study provides resources to pyramid diverse genes into locally adapted elite germplasm to improve the stripe rust resistance of bread wheat.

Molecular analysis of YR genes for yellow rust resistance in bread wheat (Triticum aestivum L. Em Thell)

Investigation was conducted to evaluate 210 recombinant inbred lines (RILs) of bread wheat to identify Yr genes using SSR markers and response to stripe rust reaction. Recombinant inbred lines were screened under epiphytotic conditions and data in terms of per cent leaf area infected were recorded using Modified Cobb’s Scale. A total of seven Yr genes, namely Yr7, Yr18, Yr26, Yr29, Yr36, Yr47 and Yr53 were found to be linked to yellow rust resistance. Out of 40 Yr specific SSR markers, seven (Xgwm130 (Yr7), Xbarc 352 (Yr18), Xgwm 11 (Yr26), Xwmc 44 (Yr29), Xwmc 149 (Yr53), WKS1_I (Yr36) and Xcfb309 (Yr47) were found to be polymorphic in parental genotypes and RIL population indicating the presence or absence of Yr genes. Three RILs namely, 51, 52, 55 had largest number of resistant genes (4Yr genes) followed by six lines namely, RIL No. 20, 21, 23, 29, 39, 53 had 3Yr genes having disease score (0S). These RILs may be utilized for incorporation of specific Yr genes in well adapted genotypes for improvement of disease resistance in bread wheat.

Yield responses in spring wheat and barley cultivars, varying in stripe rust resistance in central Alberta

Yield losses in wheat and barley cultivars varying in stripe rust reactions were determined under natural infection conditions in fungicide treated and untreated plots at Lacombe and Olds, central Alberta, from 2012 to 2014. Stripe rust caused by Puccinia striiformis was the major disease observed during the 3-year test. Moderate stripe rust severity caused significant yield losses of over 20% in a highly susceptible wheat, ‘AC Crystal’, while no severe yield losses occurred for resistant and moderately resistant cultivars/lines. However, a moderate level of stripe rust and the leaf spot complex caused reductions in thousand kernel weight (TKW) ranging from 5.4% to 13.5% for all wheat and barley cultivars/lines tested except for ‘Lillian’ wheat and ‘Seebe’ barley. When stripe rust developed early in the growing season resulting in severe disease, there were significant yield losses ranging from 17.6% to 91% for five wheat and two barley genotypes. Severe TKW and test weight losses also occurred for susceptible wheat and barley. The magnitude of yield losses in resistant and intermediate cultivars was smaller than in susceptible cultivars in the same tests. The present study demonstrated that one foliar-applied fungicide application was generally capable of protecting susceptible wheat and barley cultivars from stripe rust, while the use of resistant and intermediate cultivars was necessary for yield protection when moderate levels of disease occurred. Fungicide application was needed to reduce yield losses for resistant and moderately resistant cultivars when leaf spot diseases were prevalent and an early onset of stripe rust was observed, which can lead to severe stripe rust development even in resistant varieties.

Wheat-associated microbiota and their correlation with stripe rust reaction.

