Use of large-scale computational resources has permitted the first quantitative study of airborne migration routes of fungal spores between numerous key epidemiological hot-spots of wheat stem rust in Africa, the Middle East and the Indian subcontinent. By coupling a state-of-the-art Lagrangian particle dispersion model (NAME) with mechanistic epidemiological models, we simulate turbulent atmospheric transport of large ensembles of fungal spores from source sites. The models use highly resolved global meteorological datasets from the UK Meteorological Office. We consider release of P. graminis uredinospores from numerous source locations over an 11 year period (2003-2014) and simulate atmospheric trajectories over a 10 km2 spatial sampling grid to elucidate spore deposition rates at national, regional, and continental spatial scales. Our systematic exploration permits the first quantitative perspective and ranking of likely airborne transmission routes of wheat stem rust. We identify migration trends within and between the “Rift valley epidemiological zone”, the Middle East, the Indian Subcontinent, as well as South Africa. Our results indicate (I) consistent seasonal dispersal patterns, (II) likely airborne transmission of stem rust from the Middle East to North-East Africa, and (III) suggest that there is considerable risk of spread of Ug99 or other virulent races from Eastern Yemen to the Indian subcontinent. Model results indicate that over the 11 year study period, viable spore deposition occurred between Eastern Yemen and Pakistan on average 22 days per year during overlapping wheat growing seasons. The validity of the modelling framework has been successfully tested by comparison with survey data from the 2013 epidemic outbreak in Ethiopia, and was recently used as a risk assessment tool to provide rapid response advice in different East-African countries. Known stem rust race distributions are also supportive of the model outputs. The research we have been doing allows a quantitative perspective on likely airborne transmission routes of Ug99 or other virulent races of wheat rust. By that we hope to provide new insights and recommendations for future risk assessment, survey and control strategies and also to contribute to fundamental understanding of epidemiological spread on regional and continental scales. The work we would like to present is the result of a joint effort of Dr Laura Burgin and Dr Matt Hort from the UK Meteorological office, Dr Dave Hodson from CIMMYT, and Dr James Cox, Matthew Hitchings and me from the Epidemiology and Modelling group of Prof Gilligan in Cambridge.
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In response to the threat posed by Ug99 (race TTKSK) and a global expert panel assessment, the Borlaug Global Rust Initiative (BGRI) was formed in 2005. This represented one of the most comprehensive global programs to address an emerging crop pathogen threat. For the last decade, surveillance and monitoring has been a key component of the BGRI. Progress in rust surveillance and monitoring over the last ten years is critically reviewed, with a focus on stem rust. The transition from a data poor environment regarding stem rust to a fully functional, comprehensive crop pathogen surveillance system is a notable success. Key components and status of the current system are described, including; the surveillance network, the data management and information platforms, and pathogen tracking. The application of the existing surveillance and monitoring system and the current status of important stem rust races are described. The role that new technologies are playing in the monitoring and tracking of stem rust is highlighted. Recent stem rust epidemics in East Africa provide stark warning of threat that the disease poses and the clear need to continuously monitor evolving stem rust populations. Shortcomings of the existing system are examined and future directions for the surveillance and monitoring system are outlined.
Stem (black) rust is a potentially important disease in northern, western and southern Iran. A new Pgt race with virulence to gene Sr31 appeared in Iran in 2007. Similar races have spread in Africa and some CWANA countries. In 2014 stem rust was widespread in western, northern, northwestern and central Iran, but at low severities. Thirty-nine stem rust samples were collected for race analysis. After purification and increase each isolate was inoculated to a set of 20 North American differentials in the greenhouse. Infection types were recorded 12-14 days after inoculation using the scale described by McIntosh et al. (1995, Wheat Rusts: An Atlas of Resistance Genes, CSIRO, East Melbourne, Australia). Races TKSTC (59%), TKTTC (20%), TTTTC, KTTSK (virulent on plants with Sr31), TTSTC, PTTTF and TTTTF were detected. Race TKSTC was common in western, northwestern and central Iran. Except for avirulence to Sr17 this race is similar to the race (TKTT) that caused a stem rust epidemic in Ethiopia in 2013.
