Scientists fighting the epidemic of Ug99 wheat stem rust aim to breed durable resistance by shifting from major to minor resistance genes - again and again .

Since its identification 11 years ago in Uganda (hence its name), Ug99 has energized governments and scientists to protect wheat crops from this virulent stem rust pathogen. In 2008, several countries began producing seed of new, rust-resistant wheat varieties for distribution to farmers. Agricultural experts hope these high-yielding varieties will be in farmers' fields by 2011, providing a buffer against Ug99.

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. The most popular source of resistance, the gene Sr31, was later bred into most of the world's wheat. This resistance held for decades, and wheat crops flourished. Many thought stem rust was defeated, so research and funding shifted to other priorities. But now rust 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, a distinguished geneticist and pathologist with the International Maize and Wheat Improvement Center (CIMMYT by its Spanish abbreviation). "Ug99 and its four new variants now threaten major wheat growing areas on every continent," said Singh. "Eighty percent of cultivated wheat varieties worldwide are susceptible."

Within a decade of the emergence of Ug99, its deadly spores had moved into Kenya, Ethiopia, Sudan, Yemen and Iran. A damaging race of Ug99 has recently surfaced in South Africa. Photo: CIMMYT.

Experts have feared that massive global crop losses to Ug99 could undermine food security. Within a decade of the emergence of Ug99, its deadly spores had moved into Kenya, Ethiopia, Sudan, Yemen and Iran. A damaging race of Ug99 has recently surfaced in South Africa.

In response to this diverse and evolving pathogen, scientists have sought new and varied sources of resistance as part of a global initiative to fight rust. National governments have sped seed multiplication, varietal testing and approval procedures.

Experts at the Kenya Agricultural Research Institute and the Ethiopian Institute of Agricultural Research have performed a remarkable service by helping to screen each year thousands of experimental wheat lines received from breeding programs worldwide that were exposed to severe, natural infections of Ug99.

Efforts are starting to pay off. The Borlaug Global Rust Initiative (BGRI) was launched in 2005 to provide a common 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 several countries' progress in producing resistant seed. Sources included resistant lines from CIMMYT, the International Center for Agricultural Research in the Dry Areas and, in some instances, participating countries' own breeding programs or commercial suppliers. New stocks of resistant seed should be ready for distribution to farmers by 2011, significantly shortening the 10 years it usually takes for a new variety to be tested and made available.

Iran's national Seed and Plant Improvement Institute has released six wheat cultivars, five of bread wheat and one of durum, that resist Ug99, reported M.R. Jalal Kamali, a senior CIMMYT-Iran wheat scientist. Iran is expected to account for 95% of the Ug99-resistant wheat seed produced in seed multiplication efforts, according to Kamali.

Once established, the new rust resistance promised to be more durable than the old. Rust resistance has historically been based on single major genes that block the spores' entry into plant tissue. This type of resistance is highly effective in the short term but sets the stage for its own downfall by creating strong evolutionary pressure that favors more virulent rust mutants. Modern minor resistance genes offer only partial protection. They are harder to breed for, because their presence is less visible in field experiments, but most wheat experts agree that they offer better crop security.

"With minor genes, the disease is 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."

CIMMYT's work to develop and disseminate wheat varieties with stem rust resistance receives support from the Borlaug Global Rust Initiative; governments of Australia, India, Switzerland and the USA; Bill & Melinda Gates Foundation; Syngenta Foundation; Fundación Produce-Mexico; and Arab Fund for Economic and Social Development.