Keeping track of rust
'The disease that never sleeps' is how Nobel Prize winner Norman Borlaug once described wheat rust, a group of deadly, constantly changing fungal pathogens that pose a dangerous threat to food security worldwide. Globally, the three wheat rusts - stem, leaf and stripe rust - are the most economically damaging diseases of wheat. All inflict devastating losses when epidemics occur, but stem and stripe rust are the most feared, inflicting losses of 60 per cent or more under favourable conditions. Successful control of stem rust - mainly through resistance breeding - is estimated to have saved farmers worldwide over US$1 billion annually for more than four decades. However, past wheat breeding successes cannot protect the world's wheat crop indefinitely against this constantly changing enemy. With new rust threats emerging, scientists have implemented a global rust tracking system, to try to stay ahead of their evolving enemy.
The Global Cereal Rust Monitoring System was initiated in 2008 as part of the Durable Rust Resistance in Wheat (DRRW) project*. The rust tracking system now covers 42 countries, and includes most of the wheat grown in developing countries. Information from field surveys, pathotype analysis and predictive models are stored in a state-of-the-art data management system (the Wheat Rust Toolbox) and made available via the RustTracker information system developed by the International Maize and Wheat Improvement Center (CIMMYT) in collaboration with partners*.
"The aim of RustTracker.org is to provide a single source of information relating to global rust monitoring activities," says CIMMYT's Dave Hodson, who coordinates the rust monitoring work. "Regularly updated information can provide rapid notifications of important pathogen changes or movements, new outbreaks, or risks of outbreaks. No other comparable monitoring system for a major crop disease currently exists at such a wide geographical scale."
Where is Ug99?
Ug99, an especially virulent strain of stem rust first identified in Uganda in 1999, was the catalyst for developing the rust tracking system. Eight closely related variants of Ug99 now exist and have spread to 11 countries between South Africa and Sudan and across the Red Sea to Yemen and Iran. With a large proportion of the world's wheat estimated to be susceptible to the Ug99 races, the threat to South Asia is a major concern. The region, home to 1.4 billion people, produces around 20 per cent of the world's wheat; small-scale farmers without access to fungicides are particularly vulnerable.
In recent years, new, highly aggressive strains of stripe rust (also known as yellow rust) affecting many of the most widely grown wheat varieties have emerged, causing devastating epidemics in Azerbaijan, Ethiopia, Iraq, Morocco, Syria, Tajikistan and Uzbekistan, with national yield losses as high as 40 per cent. The value of RustTracker was demonstrated in Ethiopia during the 2013/14 crop season. Serious outbreaks of stripe rust were observed from North Africa to South Asia between February and May 2013, prompting an alert for the East African region and resulting in mitigation efforts in Ethiopia including: comprehensive major and minor season surveys, timely control in infected areas and frequent information exchange amongst stakeholders. Another important factor in Ethiopia was the recent, rapid multiplication and promotion of rust-resistant cultivars. By 2013 about 80 per cent of the wheat area was planted to improved cultivars with resistance to Ug99, yellow rust, or both. In contrast to 2010, when 600,000 hectares of wheat were affected by stripe rust; only limited and localised outbreaks were observed in 2013 and losses were minimal.
Within five years, RustTracker has developed into one of the most extensive and fully functional global crop disease monitoring platforms. The system is continually updated by in-country partners uploading field survey data (including by using smartphone and tablets) and providing samples for analysis. New molecular diagnostic tools developed by the US Department of Agriculture's Agricultural Research Service (USDA-ARS) Cereals Disease Laboratory, in Minnesota, are providing rapid detection of Ug99 races in samples collected from farmers' fields. However, all of the rust surveillance activities only have relevance if they inform the wheat variety improvement efforts. Hence strong linkages to core wheat cultivar databases such as the CIMMYT Wheat Atlas and GRIS are being developed to ensure that both host and pathogen information are connected.
Keeping one step ahead
Tremendous progress has been made in getting new, improved, high-yielding rust-resistant varieties into farmers' fields. Since 2008, over 40 rust resistant cultivars have been released in nine 'at-risk' countries in Africa and Asia. Similar progress has been made in the area of gene discovery; more than 35 genes that confer resistance to Ug99 have been identified, 25 of these are available in a form that can be easily used by wheat breeders.
The next steps for rust surveillance include a geographical expansion of coverage (targeting 40 countries), increased capacity for collaborating partners through enhanced training, and improved coverage of a wider range of rust diseases, especially stripe rust. Through a new collaboration with Cambridge University, UK, Hodson is hopeful that the initiative can make further progress on disease early warning and mitigation advice. In 2013, advanced risk maps from Cambridge were used to alert potentially at-risk neighbouring countries when a severe, but localised, epidemic of stem rust broke out in Ethiopia after a previously rust-resistant cultivar suddenly became susceptible.
The other major challenge is to ensure the long-term sustainability of the global rust monitoring system. This is critical given the continual emergence of new, virulent rust races. A failure of the global monitoring system, and the allied control and mitigation efforts, would undoubtedly result in re-emerging threats to wheat production and food security.
* Funded by the Bill and Melinda Gates Foundation and the UK's Department for International Development.
* Partners include the Global Rust Reference Centre, Aarhus University, Denmark, and the International Center for Agricultural Research in the Dry Areas (ICARDA).
With contributions from: Dave Hodson, CIMMYT
Date published: January 2014
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