This study was aimed at revealing the composition of microbiota in leaves, roots and rhizosphere soil of wheat plants that are resistant or susceptible to stripe rust, one of the most widely destructive leaf diseases in wheat production. A total of 36 wheat plants that showed resistant or susceptible reactions to stripe rust were sampled. Three compartments of each plant including leaves, roots and rhizosphere soil were used for whole-genomic DNA extraction and the DNA samples were subjected to high-throughput 16S rRNA gene sequencing. A total of 2885 operational taxonomic units (OTUs) were revealed from the sequencing, and they mainly distributed in the phylum of Proteobacteria. Twenty-nine OTUs formed the core microbiota of wheat plants. The differences between above- and below-ground environments could explain most of the dissimilarity of wheat-associated microbial communities. Therefore, those microbes that were able to adapt to the above-ground (leaf) environment might be more important resources for biocontrol agents against stripe rust, and they could be from genera Hymenobacter, Flavobacterium, Chitinophage, Flavisolibacter, Niastella, Mucilaginibacter, Pedobacter, Aquincola, Massilia, Citrobacter, Cronobacter, Ewingella, Acinetobacter and Pseudomonas. No matter the microbial taxa were significantly selected by the resistant or susceptible wheat plants, they contained the members with plant growth promoting (PGP) features and could be used as potential biocontrol agents to reduce stripe rust damage. The core microbiota associated with wheat plants and microbial taxa that were significantly correlated with reactions to stripe rust were identified in this study. Few studies had been done on the microbiota associated with wheat so far. Our study will not only provide fundamental knowledge about the composition of wheat-associated microbiota but also reveal the microbial taxa that have the potential to be integrated into the strategy of stripe rust management.

Genetic analysis of stripe rust resistance in a set of European winter wheat genotypes

Stripe rust disease caused by Puccinia striiformis f. sp. tritici is of much importance in North-Western Plain Zone of India (NWPZ) since the resistance breakdown of the widely grown wheat cultivar PBW 343. With the rapid emergence and spread of aggressive rust pathogens, robust and durable resistant crop cultivar development becomes the urgent need for any wheat breeding program to sustain. In order to identify new stripe rust resistance sources, a panel of European winter wheat germplasm screened against prevalent pathotypes of stripe rust pathogens of NWPZ under artificial epiphytotic conditions. Crosses were attempted between fourteen selected resistant European winter wheat genotypes and four highly susceptible spring wheat varieties. Backcross populations were developed and screened for stripe rust reaction. Genetic analysis was performed in BC1F2 populations and confirmed with single plant derived BC1F3 and BC1F4 progenies. Among fourteen European parents used for introgressing stripe rust resistance, two lines namely Acienda and Aubusson showed single dominant gene, four lines viz. Aciento, Bagatelle_007, Brilliant and Nuage displayed digenic segregation ratio. Overall genetic analysis gave indication of the major genes for stripe rust resistance.

Reaction of differential wheat and triticale genotypes to natural stripe rust [Puccinia striiformis f. sp. tritici] infection in Saskatchewan, Canada

Stripe rust of wheat (Triticum spp. L.) is an emerging problem in western Canada. The causal pathogen (Puccinia striiformis f. sp. tritici Eriks.) has the potential to overwinter, which is a concern because of green bridging leading to earlier establishment of rust epidemics. Resistance conferred by seedling/all-stage and adult plant genes is the most effective approach to manage this disease. However, many Canadian wheat varieties lack seedling or all-stage resistance. The effectiveness of Yr genes under field conditions was assessed by exposing differentials/varieties carrying Yr genes to natural inoculum in multiple environments. The differentials were planted at multiple Saskatchewan locations in 2013, 2014 and 2016 and significant stripe rust was detected at 12 site-years. Additionally, 33 winter wheat varieties and advanced breeding lines were assessed for stripe rust reaction in 2016 at Swift Current, Saskatchewan. Disease severity was assessed on flag and penultimate leaves between early milk to soft dough growth stages. The genes Yr5, Yr15, YrSP and those in ‘Yamhill’ are effective in Saskatchewan under field conditions. Genes/differentials were classified into two major groups based on cluster analysis, one with defeated genes and the other with partial resistant genes. Genes YrA, Yr6, Yr7, Yr8, Yr9, Yr27, Yr29, Yr31, Yr32 and YrSu were not effective in the field to SK races. At Swift Current, winter wheat varieties/lines carrying Yr17 were resistant in the field, whereas those carrying Yr10 were not.

