The global cereal rust monitoring system
View hodson_2009.pdf (620.69 KB)
Cereal rusts have long been the scourge of wheat farmers worldwide. Three fungal rusts are capable of inflicting serious economic damage to wheat; namely, leaf rust, stripe rust, and stem rust. Historically, stem rust was the most feared disease of wheat, but since the 1950s, effective resistance has protected crops and livelihoods. By the mid 1990s stem rust had been reduced to negligible levels worldwide. The detection of the Ug99 lineage of stem rust in Uganda in 1998 has challenged the assumption that stem rust was a conquered disease, and up to 80% of the world’s wheat is now considered stem rust susceptible. Ug99 has sparked a global effort by wheat scientists to counter the threat and has highlighted the need for effective surveillance and monitoring systems. Outside of a few developed countries, monitoring efforts are often irregular or even non-existent and no coordinated global surveillance effort currently exists. Ug99 has provided the impetus to implement a global surveillance and monitoring system that provides relevant and timely information as a global public good. Key components, current status and future plans for this emerging cereal rust monitoring system are described. The immediate concern regarding Ug99 makes it an initial priority focus, but the other cereal rusts cannot nor should be excluded. Lessons can be learned and parallels drawn from existing successful trans-boundary monitoring schemes such as the Desert Locust monitoring and early warning system implemented by the UN Food and Agriculture Organization (FAO). Successful networking, expanded capacity of partners, efficient field surveys and data handling, plus regular timely targeted information products are all components of the Desert Locust scheme that need to be transferred to a cereal rust monitoring system. Through a consortium of partners several advances have already been made targeting the Ug99 lineage of stem rust. GIS technology is forming the backbone of an emerging rust monitoring and surveillance system being developed collaboratively by international agricultural research centers, UN agencies and advanced research institutes. The system already incorporates a rapidly expanding volume of standardized geo-referenced field survey data, routine use of wind models and public domain web tools delivering information in near-real time. Several challenges still remain before a fully operational system is created, and these are outlined. The need for vigilance and a lack of complacency regarding unexpected events are highlighted. These might include; accidental assisted movements, natural long distance dispersal and the threat of rust pathogens occurring in “non-traditional” areas as a result of climate change.
Advances in host-pathogen molecular interactions: rust effectors as targets for recognition
CSIRO Plant Industry, Australia
View dodds_2009.pdf (131.27 KB)
Rust fungi can cause devastating diseases in agriculture and are particularly important pathogens of wheat. We have been using the flax (Linum usitatissimum) and flax rust (Melampsora lini) model system to study disease resistance mechanisms to this important class of pathogens. Rust resistance in flax and other plants is mediated by the plant innate immunity system in which highly polymorphic resistance (R) proteins act as receptors that recognize specific avirulence (Avr) proteins produced by the pathogen. This race-specific resistance is characterised by Flor’s “gene-for-gene” model, first proposed based on the flax rust system. In gene-for-gene resistance, recognition between the R and Avr proteins initiates defense responses leading to host resistance to infection, including a localised necrosis or hypersensitive response. Nineteen different rust resistance genes have been cloned from flax, including 11 allelic variants of the L locus, which all encode cytosolic proteins with conserved nucleotide-binding (NB) and Leucine-rich repeat (LRR) domains. Four families of Avr genes, AvrL567, AvrM, AvrP123 and AvrP4, have been identified in the flax rust pathogen and all encode small secreted proteins. Rust Avr proteins are secreted from haustoria, specialized infection structures that penetrate the host cell wall, and are translocated across the host plasma membrane and into the host cytoplasm. These proteins are probably members of a suite of disease ‘effectors’ involved in manipulating host cell biology to facilitate infection, but have become targeted for recognition by the host immune system. As yet the mechanism of Avr protein transport is unknown, but could prove to be a useful target for novel disease control strategies. Recognition of at least two of these Avr proteins is based on direct interaction with the cytoplasmic NB-LRR R proteins. One interesting observation from the flax rust system is that all of the virulent rust strains retain intact copies of the Avr genes, but have altered their sequences sufficiently to escape recognition. Thus it may be possible to re-engineer R genes to recognise new Avr gene variants. We are currently identifying haustorially expressed secreted proteins from wheat stem rust as candidate Avr/effector proteins.
