Rust Population Research Articles

Soybean production in eastern and southern Africa and threat of yield loss due to soybean rust caused byPhakopsora pachyrhizi

Soybean is a major source of oil and proteins worldwide. The demand for soybean has increased in Africa, driven by the growing feed industry for poultry, aquaculture and home consumption in the form of processed milk, baked beans and for blending with maize and wheat flour. Soybean, in addition to being a major source of cooking oil, is also used in other industrial processes such as in the production of paints and candle wax. The demand for soybean in Africa so far outweighs the supply, hence the deficit is mainly covered through imports of soybean products such as soybean meal. The area under soybean production has increased in response to the growing demand, a trend that is expected to continue in the coming years. As the production area increases, diseases and insect pests, declining soil fertility and other abiotic factors pose a major challenge. Soybean rust disease, caused by the fungusPhakopsora pachyrhizi, presents one of the major threats to soybean production in Africa due to its rapid spread as a result of the ease by which its spores are dispersed by the wind. Disease control by introducing resistant soybean varieties has been difficult due to the presence of different populations of the fungus that vary in pathogenicity, virulence and genetic composition. Improved understanding of the dynamics of rust ecology, epidemiology and population genetics will enhance the effectiveness of targeted interventions that, in turn, will safeguard soybean productivity.

PHYSIOLOGICAL RACES AND VIRULENCE DIVERSITY OF WHEAT STEM RUST FUNGUS Puccinia graminis f. sp. tritici IN EGYPT DURING 2012-2013

Stem rust collections were obtained from infected wheat stems throughout the survey of wheat fields and nurseries in (Garbia, Dakhlia, Minofia, Kafer Elshake and Sharkia governorates) during 2012/2013. The findings of this paper were based on race analysis through inoculation of stem rust populations, isolation and multiplication of single-pustule of the pathogen and race determination by inoculating on stem rust differential hosts. The phenotypic characterization of P. graminis f. sp. tritici resulted in identification of 85 isolates 40 races were identification from the collected samples were the most frequency and predominant race were BBBTC, TDBBB and BTCBB with frequency (9.44%, 7% and 7%) respectively. Race analysis from the tested samples reveled that race TDBBB was the most prevalent race with a frequency of 7% followed by TTHBB and TTHTB which gave frequency of 5.88% and 4.7% respectively at Garbia governorate. Whereas, race BTCBB was the most frequent at Dakhlia (7%) . The highest frequency during this survey was recorded with BBBJC at Minofia governorate which gave 9.41%. Cluster analysis reveled that, identified races were divided to A and B group with similarity a bout 31.10%, where A (Kafer Elshaik) and B the rest governorates. Among 20 wheat stem rust differential hosts, Sr31 was the most effective one which exhibit 100% efficacy to all races followed by Sr24 (89.5%). While Sr 9g while, Sr5 were the least effective during this study. Thus, use of effective Sr stem rust resistance genes such as Sr31 and Sr24 in single cultivar through gene pyramiding has paramount importance as the additive effects of several genes gives the cultivar a wider base stem rust resistance along with periodic race survey.

Replacement of the European wheat yellow rust population by new races from the centre of diversity in the near‐Himalayan region

Isolates of recently spreading races of yellow rust from wheat and triticale in Europe were analysed using virulence phenotypic data of 2605 isolates sampled in 12 countries between 2000 and 2014. A subset of 239 isolates was investigated by microsatellite markers. At least three races of non‐European origin, termed ‘Warrior’, ‘Kranich’ and ‘Triticale aggressive’, were identified in the post‐2011 population. The Warrior race was already present in high frequencies in the first year of detection in most European countries and to a large extent it replaced the pre‐2011 European population. In contrast, the two other exotic races were localized to certain regions and/or crop type. The presence already of at least six multilocus genotypes of the Warrior race and five genotypes of the Kranich race in the first year of detection and across large areas is consistent with a hypothesis of aerial spread from genetically diverse source populations. A comparison with reference isolates sampled from six continents suggested that the Warrior and Kranich races originated from sexually recombining populations in the centre of diversity of the yellow rust fungus in the near‐Himalayan region of Asia. However, the Triticale aggressive race was most similar to populations in the Middle East/Central Asia. The study illustrated the potential role of sexual Puccinia striiformis populations as a reservoir for new races replacing distant clonal populations.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2013.