Allelic variation and effects of 16 candidate genes on disease resistance in western Canadian spring wheat cultivars

Recently, we mapped genomic regions associated with resistance to wheat diseases and insensitivity to Pyrenophora tritici-repentis (Ptr) toxins using 81 historical and modern Canadian western spring wheat cultivars genotyped with genome-wide single nucleotide polymorphic (SNP) markers. Here, we investigate the frequency and effects of allelic variants of 50 markers associated with 16 candidate genes that regulate resistance to leaf rust (Puccinia triticina), yellow or stripe rust (P. striiformis f. sp. tritici), tan spot (P. tritici-repentis), and Ptr ToxA reaction in a subset of 70 of the 81 spring wheat cultivars. We evaluated the 70 cultivars in the field for all diseases except Ptr ToxA, which was evaluated in a greenhouse. Using Spearman rank correlation, stepwise discriminant analysis, and partial least squares regression, we identified between 4 and 11 markers as best predictors of each phenotypic trait. Overall, 23 of the 50 markers were associated with one or more of the phenotypic traits of which analysis of variance showed significant differences between allelic variants of 19 markers. In most analyses, markers for Lr34/Yr18 and Tsn1 loci were identified consistently as the best predictor of disease resistance and Ptr ToxA sensitivity, respectively. The same alleles from two Lr34/Yr18 diagnostic SNP markers (wMAS000003 and wMAS000004) not only decreased stripe rust scores up to 1.6 (on a 1 to 9 scale), but also increased grain yield up to 196 kg ha−1 without affecting maturity. Results from this study could aid spring wheat breeders in selecting the best parental combinations and/or marker-assisted selection to integrate disease resistance with early maturity and short stature.

Mapping of stripe rust resistance gene in an Aegilops caudate introgression line in wheat and its genetic association with leaf rust resistance.

A pair of stripe rust and leaf rust resistance genes was introgressed from Aegilops caudata, a nonprogenitor diploid species with the CC genome, to cultivated wheat. Inheritance and genetic mapping of stripe rust resistance gene in backcrossrecombinant inbred line (BC-RIL) population derived from the cross of a wheat-Ae. caudata introgression line (IL) T291- 2(pau16060) with wheat cv. PBW343 is reported here. Segregation of BC-RILs for stripe rust resistance depicted a single major gene conditioning adult plant resistance (APR) with stripe rust reaction varying from TR-20MS in resistant RILs signifying the presence of some minor genes as well. Genetic association with leaf rust resistance revealed that two genes are located at a recombination distance of 13%. IL T291-2 had earlier been reported to carry introgressions on wheat chromosomes 2D, 3D, 4D, 5D, 6D and 7D. Genetic mapping indicated the introgression of stripe rust resistance gene on wheat chromosome 5DS in the region carrying leaf rust resistance gene LrAc, but as an independent introgression. Simple sequence repeat (SSR) and sequence-tagged site (STS) markers designed from the survey sequence data of 5DS enriched the target region harbouring stripe and leaf rust resistance genes. Stripe rust resistance locus, temporarily designated as YrAc, mapped at the distal most end of 5DS linked with a group of four colocated SSRs and two resistance gene analogue (RGA)-STS markers at a distance of 5.3 cM. LrAc mapped at a distance of 9.0 cM from the YrAc and at 2.8 cM from RGA-STS marker Ta5DS_2737450, YrAc and LrAc appear to be the candidate genes for marker-assisted enrichment of the wheat gene pool for rust resistance.

Genetic Architecture of Resistance to Stripe Rust in a Global Winter Wheat Germplasm Collection.

