A new form of stem rust, a virulent wheat disease, has spread from East Africa and is now infecting wheat in Yemen, across the Red Sea on the Arabian Peninsula. Researchers with the Global Rust Initiative, a program coordinated by the International Maize and Wheat Improvement Center (CIMMYT, by its Spanish acronym), International Center for Agricultural Research in the Dry Areas (ICARDA) and the United States Department of Agriculture's Agricultural Research Service (USDA-ARS) have confirmed conclusively the existence of the disease in Yemen.

There is inconclusive evidence that it has also spread into Sudan. Until these discoveries, this strain of stem rust, known as Ug99, had been seen only in Uganda, Kenya and Ethiopia.

These developments follow closely on the heels of the discovery by the same team that wheat varieties that had previously shown resistance to the fungus are now susceptible, likely indicating that it has mutated again.

The last major epidemic of stem rust occurred in North America in the early 1950s, destroying as much as 40% of the continent's spring wheat crop. Out of this crisis came a new form of international cooperation among wheat scientists worldwide, spearheaded by wheat scientist and Nobel laureate Norman Borlaug. This combined effort brought the development of wheat varieties that resisted the onslaught of stem rust for more than 4 decades.

Ug99, which was first observed in Uganda 8 years ago, can attack most previously disease-resistant wheat varieties. A year and a half ago specialists in geographic information systems at CIMMYT plotted the probable trajectory of the fungus, whose spores can travel great distances on the wind. The models predicted that, if the fungus crossed from East Africa to the Arabian Peninsula, it could easily spread to the vast wheat-growing areas of North Africa, the Middle East and South Asia, where a quarter of the world's wheat is grown.

This has happened before. Another wheat disease called yellow rust emerged in East Africa in the late 1980s. Once it appeared in Yemen, it took just 4 years to reach wheat fields in South Asia. This strain of yellow rust caused major wheat losses exceeding US$1 billion in value.

There is every reason to believe that Ug99 presents a much greater risk to world wheat production. Experiments conducted over the past 2 years in Kenya and Ethiopia have demonstrated that most of the world's wheat varieties are susceptible to Ug99.

"This is a problem that goes far beyond wheat production in developing countries," warns Borlaug. "The rust pathogen needs no passport to cross national boundaries. Sooner or later, Ug99 will be found throughout the world, including in North America, Europe, Australia and South America."

Scientists from CIMMYT and ICARDA have already identified promising experimental wheat breeding materials resistant to Ug99. But moving from the first breeding trials to placing new, rust-resistant varieties on millions of hectares of farmers' fields takes time and massive effort.

"If we fail to contain Ug99, it could bring calamity to tens of millions of farmers and hundreds of millions of consumers," says Borlaug. "We know what to do and how to do it. All we need are the financial resources, scientific cooperation and political will to contain this threat to world food security."