Collections of Puccinia triticina were obtained from rust-infected leaves provided by cooperators throughout the United States and from wheat fields and breeding plots by USDA-ARS personnel and cooperators in the Great Plains, Ohio River Valley, and southeastern states in order to determine the virulence of the wheat leaf rust population in 2013. Single uredinial isolates (490 total) were derived from the collections and tested for virulence phenotype on 20 lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes. In 2013, 79 virulence phenotypes were described in the United States. Virulence phenotypes MBTNB, TNBGJ, and MCTNB were the three most common phenotypes. Phenotypes MBTNB and MCTNB are both virulent to Lr11, and MCTNB is virulent to Lr26. MBTNB and MCTNB were most common in the soft red winter wheat region of the southeastern states and Ohio Valley. Phenotype TNBGJ is virulent to Lr39/41 and was widely distributed throughout the hard red winter wheat region of the Great Plains. Isolates with virulence to Lr11, Lr18, and Lr26 were common in the southeastern states and Ohio Valley region. Isolates with virulence to Lr21, Lr24, and Lr39/41 were frequent in the hard red wheat region of the southern and northern Great Plains.

Field pathogenomics reveals the emergence of a diverse wheat yellow rust population.

BackgroundEmerging and re-emerging pathogens imperil public health and global food security. Responding to these threats requires improved surveillance and diagnostic systems. Despite their potential, genomic tools have not been readily applied to emerging or re-emerging plant pathogens such as the wheat yellow (stripe) rust pathogen Puccinia striiformis f. sp. tritici (PST). This is due largely to the obligate parasitic nature of PST, as culturing PST isolates for DNA extraction remains slow and tedious.ResultsTo counteract the limitations associated with culturing PST, we developed and applied a field pathogenomics approach by transcriptome sequencing infected wheat leaves collected from the field in 2013. This enabled us to rapidly gain insights into this emerging pathogen population. We found that the PST population across the United Kingdom (UK) underwent a major shift in recent years. Population genetic structure analyses revealed four distinct lineages that correlated to the phenotypic groups determined through traditional pathology-based virulence assays. Furthermore, the genetic diversity between members of a single population cluster for all 2013 PST field samples was much higher than that displayed by historical UK isolates, revealing a more diverse population of PST.ConclusionsOur field pathogenomics approach uncovered a dramatic shift in the PST population in the UK, likely due to a recent introduction of a diverse set of exotic PST lineages. The methodology described herein accelerates genetic analysis of pathogen populations and circumvents the difficulties associated with obligate plant pathogens. In principle, this strategy can be widely applied to a variety of plant pathogens.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-015-0590-8) contains supplementary material, which is available to authorized users.

A simulation model for epidemics of stem rust in ryegrass seed crops.

A simulation model (STEMRUST_G, named for stem rust of grasses) was created for stem rust (caused by Puccinia graminis subsp. graminicola) in perennial ryegrass grown to maturity as a seed crop. The model has a daily time step and is driven by weather data and an initial input of disease severity from field observation. Key aspects of plant growth are modeled. Disease severity is modeled as rust population growth, where individuals are pathogen colonies (pustules) grouped in cohorts defined by date of initiation and plant part infected. Infections due to either aerial spread or within-plant contact spread are modeled. Pathogen cohorts progress through life stages that are modeled as disease cycle components (colony establishment, latent period, infectious period, and sporulation) affected by daily weather variables, plant growth, and fungicide application. Fungicide effects on disease cycle components are modeled for two commonly used active ingredients, applied preinfection or postinfection. Previously validated submodels for certain disease cycle components formed the framework for integrating additional processes, and the complete model was calibrated with field data from 10 stem rust epidemics. Discrepancies between modeled outcomes and the calibration data (log10[modeled]-log10[observed]) had a mean near zero but considerable variance, with 1 standard deviation=0.5 log10 units (3.2-fold). It appears that a large proportion of the modeling error variance may be due to variability in field observations of disease severity. An action threshold for fungicide application was derived empirically, using a constructed weather input file favorable for disease development. The action threshold is a negative threshold, representing a level of disease (latent plus visible) below which damaging levels of disease are unable to develop before the yield-critical crop stage. The model is in the public domain and available on the Internet.