Virulence shifts in populations of Puccinia striiformis f. sp. tritici (Pst), the causal pathogen of wheat stripe rust, are a major challenge to resistance breeding. The majority of known resistance genes are already ineffective against current races of Pst, necessitating the identification and introgression of new sources of resistance. Germplasm core collections that reflect the range of genetic and phenotypic diversity of crop species are ideal platforms for examining the genetic architecture of complex traits such as resistance to stripe rust. We report the results of genetic characterization and genome-wide association analysis (GWAS) for resistance to stripe rust in a core subset of 1175 accessions in the National Small Grains Collection (NSGC) winter wheat germplasm collection, based on genotyping with the wheat 9K single nucleotide polymorphism (SNP) iSelect assay and phenotyping of seedling and adult plants under natural disease epidemics in four environments. High correlations among the field data translated into high heritability values within and across locations. Population structure was evident when accessions were grouped by stripe rust reaction. GWAS identified 127 resistance loci that were effective across at least two environments, including 20 with significant genome-wide adjusted P-values. Based on relative map positions of previously reported genes and QTL, five of the QTL with significant genome-wide adjusted P-values in this study represent potentially new loci. This study provides an overview of the diversity of Pst resistance in the NSGC winter wheat germplasm core collection, which can be exploited for diversification of stripe rust resistance in breeding programs.

Stripe rust reaction and yield response of winter cereals in bio - versus conventional farming

In 2014, was an extremely early and heavy yellow rust (Puccinia striiformis var. striiformis) epidemic in Hungary. Significant differences were among locations, years and genotypes in the severity of infection. Ratio of the resistant and moderately resistant genotypes was higher under bio environment. The yellow rust epidemic caused significant yield decreasing in the tested winter cereals.

Stripe rust resistance and genes in Chinese wheat cultivars and breeding lines

AbstractStripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases on wheat in China. To assess resistance in wheat cultivars and breeding lines in China, 330 leading cultivars and 164 advanced breeding lines were evaluated with stripe rust. In the greenhouse tests, seedlings of the entries were inoculated separately with several Pst pathotypes. In the field tests, the entries were evaluated for stripe rust resistance in Yangling, Shaanxi Province artificially inoculated and in Tianshui, Gansu Province under natural infection of Pst. The oversummering/wintering and spring epidemic zones of resistance genes were postulated using molecular markers for Yr5, Yr9, Yr10, Yr15, Yr17, Yr18, and Yr26, in combination with resistance spectra. Out of the 494 wheat entries, 16 (3.24 %) entries had all-stage resistance (ASR) in all race tests, 99 (20.04 %) had adult-plant resistance (APR), 28 (5.67 %) were considered to have slow-rusting (SR), and 351 (71.05 %) were susceptible to one or more races in both seedling and adult-plant stages. Advanced breeding lines had a higher percentage (37.2 %) of resistant entries (The sum of ASR, APR and SR) than leading cultivars (24.85 %). Among the epidemic regions, southern Gansu had a higher percentage of resistant entries than any other regions. Based on stripe rust reactions and molecular markers, two cultivars were found to possibly have Yr5 while no entries have Yr10 or Yr15. Resistance genes Yr9, Yr17, Yr18, and Yr26 were found in 134 (29.4 %), 45 (9.1 %), 10 (2 %), and 15 (3 %) entries, respectively.

Use of SSR markers to determine the genetic purity of a popular Australian wheat variety and consequences for stripe rust reactions

Sunvale is a popular bread wheat variety grown throughout central and northern New South Wales in Australia which has moderate resistance to the fungal leaf disease stripe rust. However, significant levels of stripe rust have been reported in Sunvale crops over the past few seasons. Sunvale is a 17-year-old variety so genetic purity may also be involved in the numerous reports of increased disease susceptibility in grower crops. Twelve SSR markers were used to determine the genetic purity of 23 grower-retained seed lots of cv. Sunvale. Sixteen of the 23 grower-retained Sunvale seed lots (70%) contained at least 5% contamination from a different genotype, while seven samples (30%) contained > 10% contamination. The SSR analysis was complemented by field observations of the various seed lots in a replicated small plot trial. In 17 of the 19 grower-retained Sunvale seed lots that had some level of contamination with another genotype(s), at least one contaminant was very susceptible to stripe rust. The pure Sunvale plants in the plots showed a moderately resistant reaction. This is the first report of seed impurity being definitively determined as the cause of unexpected stripe rust responses within a wheat variety.