Global stem rust surveillance in practice
Department of Plant Sciences, University of the Free State, South Africa
View pretorius_2009.pdf (118.71 KB)
An assessment was made of stem rust race analysis on a global scale. Responses were obtained from 23 rust workers representing 21 countries. Five laboratories have an institutional history in stem rust race analysis of more than 60 years, whereas personal experience in this field ranged from 0 to 35 years. The number of stem rust samples processed from 2006 to 2008 varied greatly between countries. For the three year period most collections were characterized in Canada, followed by Georgia, USA, South Africa and Australia. Most laboratories use the North American differential set and nomenclature system. However, these entries are often supplemented by additional tester lines from the Stakman set, other single gene lines or local cultivars. Differential sets varied between eight and 50 entries. More than half of the respondents indicated that they often encounter seed mixtures amongst their differentiating lines. In recent surveys most races were detected in Ethiopia, followed by Georgia and China. One race dominated the USA and Canadian stem rust population. In South America and Australia stem rust has been rare in commercial wheat for many years. Races within the Ug99 cluster were frequently identified in stem rust collections from Kenya and Ethiopia. Two races related to Ug99, but avirulent on Sr31, occur in South Africa. Several laboratories are in the process of purifying and bulking differential seed, which appears to be one of the major limiting factors in reliable stem rust race analysis. Improvement of infrastructure and training of individuals inexperienced with stem rust should improve global surveillance efforts. In addition, countries doing race analysis should keep viable culture collections in long-term storage.
Race nomenclature systems: Can we speak the same language?
View fetch_2009.pdf (114.05 KB)
The first system describing physiologic specialization in the cereal rust fungi was that by Stakman and Levine (1922) for the wheat stem rust pathogen. Thirty seven biologic forms or “races” were identified using 12 differential wheat lines. Since then, additional variability in physiologic specialization was found and several systems evolved to describe this variation using numbers, letters, or combinations of both. This led to difficulties in comparing races, most often because of differences in the system that is used and the differential lines employed. A system that describes virulence succinctly and allows easilymade comparisons between races is highly desirable. Additionally, differential lines should be monogenic or near-isogenic so that virulence is classified on a genetic basis. Wherever near-isogenic stocks are used, it is vital that the recurrent parent is included. The systems that appear to be best suited to describing virulence with the above parameters are the letter-code and octal nomenclature. Of these, the letter-code system is the most commonly used based on a survey of research scientists working on stem rust. Thus, the letter-code system that uses 20 differential host lines is proposed to describe the nomenclature of Puccinia graminis f. sp. tritici on a worldwide basis. In addition, the source seedstock line for each differential gene is provided.
Are rust pathogens under control in the Southern Cone of South America?
National Institute of Agricultural Research [INIA], Uruguay
View german_2009.pdf (238.77 KB)
Approximately nine million ha of wheat (Triticum aestivum and T. durum) were sown annually in the Southern Cone of South America (Argentina, Brazil, Chile, Paraguay and Uruguay) during 2003-2007. Presently, leaf rust (caused by Puccinia triticina) is the most important rust of wheat throughout the region. The pathogen population is extremely dynamic leading to short-lived resistance in commercial cultivars. Leaf rust management relies on the use of resistant cultivars and fungicides. Sources of adult plant resistance conferred by minor additive genes have been increasingly used in breeding programs to obtain cultivars with more durable resistance. Stripe rust (P. striiformis f. sp. tritici) is endemic in central and southern Chile, where fungicides are required to control the disease on susceptible cultivars. Stem rust (P. graminis f. sp. tritici) has not caused widespread epidemics in the last 25 years due to the use of resistant cultivars. Virulence to Sr24 and Sr31, the most important genes conferring resistance to local races, has not been reported in the region. The areas sown with cultivars susceptible to local races in Argentina and Uruguay have increased in recent years. Since most varieties sown in the region are susceptible to Ug99 or derived races, testing and selection for resistance in Kenya, facilitated by the Borlaug Global Rust Initiative, is highly relevant for research aimed at preventing epidemics, which may occur if these races migrate, or are accidentally introduced to our region. The resistances identified in east Africa will also contribute to increasing the levels of resistance to current local races.
The development and application of near-isogenic lines for monitoring cereal rust pathogens
The University of Sydney, Plant Breeding Institute, Australia
View wellings_2009.pdf (174.99 KB)
The purpose of monitoring cereal rust pathogens is to provide a basis for disease control strategies that include breeding for resistance, predicting disease response in commercial cultivars and responding to the dynamics of pathogen change. The means of achieving this vary from regular collection surveys based on assessments of sample collections in greenhouse tests, to monitoring and recording static trap plots. Factors governing the method of approach include the size of the target region, the available research resources and the experience of staff involved. This paper is a brief review of the development of near-isogenic lines as a means of monitoring cereal rust pathogens. Emphasis will be given to wheat stripe/ yellow rust and the development and application of a near-isogenic set of materials based on the spring wheat cultivar Avocet. This parent was selected because of its high degree of susceptibility to the disease, agronomic adaptability (semidwarf, spring habit, moderate vernalisation and day-length requirements), and resistance to stem rust. The relative benefits of using these materials will be discussed in the context of available data.