Virulence Spectrum of Puccinia hordei of Barley in Western and Central Highlands of Ethiopia

Virulence surveys of Puccinia hordei of barley were conducted in the main and off crop seasons of 2010/11 and 2011/12 in West Shewa, Wellega (western part of the country) and Arsi (central part of the country) zones of Oromiya region, Ethiopia to determine the virulence spectrum of the pathogen, and identify the effective resistance genes to the pathotypes. In the two crop seasons, 56 leaf (brown) rust samples in the main and 32 in the offseason were collected. From each barley field, single pustule descent spores were multiplied and inoculated onto the seedlings of 12 leaf rust differentials carrying Rph1 - Rph12 genes to designate the pathotypes. A total of 88 leaf rust isolates were processed and based on infection phenotype on the resistance genes, 7 pathotypes (ETPh6631, ETPh6611, ETPh6671, ETPh7671, ETPh7631, ETPh7611 and ETPh7651) were identified. The most frequently isolated pathotype was ETPh6631 with 43.2% followed by ETPh6611 with 19%. Moreover, virulence spectrum of P. hordei pathotypes identified in this study was diverse. Resistance genes with Rph1 (Sudan), Rph4 (Gold), Rph8 (Egypt4), Rph9 (Hor 2596), Rph11 (Clipper BC68) and Rph12 (Triumph) were non-effective to all pathotypes identified whereas genes Rph5 (Magnif), Rph6 (Bolivia) and Rph10 (Clipper BC 8) were effective to 26.1, 73.9 and 78.4% of the isolates, respectively. Virulence against Rph2 (Peruvian), Rph3 (Estate) and Rph7 (Cebada Capa) was absent. Therefore, the effective major genes to the existing leaf rust populations could be utilized as sources of resistance in the barley breeding program.

The virulence spectrum of the wheat leaf rust population analyzed in the Czech Republic from 2002 to 2011

The research report presents a summary of wheat leaf rust virulence surveys in the Czech Republic from 2002 to 2011. Determination of virulence was based on infection types on Thatcher near-isogenic lines (NILs) with the resistance genes Lr1, Lr2a, Lr2b, Lr2c, Lr3a, Lr9, Lr10, Lr11, Lr13, Lr15, Lr17, Lr19, Lr21, Lr23, Lr24, Lr26 and Lr28, respectively. Virulence for Lr9 and Lr 19 occurred only sporadically in the investigated period. On average, virulence for Lr2a, Lr2b, Lr24 and Lr28 was low. The highest frequency of virulence was found for Lr3a, Lr10, Lr11, Lr13, Lr15, Lr17, Lr21, Lr23 and Lr26. During the investigation period we recorded a shift from prevailing virulence for Lr2c and avirulence for Lr1 to avirulence for aLr2c and virulence for Lr1. Ther virulences in the leaf rust population were compared with matching resistance genes  in the registered wheat cultivars. The most frequent resistance gene was Lr37, while the genes Lr3a and Lr26 were less frequent.

Integrating pest population models with biophysical crop models to better represent the farming system

Farming systems frameworks such as the Agricultural Production Systems simulator (APSIM) represent fluxes through the soil, plant and atmosphere of the system well, but do not generally consider the biotic constraints that function within the system. We designed a method that allowed population models built in DYMEX to interact with APSIM. The simulator engine component of the DYMEX population-modelling platform was wrapped within an APSIM module allowing it to get and set variable values in other APSIM models running in the simulation. A rust model developed in DYMEX is used to demonstrate how the developing rust population reduces the crop's green leaf area. The success of the linking process is seen in the interaction of the two models and how changes in rust population on the crop's leaves feedback to the APSIM crop modifying the growth and development of the crop's leaf area. This linking of population models to simulate pest populations and biophysical models to simulate crop growth and development increases the complexity of the simulation, but provides a tool to investigate biotic constraints within farming systems and further moves APSIM towards being an agro-ecological framework.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2012.