Multivariate analysis of stripe rust assessment and reactions of barley in multi-location nurseries

Xi, K., Chen, X. M., Capettini, F., Falconi, E., Yang, R. C., Helm, J. H., Holtz, M. D., Juskiw, P., Kumar, K., Nyachiro, J. and Turkington, T. K. 2013. Multivariate analysis of stripe rust assessment and reactions of barley in multi-location nurseries. Can. J. Plant Sci. 93: 209–219. A total of 1357 entries, mainly consisting of hulled two-row, hulled six-row and hulless barley, were evaluated in stripe rust nurseries at Toluca, Mexico during 2007, Quito, Ecuador during 2007 and 2008, and Pullman and Mt. Vernon, USA [Pacific Northwest (PNW)] during 2007–2009. Disease screening data for barley stripe rust resistance at multiple locations and seasons were analyzed using factor analysis (FA), principal component analysis (PCA) and analysis of variance (ANOVA). Factor analysis led to the removal of a number of disease assessment variables that had inadequate disease levels or an unsuitable rating scale. The PCA scores revealed that the two-row types of barley were generally more resistant than the six-row and hulless types. ANOVA indicated that the effect of seasonal influence on screening was small, while stripe rust susceptible and resistant barley types were differentiated significantly on mean values averaged on infection type (IT) and percentage diseased leaf area (disease severity, DS) during the 3-yr tests in multiple screening nurseries. The significant correlations in disease assessments between IT and DS suggest that either assessment can be used to replace the other without a significant loss of information regarding barley stripe rust reactions. The first principal component is a useful criterion for assessing stripe rust reactions in barley lines.

QTL Mapping for Adult-Plant Resistance to Stripe Rust in a Common Wheat RIL Population Derived from Chuanmai 32/Chuanyu 12

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a devastating wheat disease worldwide. The Chinese wheat cultivar Chuanmai 32 has shown stable resistance to stripe rust for 10 yr in Sichuan Province, a hotspot for stripe rust epidemics. The objective of the present study was to map quantitative trait loci (QTL) for adult-plant resistance (APR) to stripe rust in a population of 140 recombinant inbred lines (RILs) derived from Chuanmai 32/Chuanyu 12. Field trials were conducted in Chengdu and Yaan, Sichuan, from 2005 to 2008, providing stripe rust reaction data for 6 environments. 797 simple sequence repeat (SSR) markers were screened for association with stripe rust reaction, initially through bulked segregant analysis (BSA). Based on the mean disease values averaged across environments, the broad-sense heritability of maximum disease severity (MDS) was 0.75. Two QTLs for stripe rust resistance were detected by composite interval mapping (CIM). They were designated QYr.caas-3BL and QYr.caas-3BS and explained from 6.6 to 20.1%, respectively, of the phenotypic variance across environments. QYr.caas-3BL came from Chuanmai 32; QYr.caas-3BS with lower effect was from the susceptible parent Chuanyu 12. Both QTLs appear to be new.

QTL Mapping of Adult-Plant Resistance to Stripe Rust in Chinese Wheat Cultivar Chuanyu 16

