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Stem Rust Ug99–the Agricultural Bully

The views expressed are those of the author and are not necessarily those of Scientific American.


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Remember 1999? It was the year in which the European Union first unveiled its uniform currency and Y2K threatened to bring the technological rapture to global information systems. 1999, the year the artist then-known as Prince declared the benchmark for partying (although he sang it in 1982).

It also marked the identification of a new race of an aggressive wheat fungus in Uganda that continues to plague farmers in East Africa, the Middle East and Central Asia.

Right: Stem rust can devastate world wheat supplies, and a relatively new race known as Ug99 has been especially tough for farmers to control. Photo: U.S. Department of Agriculture.

Ug99, named for the country and year of its identification, is a race of stem rust, a pathogen that attacks the stalks of wheat and barley. Since it was found in Uganda, Ug99 has meandered in all directions, emerging out of the continent into the Middle East and Central Asia. To date, Ethiopia Iran, Kenya, South Africa, the Sudan, Uganda, Yemen and Zimbabwe have all been touched by rust.Traveling by wind, it can spread up to several thousand kilometers before infecting a new field of wheat. The big fear is that it will eventually reach the expansive fields of India and China, the world’s two largest producers by yield. Amidst droughts, erratic weather and fires, rust has grown to become a global threat to supply.

As the most important food crop in terms of caloric contribution to diets across the world, any threat to wheat– biological, physical or environmental– is a wake-up call for those in international agriculture.

According to the UN Food and Agriculture Organization (FAO), about 90 percent of the commercial wheat grown in the world is at risk. Under the right conditions – moist morning air, warm climate and a steady wind – Ug99 can devastate more than 70 percent of a given wheat yield. The FAO runs Rust SPORE, a tracking system for stem rust.

So why, in 12 years, have scientists been unable to control the ‘super-race" of Ug99? Curious, I contacted David Hodson, FAO’s lead expert on wheat rust for some insight and a refresher course in plant genetics.

Left: Wheat stem that has been 10 to 15 percent affected by rust. Photo: CIMMYT.

The sheer variety of Ug99 strains have made a big part of the world’s crop susceptible, said Hodson. Once rust is established, control with fungicides is difficult to achieve. In Kenya, some farmers use up to five sprays and still cannot control the rust.

"The unique thing about Ug99 is the broad spectrum of virulence it exhibits – it is very different from any previous known stem rust races," he wrote in an e-mail. "It is unique in that it has defeated the Sr31 resistance gene – a gene that has been effective in wheat for over 30 years and very widely used – but also most of the resistance genes of wheat origin have also been defeated and other key genes like Sr38 from related species."

The Sr31 resistance gene– originally identified in wheat’s cousin, rye– was found to be extremely effective against common stem rust, pre-1999. Wheat breeders introduced it to fields across Africa to guard farmers against the fungus.

Alas, Sr31 provided too much of a good thing, and the lack of wheat diversity created an open door for the mutant Ug99 to devastate.

There are about 50 different stem resistant genes besides Sr31 in all of the major varieties of wheat. After Sr31 varieties fell, scientists found evidence of an attack on another important resistance gene– Sr21– in einkorn wheat. In the years following 1999, key resistant genes in wheat were lost as Ug99 gained momentum. Ug99 is an agricultural bully– pushing all the buttons to break its victim down– and a quickly-evolving one at that.

But as fast as it may run, Ug99 may have an Achilles heel. News broke last week about a new variety of wheat that could resist Ug99– which can kill off 70 percent of a farmer’s crop. The official announcement is expected to be made this week at a meeting in St. Paul, Minn. at a meeting dedicated to Ug99.

Right: Wheat stem that has been 90 percent affected by rust. Photo: CIMMYT.

According to the Greenwire article, Ravi Singh, a researcher at the International Wheat and Maize Improvement Center (CIMMYT) headquartered in Mexico, knew that the heavy reliance on Sr31 to resist wheat would not last forever. He decided to breed four to five genes to create 298 varieties. The seeds of 15 to 20 of these have been dispatched to various sites for field tests, said Hodson.

In addition, the new varieties offer a 10 to 15 percent increase in yield over resistant varieties, a very attractive package for farmers fighting a multitude of other stresses like pests and drought, much of it attributed to climate change.

So will Singh’s findings be the end-all to Ug99’s devastation? It’s promising, says Hodson, but a permanent solution is still very much in the long-term.

"The complex resistance to Ug99 in these materials, and also similar resistance to other rusts, combined with high yield performance should make them both attractive to farmers and also very difficult targets for the pathogen to attack," he said.

The 15 to 20 different samples being tested throughout vulnerable places will also ensure a genetic diversity of cultivars. "So the nature of the resistance, combined with the choice available should really help to reduce the cycle of susceptibility," explained Hodson.

If the field tests give reason to ring the bell of disease resistance, it will be a triumph for averting poverty and maintaining food security, and a huge weight off the collective shoulders of the wheat breeding community.

About the Author: Tiffany Stecker writes about climate change, agriculture and forestry in Washington, D.C., via London, Paris and San Diego. Wheat plays an important role in her Saturday morning pancake-making ritual. She keeps an informal blog Steckarrr and claims the occasional byline at www.climatewire.net. She also tweets as @TiffanyStecker.

 

The views expressed are those of the author and are not necessarily those of Scientific American.






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