Collections of Puccinia triticina were obtained from rust-infected leaves provided by cooperators throughout the United States and from wheat fields and breeding plots by United States Department of Agriculture-Agricultural Research Service personnel and cooperators in the Great Plains, Ohio River Valley, southeastern states, and Washington State and Idaho in order to determine the virulence of the wheat leaf rust population in 2012. Single uredinial isolates (501 in total) were derived from the collections and tested for virulence phenotype on 20 lines of 'Thatcher' wheat that are near-isogenic for leaf rust resistance genes. In 2012, 74 virulence phenotypes were described in the United States. Virulence phenotypes TNBGJ, TCRKG, and MBTNB were the three most common phenotypes. Phenotype TNBGJ is virulent to Lr39/41 and was widely distributed throughout the hard red winter wheat region of the Great Plains. Phenotype TCRKG is virulent to Lr11, Lr18, and Lr26 and was found mostly in the soft red winter wheat region in the eastern United States. Phenotype MBTNB is virulent to Lr11 and was also found mostly in the soft red winter wheat region. The frequency of isolates with virulence to Lr39/41, which is present in many hard red winter wheat cultivars in the Great Plains region, continued to increase. Isolates with virulence to Lr21, which is present in many hard red spring wheat cultivars, also continued to increase in frequency in the northern Great Plains region.

Molecular mapping of Asian soybean rust resistance in soybean landraces PI 594767A, PI 587905 and PI 416764

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is one of the most serious diseases of soybean. The soybean landraces PI 594767A, PI 587905 and PI 416764 previously showed high levels of resistance to a wide range of ASR fungus, while the genetic basis of the resistance has yet to be understood. In this study, the ASR resistance loci were mapped using three independent mapping populations, POP‐1, POP‐2 and POP‐3 derived from crosses BRS184 × PI 594767A, BRS184 × PI 587905 and BRS184 × PI 416764, respectively. In each population, the resistance to ASR segregated as a single gene, but the resistance was dominant in PI 594767A and PI 587905 and incompletely dominant in PI 416764. The resistance genes from both PI 594767A and PI 587905 were mapped on chromosome 18 corresponding to the same location as known resistance locus Rpp1. Quantitative trait locus (QTL) analysis performed on POP‐3 identified the putative ASR resistance locus in PI 416764 on the defined region of chromosome 6 where Rpp3 was located. The QTLs detected by the mapping explained about 67–72% of the phenotypic variation in POP‐3. Cluster analysis based on disease reactions to 64 ASR populations demonstrated the presence of at least two types of functional resistant Rpp1 alleles: strong and weak allele(s), e.g. soybean accession PI 594767A and PI 587905 carry the strong resistant Rpp1 allele(s). Introducing or pyramiding strong Rpp1 allele(s) in elite soybean cultivars is expected to be useful against the South American rust population.

Population Diversity of Puccinia graminis is Sustained Through Sexual Cycle on Alternate Hosts

