New Agriculturist: Developments - New virulent rust threatens world's wheat

Home
Developments
New virulent rust threatens world's wheat

New virulent rust threatens world's wheat

About 90 per cent of the world's wheat is estimated to be susceptible to stem rust
USDA

Widespread outbreaks of a new aggressive strain of wheat yellow rust, in Central and West Asia, North Africa and the Caucasus, could cause billions of dollars in crop losses in 2010. The news came as scientists gathered in St. Petersburg for a global wheat event in June, where it was also announced that four new mutations of Ug99 - a strain of wheat stem rust - had overcome two of the most important stem rust-resistant genes used in breeding programmes. While outbreaks of yellow and leaf rust cause significant economic losses by reducing yields, an outbreak of Ug99 in any significant wheat growing area could wipe out 40 per cent of the crop.

Under the umbrella of the Borlaug Global Rust Initiative (BGRI), wheat researchers and breeders from around the world have come together to monitor the spread of wheat rusts and develop and distribute new varieties of wheat with durable resistance. "We need to have a sustainable system worldwide that not only deals with wheat rust, but the improvement of wheat in general," explains Ronnie Coffman, vice chairman of the BGRI.

Small farmers under threat

First discovered in Uganda in 1998, the wind-borne fungus attacks wheat stems, causing plants to fall over; it is capable of destroying an entire harvest. In Kenya, up to 80 per cent of wheat was destroyed during several cropping seasons in the late 1990s. The original Ug99 has spread so far to Kenya, Ethiopia, Sudan, Yemen and Iran. "Given favourable conditions [Ug99] threatens to spread into other wheat-producing regions of Africa and Asia, and potentially, the entire world," says Arun Kumar Joshi, a scientist with the International Maize and Wheat Improvement Center (CIMMYT). "The threat is particularly acute in South Asia, which produces 20 per cent of the world's wheat for a population of 1.4 billion people."

About 90 per cent of the world's wheat is estimated to be susceptible to stem rust. Smallscale farmers without access to fungicides are the most vulnerable. "You can go for years and not have the conditions right for an epidemic, but when it occurs it's just devastating," describes Coffman. "East Africa and South Asia, where there are very large numbers of poor people and low income farmers who depend on wheat, are big worries, but having it occur in any significant wheat growing area will spike commodity prices and will have a serious economic impact."

Combining resources

Potential Ug99-resistant varieties are being screened at nurseries in Kenya and Ethiopia
University of Minnesota/David Hansen

Scientists hope that using molecular techniques to identify and pool several resistant genes in one wheat variety will make it much harder for the fungus to overcome new varieties. "Previously, we've been limited to deploying single genes which tend to be overcome quickly," Coffman states. "But the probability that the fungus would have four mutations and overcome four genes simultaneously is very low."

The BGRI strategy is to deploy as much rust-resistant wheat seed as possible so that when the pathogen strikes at least some farmers in each country will have resistant wheat seeds. "It's very hard in the absence of disease to convince farmers that they should replace the varieties they are growing," Coffman remarks. "But we are hoping to cover at least five per cent of the wheat growing area of countries likely to be affected by rust."

In addition to developing resistant genetic material, a number of projects are underway to track the spread of the disease. The UN's Food and Agriculture Organization has created a website, 'Rust Spore', to provide up-to-date information about the status of wheat stem rust, monitor new strains and provide access to reliable data. With support from the Danish government, a global reference laboratory is also being established so that countries are able to send rust samples for analysis.

Preparing for the worst

Scientists, who recently met at the eighth International Wheat Conference (IWC) in June 2010, said that collaborative research and breeding programmes are producing promising new lines that exhibit excellent defences against Ug99 and the new strains. "But" says Dr. Ravi Singh, senior scientist with CIMMYT, "with the new mutations we are seeing, countries cannot afford to wait until rust 'bites'. The variant of Ug99 identified in Kenya, for example, went from first detection in trace amounts in one year to epidemic proportions in the next."

Collaborative research and breeding programmes are producing promising new lines
Petr Kosina/CIMMYT

"All governments of wheat-growing countries need to be sensitised because the varieties that farmers are cultivating now will definitely be overcome in time," predicts Coffman. Therefore, in addition to establishing surveillance teams of plant pathologists to track the disease, Coffman states that governments should also ensure that scientists are engaged in wheat improvement in their country and collaborating with colleagues across the world to ensure that they have the best sources of resistance. "Attention to seed systems is the third vital step," he continues. "When stem rust does come, will they be ready to multiply seed and deliver it to their farmers in a timely way? Because it will be needed and needed urgently."

Date published: June 2010