History and status of the wheat rusts
The University of Sydney, Plant Breeding Institute, Australia
View Mcintosh_2009.pdf (273.55 KB)
The rusts have been ongoing problems for wheat production probably since domestication of the crop about 8,000 years ago. Epidemics vary in size and frequency with host genotype and environment, wet years being ‘rust’ years. Although partial control in modern agriculture was achieved with resistant varieties, conditions favoring epidemics were made worse with the intensification of production and greater resistance gene uniformity in the host. The current Ug99 incident illustrates the situation of very widely adapted successful genotypes grown across huge areas in the presence of an ongoing threat from a recently emerged widely virulent and obviously highly aggressive pathotype of the stem rust pathogen. This paper addresses some of the history of cereal rusts and reviews underlying principles of host pathogen genetics, some of which are being neglected in the period of modern genetics.
Using race survey outputs to protect wheat from rust
The University of Sydney, Plant Breeding Institute, Australia
View park_2009.pdf (140.72 KB)
Race (pathotype) surveys of cereal rust pathogens have been conducted in many parts of the world since the early 1900s. The only way to identify rust pathotypes remains virulence testing in greenhouse tests using genotypes (“differentials”) carrying different resistance genes. Virulence determinations have rarely targeted genes conferring adult plant resistance because of the technical difficulties of working with adult plants under controlled conditions. Where pathotype surveys have been conducted in a robust and relevant way, they have provided both information and pathogen isolates that underpinned rust control efforts, from gene discovery to post-release management of resistance resources. Information generated by pathotype surveys has been used to: devise breeding strategies; indicate the most relevant isolates for use in screening and breeding; define the distribution of virulence and virulence combinations; allow predictions of the effectiveness/ ineffectiveness of resistance genes; and issue advance warning to growers by identifying new pathotypes (both locally evolved and introduced) before they reach levels likely to cause significant economic damage. To be most effective, pathotype surveys should also provide fully characterized isolates (defined pathotypes) for use in identifying new sources of resistance and screening breeding material. Although constrained to some extent by a lack of markers, particularly those not subject to natural selection, surveys have also provided considerable insight into the dynamics of rust pathogen populations, including the evolution and maintenance of virulence, and migration pathways, including periodic long-distance migration events.
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
View jin_2009.pdf (111.81 KB)
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.
Pyramiding Slow Rusting Genes for Durable Resistance to Leaf Rust in Durum Wheat
Variants of Puccinia triticina race BBG/BN, separately overcoming three resistance genes, were identified from durum wheat (Triticum turgidum ssp. durum) fields in northwestern Mexico since its introduction in 2001. Major genes available for use in breeding programs are limited and an alternative strategy is required. Previous studies indicated that slow rusting resistance in eight CIMMYT durums was determined by 2 to 3 minor genes with additive effects. Twenty-eight 4-way crosses were made between these lines with the aim of developing new germplasm with enhanced levels of resistance through pyramiding diverse minor genes. Plants in F1 (4-way) through F3 generations were selected for slow rusting under high leaf rust pressure at the Cd. Obregon and El Batan field sites in Mexico and spikes from selected plants were harvested as bulks. Plants in the F4 generation were individually harvested and1,843 advanced lines obtained, among which 106 lines with enhanced resistance, and desirable agronomic and grain characteristics were selected for non-replicated yield and leaf rust evaluation trials at Obregon during the 2007-2008 season. The best 19 lines, exhibiting near-immunity but with the presence of a few susceptible type pustules, parents and susceptible checks were evaluated for leaf rust resistance under very high disease pressure in replicated trials sown on two dates (16 May and 6 June) at El Batan during 2008. Spreader rows of susceptible cultivar ‘Banamichi C2004’, sown as border and as hills on one side of each plot, were inoculated with P. triticina race BBG/BP. Leaf rust severities, and host responses to infection were determined from weekly readings, and area under the disease progress curves (AUDPC) were calculated. Several lines were identified with significantly lower final leaf rust severity responses and AUDPC values than the most resistant parent in each cross. Our results show that enhanced levels of slow rusting can be generated by pyramiding diverse genes present in different parents. The trial is being repeated during the 2008-2009 season at Obregon to validate the results. In addition these lines are being used for transferring slow rusting resistance into high yielding, superior quality adapted backgrounds using the single-backcross approach.