A high degree of virulence diversity has been maintained in the population of Puccinia graminis f. sp. tritici (Pgt) in northwestern United States. Although Berberis vulgaris is present in the region and Pgt has been isolated from aecial infections on B. vulgaris, the population is too diverse to be explained by the limited presence of B. vulgaris alone. Since 2008, we have isolated P. graminis from aecial infections on fruits of Mahonia repens and Mahonia aquifolium from northwestern United States. These two native woody shrub species, widely distributed in western North America, were once classified as resistant to P. graminis based on artificial inoculations. By isolating P. graminis from aecia, we established that M. repens and M. aquifolium along with B. vulgaris (albeit infrequent) serve as the alternate hosts of P. graminis in the region. The isolates of P. graminis from Mahonia of North America had diverse virulence patterns and most of the isolates could be differentiated on Morocco, Line E, Chinese Spring, Little Club, LMPG-6, Rusty, and other genotypes that are considered to be universally susceptible to most Pgt isolates. This discovery explained the persistence of virulence diversity of Pgt observed in isolates derived from uredinia on cereal crops in the region. In addition to cereal crops, uredinial stage of the P. graminis population is sustained by wild grasses, especially Elymus glaucus, a native grass sharing the same habitat with the rusted Mahonia spp. Although virulence to some important stem rust resistance genes was observed in some isolates derived from Mahonia of North America when tested against single stem rust resistance gene stocks, the overall virulence is very limited in these isolates. This is likely a result of limited selection pressure on the rust population. In contrast to northwestern United Sates, the Pgt population in east of the Rocky Mountains of North America has declined steadily with a single race, QFCSC, being predominant in the last decade. This decline is likely due to a combination of factors, of which a lack of sexual recombination in the region is perhaps the most important one.

Response of Improved Durum Wheat (Triticum durum L.) Varieties to Wheat Stem Rust in Central Ethiopia

Wheat stem rust caused by Puccinia graminis f.sp. tritici is amongst the biotic factors which causes up to 100% yield loss during epidemic years. Therefore, the present study was carried out to observe the reaction of improved durum wheat varieties to virulent stem rust isolates at seedling growth stage and to stem rust population at adult growth stages. The finding of this experimental study was based on isolation and multiplication of virulent races of P. graminis, then ten durum wheat varieties including the susceptible check morocco were tested for the virulent isolates at seedling growth stage; and the reaction of these varieties under natural field conditions were evaluated in the Eastern and Western Showa of Central Ethiopia at adult growth stages. At seedling growth stage, 60% of varieties tested with the virulent stem rust isolate from Debrezeit exhibited resistance, while all varieties were resistant to Ambo isolate. Ten durum wheat varieties evaluated at adult plant growth stage to stem rust population of Debrezeit (Eastern Showa) showed severities of 20S in the variety Geredo to 70S in the variety Foka, while at Ambo (Western Showa) only the susceptible check, Morocco was infected up to 20S. The result indicated that stem rust from Debrezeit was virulent to durum wheat varieties as compared to the Ambo population.

Genetic Variability in Puccinia psidii Populations as Revealed by PCR-DGGE and T-RFLP Markers.

Eucalyptus rust caused by Puccinia psidii is responsible for losses of approximately 20% of young Eucalyptus plants, depending on the environmental conditions and the geographic location. Despite its economic importance, there are few studies describing the genetic variability in P. psidii populations that infect different host plants. In the present study, we evaluated the ribosomal DNA internal transcribed spacer region (rDNA-ITS) using polymerase chain reaction denaturing gradient gel electrophoresis and terminal restriction fragment length polymorphism to assess the genetic variability in P. psidii populations infecting different Eucalyptus spp. and hybrids, as well as guava, jabuticaba, and syzygium. These culture-independent methods were efficient in differentiating populations based on the host species from which they were collected. In general, the results from both techniques showed that the populations collected from guava, jabuticaba, and syzygium were different from and had a greater level of diversity than the Eucalyptus rust populations. The sequencing of cloned rDNA-ITS fragments confirmed that the vast majority of the profiles generated were from P. psidii. This analysis also revealed interesting single-nucleotide polymorphisms. Therefore, these culture-independent methods are suitable for the rapid assessment of genetic variability within and between populations of this biotrophic fungus on a variety of host species and could be a tool to study the evolution of this pathogen and its interactions with host plants.