Progress and prospects in discovery and use of novel sources of stem rust resistance
USDA-ARS, Cereal Disease Laboratory
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A number of stem rust resistance genes derived from wild relatives of wheat appeared to be more effective against race TTKSK (Ug99) of Puccinia graminis f. sp. tritici than Sr genes of wheat origin. In an attempt to identify sources of stem rust resistance genes effective against TTKSK, we evaluated several cultivated and wild relatives of wheat for resistance to TTKSK and other stem rust races with broad virulence in seedling tests. Preliminary results indicated that TTKSK resistance could readily be found, but frequencies of resistance varied among the species. Aegilops speltoides had the highest frequency of resistance (nearly 100%). Other species having high frequencies of TTKSK resistance included triticale (77.7% of 567 accessions), Triticum urartu (96.8% of 205 accessions), and T. monococcum (61% of 1020 accessions). Frequencies of TTKSK resistance in other species were: 14.7% in Ae. tauschii (456 accessions), 15% in T. timopheevii (298 accessions), and 17% in T. turgidum ssp. dicoccoides (157 accessions). Based on specific infection types to several races, we postulated that novel genes for resistance to TTKSK are present in some of these species. Accessions with putatively new resistance genes were selected to develop crosses for introgressing resistance into wheat and for developing mapping populations.
Cytogenetic manipulation to enhance the utility of alien resistance genes
Department of Crop and Soil Sciences, Washington State University, USA
View pumphrey_2009.pdf (236.58 KB)
Although many wild relatives in the Triticeae tribe have been exploited to transfer stem rust resistance genes to wheat, the derived germplasms have often not been immediately useful in wheat breeding programs. Too frequently, large chromosome segments surrounding desirable genes also harbor deleterious genes that result in unacceptable yield or quality. Recombination between chromosomes of wheat and chromosomes of distant relatives is very rare due to genetic restrictions on chromosome pairing in polyploid wheat. However, chromosome pairing can be manipulated by utilizing mutant stocks that relax this tight genetic control. The ph1b mutant produced by E.R. Sears over 30 years ago is an invaluable chromosome engineering tool, readily employed in the age of high-throughput molecular genetics. Shortened translocations have already been produced for stem rust resistance genes Sr26 and SrR using ph1b-induced homoeologous recombination. We are currently using induced-homoeologous recombination to reduce the sizes of alien chromosome segments surrounding TTKSK-effective genes Sr32, Sr37, Sr39, Sr40, Sr43, Sr47, SrTt3, Sr2S#1 and SrAeg5 to eliminate linkage drag putatively associated with these genes. Additional TTKSK-effective genes Sr44, SrHv6, SrAsp5, and SrAse3 were first targeted for development of compensating translocation stocks and then for shortening the size of each alien segment. Population development is also underway to characterize several potentially new sources of resistance.
Cloned rust resistance genes and gene based molecular markers in wheat: Current status and future prospects
CSIRO Plant Industry, Australia
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Two broad categories of resistance genes in wheat have been described. One group represents the so called seedling resistance or the ‘gene for gene’ class that often provides strong resistance to some but not all strains of a rust species. The other category referred to as adult plant resistance provide partial resistance that is expressed in adult plants during the critical grain filling stage of wheat development. A few seedling rust resistance genes have been cloned in wheat and other cereals and are predominantly from the nucleotide binding site/leucine rich repeat class which is associated with localized cell death at the pathogen entry site. Until recently, the molecular basis of race non-specific, partial and slow rusting adult plant resistance genes were unknown. Gene products that differ from known plant resistance genes were revealed from the recent cloning of the Yr18, Yr36 and Lr34 adult plant genes in wheat. The available range of diverse resistance gene sequences provide entry points for developing genebased markers and will facilitate selection of germplasm containing unique resistance gene combinations.
Molecular-genetic dissection of rice nonhost resistance to wheat stem rust
CSIRO Plant Industry, Australia
View ayliffe_2009.pdf (244.47 KB)
Rust diseases remain a significant threat to the production of most cereals including wheat. New sources of resistance are continually sought by breeders to combat the emergence of new pathogen races. Rice is atypical in that it is an intensively grown cereal with no known rust pathogen. The resistance of rice to cereal rust diseases is referred to as nonhost resistance (NHR), a resistance mechanism that has only recently become genetically tractable. In this report, the mechanisms of rice NHR to wheat stem rust and other cereal rust diseases are explored and the potential for transferring this durable disease resistance to wheat is considered. Approaches being undertaken for the molecular-genetic dissection of rice NHR to rust are described.