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July 26, 2010

New rust resistant wheat seed on its way to farmers

Stem rust rises again… and again

Stem rust is an old foe. Norman Borlaug, the late Nobel Peace Prize winner and father of the Green Revolution, battled this fungal disease in the 1950s. After years of painstaking plant breeding, he and his team endowed improved wheat varieties with rust resistance genes. The most popular source of resistance, gene Sr31, was later bred into most of the world’s wheat. For decades its resistance held and wheat crops flourished. Many thought stem rust was defeated and research and funding shifted to other priorities.

But now it is back. And mutating. Four new strains able to overcome previous forms of genetic resistance have crept into wheat fields, causing international alarm, according to Ravi Singh, CIMMYT distinguished scientist and geneticist/pathologist. "Ug99 and its four new variants now threaten major wheat growing areas in every continent," says Singh. "Eighty percent of cultivated wheat varieties worldwide are susceptible."

Since Ug99 appeared in Uganda in 1998 and overcame resistance genes, experts have feared the worst: massive global crop losses leading to increased food insecurity. Within a decade, its deadly spores had moved into Kenya, Ethiopia, Sudan, Yemen, and Iran. A damaging race of the Ug99 family has recently appeared in South Africa.

In response, the world has rallied: scientists have sought new sources of resistance and joined in global initiatives to fight rust; national governments have sped seed multiplication and varietal testing and approval procedures.

Experts from the Kenya Agricultural Research Institute (KARI) (shown here) and the Ethiopian Institute of Agricultural Research (EIAR) have performed a remarkable service by helping screen thousands of experimental wheat lines from breeding programs worldwide each year under severe, natural infections of Ug99 stem rust.

Efforts are starting to pay off. The Borlaug Global Rust Initiative (BGRI) was founded in 2005 and provides a key venue for the world's wheat and rust experts to exchange information about the disease and its movements, as well as about resistant wheat lines. At a recent meeting, BGRI participants discussed progress by several countries in producing resistant seed. Sources included resistant lines from CIMMYT, from the International Center for Agricultural Research in the Dry Areas (ICARDA), and in some instances from their own breeding programs or commercial suppliers. According to reports, new stocks of resistant seed should be ready for distribution to farmers by 2011—significantly sooner than the 10 years it usually takes for a new variety to be released, tested, and made available.

The release of a new variety is usually slow and subject to tough criteria that vary from country to country. After being developed and selected in breeding programs, candidate varieties undergo years of tests. The best are moved into multiplications trials, where a larger amount is planted for seed production. To get Ug99 resistant seed to farmers more quickly, several countries are testing promising varieties and increasing their seed at the same time—an expensive approach, as only seed of the few varieties selected will finally be used.

The Seed and Plant Improvement Institute (SPII), Iran’s national wheat breeding program, has released five bread wheat and one durum wheat cultivars that are resistant to Ug99, says M.R. Jalal Kamali, CIMMYT-Iran senior wheat scientist. Iran is expected to account for 95% of the Ug99 resistant wheat seed produced in seed multiplication efforts, according to Kamali.

In an initiative supported through the USAID Famine Fund, six countries (Afghanistan, Bangladesh, Egypt, Ethiopia, Nepal, and Pakistan) embraced this quicker production method and sowed 52 hectares with 11 varieties, producing nearly 145 tons of Ug99 resistant seed in the 2008-09 crop cycle. In the same growing season, Iran planted 34,000 hectares and produced 80,000 tons of Ug99 resistant wheat seed. Large-scale seed production continued into 2009-10 and combined the seven countries have sown over 47,000 hectares, anticipated to yield 118,000 tons of improved seed. If seed production continues as anticipated, Bangladesh, Egypt, and Iran will have enough Ug99 resistant seed to sow at least 5% of their national wheat area. One hectare of wheat produces enough seed to sow 20 hectares, so 5% is the safeguard threshold for replacing susceptible varieties in case of a Ug99 outbreak. Four additional countries will likely reach this target by the end of 2010.

So what is the difference between these new Ug99 resistant varieties and those of the past? Rust resistance has historically been based on major genes. Single major genes block entry of spores into plant tissue. This type of resistance is highly-effective in the short term, but also sets the stage for its own downfall, creating strong evolutionary pressure that favors more virulent rust mutants.

Minor genes offer partial protection. In this sense, they are harder to breed for, because their presence is less visible in field experiments. But most wheat experts agree they represent the safest path to crop security. “With minor genes, the disease it not eliminated, but its attack on the plant is slowed," explains Singh. "Like the code for a combination lock, several minor genes in tandem in the same variety are hard for the pathogen to 'decipher' and provide more durable resistance. CIMMYT’s strategy has been to identify and breed minor genes into wheat varieties, as well as assisting partners in this challenging task.”


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