English

Puccinia triticina, Puccinia graminis and Puccinia striiformis cause leaf, stem and yellow rust, respectively. Wheat rusts can cause losses as high as 70%. The rusts ability to evolve fungicide resistance has resulted in the use of resistant cultivars as the primary method of control. Breeding resistant cultivars is a long process and requires an accurate picture of the current and future pathogen population. Differentiation of wheat rust pathotypes using conventional plant pathology techniques is time consuming, labour intensive and requires the services of a highly skilled and experienced plant pathologist. Modern molecular biology techniques have the potential to aid the conventional techniques and provide fast, accurate same-day results. Microsatellite markers were used to differentiate P. triticina and P. striiformispathotypes. Amplified fragment length polymorphisms (AFLP) were used to differentiate stem rust P. graminis pathotypes. Phylogenetic trees were created for leaf and stem rust pathotypes. Field isolates of leaf, stem and yellow rust were collected from eleven sites across the Western Cape Province. Microsatellite markers were used to type leaf and yellow rust isolates. AFLP markers could not be used on field isolates due to the presence of plant DNA. Novel alleles found in the Leaf and yellow rusts isolates prevented the assigning of a specific pathotype to each isolate. UVPrt10 (25.2%) and UVPrt9 (21.5%) were the most prevalent leaf rust pathotypes. Only 6E16A+ was identified in the yellow rust isolates. Pathotype incidence was similar to previous studies. The prevalence of multiple pathotypes with a variety of virulence genes in the rust population shows that breeding lines with single major resistance genes will not be effective and breeding programmes should concentrate on lines that exhibit quantitative resistance.   Key words: Prevalence, microsatellite, amplified fragment length polymorphisms (AFLP), phylogeny, Puccinia.

Physiological specialization of wheat leaf rust (Puccinia triticina Eriks.) in the Czech Republic in 2009-2011

In 2009–2011 virulence of the wheat leaf rust population was studied on Thatcher near isogenic lines with Lr1, Lr2a, Lr2b, Lr2c, Lr3a, Lr9, Lr11, Lr13, Lr15, Lr17, Lr19, Lr21, Lr23, Lr24, Lr26 and Lr28. Samples of leaf rust were obtained in different parts of the Czech Republic. A total of 164 wheat leaf rust isolates were analysed. Resistance gene Lr9 was effective to 98% of all tested isolates. No virulence to Lr19 was found. Gene Lr24 was effective to 93% of isolates. A lower frequency of virulence to Lr2a, Lr2b and Lr28 was also observed. Recently registered cultivars were tested with six older and five most widespread leaf rust pathotypes at present. Winter wheat cultivars Carroll and Citrus were resistant to all tested older pathotypes at the seedling stage and they were also resistant to almost all pathotypes widespread at present. They displayed resistance also in official yield trials. Genes Lr34, Lr37, Lr10, Lr24 were determined in the tested cultivars by molecular markers.

Assessing the potential of the rust fungus Puccinia spegazzinii as a classical biological control agent for the invasive weed Mikania micrantha in Papua New Guinea

The rust fungus Puccinia spegazzinii was introduced into Papua New Guinea (PNG) in 2008 as a classical biological control agent of the invasive weed Mikania micrantha (Asteraceae), following its earlier release in India, mainland China and Taiwan. Prior to implementing field releases in PNG, assessments were conducted to determine the most suitable rust pathotype for the country, potential for damage to non-target species, most efficient culturing method and potential impact to M. micrantha. The pathotype from eastern Ecuador was selected from the seven pathotypes tested, since all the plant populations evaluated from PNG were highly susceptible to it. None of the 11 plant species (representing eight families) tested to confirm host specificity showed symptoms of infection, supporting previous host range determination. A method of mass-producing inoculum of the rust fungus, using a simple technology which can be readily replicated in other countries, was developed. Comparative growth trials over one rust generation showed that M. micrantha plants infected with the rust generally had both lower growth rates and lower final dry weights, and produced fewer nodes than uninfected plants. There were significant correlations between the number of pustules and (a) the growth rate, (b) number of new nodes and (c) final total dry weight of single-stemmed plants placed in open sunlight and between the number of pustules and number of new nodes of multi-stemmed plants placed under cocoa trees. The trials suggest that field densities of M. micrantha could be reduced if the rust populations are sufficiently high.