The discovery of Ug99 stem rust with virulence on most widely grown wheat cultivars worldwide triggered substantial new research on host resistance genes and associated virulence dynamics in the pathogen. Ug99 is mutating and migrating, with eight variants presently known, and has spread throughout eastern Africa, across the Red Sea to Yemen and Iran, and to South Africa. It has been speculated that further movement of Ug99 spores from South Africa to South America could happen on prevailing winds that occur about eight days per month on average. While Ug99 is not yet present in South America, this is a critical entry point into the Western Hemisphere as demonstrated by introduction of soybean rust to Paraguay in 2001. Thus, work was initiated to engage countries in South America to participate in monitoring for its occurrence. Stem rust surveys are currently conducted in Argentina, Brazil, and Uruguay on a regular basis. Each country has a national agricultural institute with adequate to good capacity to perform pathotyping work, but have limitations due to inadequate greenhouse cooling. We will present the current virulence dynamics of Pgt in each country. In addition to surveys for rust, we searched for the presence of Berberis spp. in Brazil. Berberis laurina was abundantly distributed in the Rio Grande du Sul state near the city of Caçapava. Leaves sampled in October displayed low to moderate aecial infections. Determination of the pathogen species infecting B. laurina is currently being determined by physiologic and molecular methods.
The risk of aerial long-distance transport of rust pathogens from potential source locations in the Eastern Hemisphere to the Western Hemisphere and from subtropical to continental interior regions within North America is investigated. Simulations of longdistance transport of rust spores using the Integrated Aerobiology Modeling System indicate that the frequency of transport and deposition in the Western Hemisphere of viable rust spores originating from potential sources in tropical Africa, at high latitudes in Europe, and throughout eastern Asia is low. However, the frequency of trans-oceanic transport and deposition of viable rust spores in the Western Hemisphere is high for potential African source locations poleward of the tropics. The relatively short distance between Western Africa and northeastern South America coupled with the presence of persistent Northeasterly Trade Winds create an active pathway for spore transport. Western Hemisphere regions that are influenced by the Intertropical Convergence Zone have the highest likelihood of receiving viable rust spores from the Eastern Hemisphere. The risk of aerial transport of viable rust spores to U.S. regions from potential Eastern Hemisphere source regions is low. Analysis of wind streamline maps for North America indicate that strong low-level advection of air northward from the subtropics is prevalent east of the Rocky Mountains from early April to mid-May providing opportunities for long-distance transport of rust pathogens into the continental interior. After mid-June, the number of days with strong lowlevel advection of air from south to north across these regions and thus opportunities for long-distance spore transport decrease dramatically.
Detection of stem rust race TTKSK (Ug99) from Uganda in 1998/99 highlighted not only the extremely high vulnerability of the global wheat crop to stem rust but also a lack of adequate global systems to monitor such a threat. Progress in the development and expansion of the Global Cereal Rust Monitoring System (GCRMS) is described. The current situation regarding the Ug99 lineage of races is outlined and the potential for expansion into important wheat areas is considered. The GCRMS has successfully tracked the spread and changes that are occurring within the Ug99 lineage and is now well positioned to detect and monitor future changes. The distribution of Ug99 variants possessing combined virulence to Sr31 and Sr24 is expanding rapidly and future spread outside of Africa is highly likely. Efficient and effective data management is now being achieved via the Wheat Rust Toolbox platform, with an expanding range of dynamic information products being delivered to endusers. Application of new technologies may increase the efficiency of the GCRMS, with mobile devices, molecular diagnostics and remote sensing all seen to have potential application in the medium to longterm. Expansion of the global capacity for race analysis is seen to be critical and integration of the Global Rust Reference Centre into the stem rust monitoring network is seen as a positive development. The current acute situation with severe epidemics of stripe rust in many countries indicates a clear need for more effective global monitoring systems and early warning for this pathogen. The existing GCRMS for stem rust is seen as a good foundation for this to occur.