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a serious wheat fungal disease, causing significant annual yield losses worldwide. The Chinese wheat cultivar Chuanyu 16 has shown good adult-plant resistance (APR) to stripe rust in Sichuan province, a hotspot for stripe rust epidemics. Chuanyu 16 was crossed with Chuanyu 12 and Chuanmai 32. Two populations, each with 140 recombinant inbred lines (RILs), were developed by single-seed descent, and used for quantitative trait locus (QTL) mapping. Field trials were conducted in Chengdu and Yaan from 2005 to 2008, providing stripe rust reaction data for six environments. Seven hundred and thirty one simple sequence repeat (SSR) markers were screened for association with stripe rust reaction, initially through bulked segregant analysis (BSA). Three QTLs for stripe rust resistance derived from Chuanyu 16 were detected in the first cross. They were detected by inclusive composite interval mapping (ICIM) and designated QYr.caas-1BL.1, QYr.caas-1BL.2 and QYr.caas-2AS. They explained 6.0 - 12.8%, 4.5 - 5.8% and 14.9 - 43.0%, respectively, of the phenotypic variance across environments. One digenic epistatic QTL between QYr.caas-1BL.2 and QYr.caas-2AS explained 4.3 - 10.4% of the phenotypic variance. QYr.caas-2AS was also detected in Chuanmai 32/Chuanyu 16, explaining 27.9 - 57.2% of the phenotypic variance across six environments. This QTL showed a major effect against stripe rust in Chuanyu 16, and was located in a similar position to Yr17. Specific markers indicated the presence of a segment from chromosome 2N of Triticum ventricosum that carries Yr17. Despite the lack of evidence for Yr17 in Chuanyu 16 based on pedigree, and inconsistencies in stripe rust response relative to a near-isogenic reference stock with the gene, we concluded that QYr.caas-2AS is Yr17. QYr.caas-1BL.1 and QYr.caas-1BL.2 showed minor effects for APR against stripe rust. QYr.caas-1BL.1 is probably a new gene for APR to stripe rust.

Evaluation of Pakistan wheat germplasms for stripe rust resistance using molecular markers

Wheat production in Pakistan is seriously constrained due to rust diseases and stripe rust (yellow) caused by Puccinia striiformis f. sp. tritici, which could limit yields. Thus development and cultivation of genetically diverse and resistant varieties is the most sustainable solution to overcome these diseases. The first objective of the present study was to evaluate 100 Pakistan wheat cultivars that have been grown over the past 60 years. These cultivars were inoculated at the seedling stage with two virulent stripe rust isolates from the United States and two from Pakistan. None of the wheat cultivars were resistant to all tested stripe rust isolates, and 16% of cultivars were susceptible to the four isolates at the seedling stage. The data indicated that none of the Pakistan wheat cultivars contained either Yr5 or Yr15 genes that were considered to be effective against most P. striiformis f. sp. tritici isolates from around the world. Several Pakistan wheat cultivars may have gene Yr10, which is effective against isolate PST-127 but ineffective against PST-116. It is also possible that these cultivars may have other previously unidentified genes or gene combinations. The second objective was to evaluate the 100 Pakistan wheat cultivars for stripe rust resistance during natural epidemics in Pakistan and Washington State, USA. It was found that a higher frequency of resistance was present under field conditions compared with greenhouse conditions. Thirty genotypes (30% of germplasms) were found to have a potentially high temperature adult plant (HTAP) resistance. The third objective was to determine the genetic diversity in Pakistan wheat germplasms using molecular markers. This study was based on DNA fingerprinting using resistance gene analog polymorphism (RGAP) marker analysis. The highest polymorphism detected with RGAP primer pairs was 40%, 50% and 57% with a mean polymorphism of 36%. A total of 22 RGAP markers were obtained in this study. RGAP, simple sequence repeat (SSR) and sequence tagged site (STS) markers were used to determine the presence and absence of some important stripe rust resistance genes, such as Yr5, Yr8, Yr9, Yr15 and Yr18. Of the 60 cultivars analyzed, 17% of cultivars showed a RGAP marker band for Yr9 and 12% of cultivars exhibited the Yr18 marker band. No marker band was detected for Yr5, Yr8 and Yr15, indicating a likely absence of these genes in the tested Pakistan wheat cultivars. Cluster analysis based on molecular and stripe rust reaction data is useful in identifying considerable genetic diversity among Pakistan wheat cultivars. The resistant germplasms identified with 22 RGAP markers and from the resistance evaluations should be useful in developing new wheat cultivars with stripe rust resistance.