Diversity of virulence phenotypes among annual populations of wheat leaf rust in Israel from 1993 to 2008

Leaf rust, caused by the fungus Puccinia triticina, is the most common rust disease of wheat in wheat‐producing areas worldwide. The Israeli population of wheat leaf rust has been consistently monitored since 1993. A total of 840 single urediniospore isolates from Triticum aestivum (567), T. dicoccoides (119) and T. durum (154) were analysed during 1993–2008. The structure of the pathogen population has changed to a large extent since 1993. The annual populations of P. triticina were separated into two distinct groups: 1993–1999 and 2000–2008. Differentiation among the annual pathogen populations, as well as between the overall populations of the 1990s and 2000s, could be mainly attributed to the following forces: (i) migration of leaf rust urediniospores from neighbouring regions; and (ii) selection pressure of new yellow rust‐resistant wheat cultivars that have been introduced into Israel since 1997. Genetic multiplicity of wild emmer contributes to P. triticina variability in Israel. Leaf rust populations collected from common wheat, wild emmer and durum wheat differed. The population that originated from T. durum was rather stable during the years of the survey, whereas that from T. aestivum changed significantly from the 1990s to the 2000s. Diversity within the annual populations of P. triticina was highest in 1994 when many new pathotypes and associations between virulences were observed. Single‐step derivatives of the new pathotypes became dominant after 2000. Significant changes in virulence frequency to a number of Lr genes (e.g. Lr2a, Lr15, Lr17, Lr21, Lr26) were also registered in 2000–2008.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2011.

Collections of Puccinia triticina were obtained from rust-infected leaves provided by cooperators throughout the United States and from wheat fields and breeding plots by USDA-ARS personnel and cooperators in the Great Plains, Ohio River Valley, southeastern states, Oregon, and Washington State in order to determine the virulence of the wheat leaf rust population in 2011. Single uredinial isolates (440 total) were derived from the collections and tested for virulence phenotype on 18 lines of Thatcher wheat and a winter wheat line that are near-isogenic for 19 leaf rust resistance genes. In 2011, 87 virulence phenotypes were described in the United States. Virulence phenotypes TBBGJ, MLDSD, and TCRKG were the three most common phenotypes. Phenotype TBBGJ is virulent to Lr39/41 and was widely distributed throughout the hard red winter wheat region of the Great Plains. Phenotype MLDSD is virulent to Lr17 and Lr39/41 and was widely distributed throughout the United States. Phenotype TCRKG is virulent to Lr11, Lr18, and Lr26 and was found mostly in the soft red winter wheat region in the eastern United States. Isolates with virulence to Lr39/41 and avirulence to Lr9 were prevalent in the Great Plains region for the first time. Virulence to Lr21 was present in five virulence phenotypes mostly from the spring wheat region of the northern Great Plains.

Chromosome location and allele-specific PCR markers for marker-assisted selection of the oat crown rust resistance gene Pc91

Race-specific seedling resistance genes are the primary means of controlling crown rust of oat caused by Puccinia coronata Corda f. sp. avenae Eriks in Canada. Pc91 is a seedling crown rust resistance gene that is highly effective against the current crown rust population in North America. A number of race-specific resistance genes have been mapped and markers that are closely linked to them have been identified. However, the use of these markers in oat breeding has been limited by the economics of marker-assisted selection (MAS). A crucial step in the successful application of MAS in breeding programs is the development of inexpensive and easy-to-use molecular markers. The primary objective of this study was to develop co-dominant KBioscience competitive allele-specific PCR (KASP) markers linked to Pc91 for deployment in high-throughput MAS in oat breeding programs. The allele-specific marker showed consistent diagnostic polymorphism between the selected 16 North American oat breeding lines. The developed co-dominant marker was also validated on three F2 populations (AC Morgan × Stainless; SW Betania × Stainless; AC Morgan × CDC Morrison) and one recombinant inbred line population (CDC Sol-Fi × HiFi) segregating for Pc91 using KASP genotyping technology. We recommend the simple, low-cost marker as a powerful tool for pyramiding Pc91 with other effective crown rust resistance loci into a single line. The mapping results indicate that crown rust resistance gene Pc91 resides on the translocated oat chromosome 7C-17A.