Surveillance of wheat rust pathogens, including assessments of rust incidence and virulence characterization via either trap plots or race (pathotype) surveys, has provided information fundamental in formulating and adopting appropriate national and international policies, investments and strategies in plant protection, plant breeding, seed systems, and in rust pathogen research. Despite many successes from national and regional co-ordination of rust surveillance, few attempts were made to extend rust surveillance to international or even global levels. The Global Cereal Rust Monitoring System was established to address this deficiency. It is underpinned by an information platform that includes standardized protocols for methods and systems used in surveys, preliminary virulence testing, data, sample transmission and management at the field and national and global levels, and includes two web-based visualization tools. While considerable progress has been made towards a global system for monitoring variability in the wheat stem rust pathogen, and linking this to the threat posed by this pathogen to regional wheat production, some challenges remain, including ongoing commitment to support rust surveillance, and the ability to share and compare surveillance data.
Limited but targeted stem rust race characterization was undertaken in Kenya in 2004 and 2005 which led to the detection of Ug99 present in Kenya and designation of Ug99 as race TTKS (based on North American stem rust race nomenclature system). Further surveillance in 2006 and 2007 detected variants of TTKS with virulence on Sr24 (TTKST) and Sr36 (TTTSK), respectively. Stem rust surveillance was undertaken at an extended level in 2008 and 2009 within predominant wheat growing regions of Kenya. Three hundred and sixty farms were surveyed from regional districts of Naivasha, Narok, Nakuru, Laikipia, Meru, Uasin-Gishu, Nandi, Elgeyo and Trans-Nzioa, during 2008 main season (May to September and December). The information from farmers indicated that more than 95% of these farms were sprayed with fungicides. Despite the use of fungicides, stem rust was detected in 67% of the surveyed farms. Stem rust ranged from trace amount -100% in severity with minimum infection in Naivasha district (40%) and maximum in Narok district (90%). Yellow rust was detected in 22% of the farms. Out of one hundred and twenty-six stem rust samples collected, 37 and 39 (a total of 76 ) samples were sent to Cereal Disease Laboratory (CDL) Minnesota, USA and Cereal Research Laboratory of Agriculture and Agri-Food Canada respectively, for race typing using the respective differentials used by these labs. From the 39 collections sent to Canada, 17 (43%) survived, of which majority were typed to TTKST (65%) followed by TTKSK (18%), PTKST (12%) and mixture of TTKST and TTKSK (5%). The CDL typed vast majority of pathotypes as TTKSK (84%) followed by TTKST and TTTSK (7% each). The combined results of two labs indicated that predominant frequency in Kenya in 2008 was TTKSK (51%) followed by TTKST (31%), PTKST (6%) and TTTSK (6%). The frequency of TTKST significantly increased in 2008 compared to 2007 which is not surprising, given that Sr24 carrying wheat cultivar KS Mwamba is cultivated on large acreage in Kenya. In 2009, 262 farms were surveyed from regional districts of Narok, Laikipia, Nyandarua, Meru, Uasin-Gishu, Nandi, Elgeyo and Trans-Nzioa. The 2009 season experienced heavy drought in many areas. Nevertheless, stem rust was detected in 79% of the farms with disease severity ranging from trace to 100%. Yellow rust was detected in 15% of the farms. Stem rust infection ranged from 0 to 100% with minimum infection in Nyandarua (18%), Laikipia (42%) and maximum in Uasin-Gishu and Elgeyo (100% each). Out of seventy-four stem rust samples collected, 55 samples were sent to Canada for race typing. Only 20% of the samples survived, of which majority were typed to TTKST (50%), PTKST (34%) and PTKSK (16%). Borlaug Global Rust Initiative 2010 Technical Workshop / Poster Abstracts 7 The 2009 results did not depict real situation of predominance of pathogenic variability because of small sample size, however it provided fair indication that race TTKST is still the most prevalent. This information generated on the distribution of stem rust races, and the incidence of stem rust is important for anticipatory breeding and release of cultivars with effective sources of resistance in Kenya, and at same time mitigating global threat of stem rust by reducing intensity of stem rust inoculum in East Africa.
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.
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.