Just when scientists thought they had managed to curb a mutant fungus, Ug99, a variation of wheat stem rust, four new forms of the killer pathogen have surfaced in the last three years, posing a significant threat to the world's most consumed cereal.
The newest mutation, or race, of Ug99 was discovered in 2009 in South Africa; another was found in the wheat-growing belt of South Africa's Western Cape Province in 2007, said Zak Pretorius, professor of plant pathology at University of the Free State.
The other two variations of Ug99 were found in 2006 and 2007 in Kenya, from where they spread rapidly to Ethiopia and then across the Red Sea to Yemen and Iran.
Ethiopia and Kenya had serious wheat rust epidemics with considerable yield losses in 2007, according to the UN Food and Agriculture Organization (FAO), which has estimated that up to 80 percent of all wheat varieties planted in Asia and Africa are susceptible to Ug99.
Wheat stem rust, also known as wheat black rust, can destroy entire fields and even whole crops. The pathogen enters the stems of the plant and destroys the vascular tissue.
There are three types of rust that can harm wheat, but stem rust, of which Ug99 is a variant, is the most feared, according to the Borlaug Global Rust Initiative, named after Nobel Laureate wheat scientist Norman Borlaug, who drew attention to the threat posed by the fungus before his death in 2009.
The initiative led by Borlaug and other international wheat scientists was formed after the last major stem rust epidemic swept across North America's wheat fields in the early 1950s, destroying up to 40 percent of the crop. The initiative contributed to the development of rust-resistant varieties until Ug99 surfaced in Uganda in 1999, which it named.
"We will now have to make sure that every new wheat variety we release has iron-clad resistance to both Ug99 and the new races," said Ravi Singh, senior scientist in plant genetics and pathology at the Mexico-based International Maize and Wheat Improvement Centre (CIMMYT).
|We will now have to make sure that every new wheat variety we release has iron-clad resistance to both Ug99 and the new races|
"With the new mutations we are seeing, countries cannot afford to wait until rust 'bites' them," he said. "The variant of Ug99 identified in Kenya, for example, went from first detection in trace amounts in one year  to epidemic proportions the next year."
The new mutants
The new mutations or "races" have acquired the ability to defeat two of the most important stem rust-resistant genes, used widely in most of the world's wheat breeding programmes. Singh said when the new race of Ug99 surfaced in Kenya, CIMMYT had to delay releasing new lines of wheat that had only one of the stem-rust resistant genes.
The rust fungus, which causes rust-coloured patches in the infected parts of the plant, spreads by spores that can survive harsh winters but germinate in warmer conditions and are usually transported by the wind - but sometimes even on clothing - over long distances and across continents.
South African scientist Pretorius, who characterized the new races, will present his findings at a two-day meeting of global wheat experts on fighting stem rust starting on 30 May in St Petersburg, Russia.
"My greenhouse work showed that from a collection of 129 South African commercial cultivars and advanced breeding lines tested, 47 percent are susceptible in the seedling stage to one or both of the new stem rust races," said Pretorius.
The good news is that the disease can be prevented by using fungicides. In South Africa, wheat is grown by commercial farmers who are aware of the threat, but small-scale farmers in developing countries, who often do not have access to the fungicides or information about the disease, are most susceptible.
South Africa grows about two million tonnes of its annual wheat requirement of between 2.8 million tonnes and three million tonnes, most of which is used to produce bread. "An outbreak could have serious consequences, as we already depend on imports," Pretorius said. "The bigger threat is that of the spores spreading to neighbouring countries and continents, where small-scale farmers with limited resources grow wheat."
The safest option is to keep on developing varieties that are resistant to stem rust. CIMMYT's Singh explained that the centre had developed several minor rust-resistant genes, which were pooled together to counter the infection, giving them an edge over single rust-resistant genes. "Several minor rust-resistant genes makes it more difficult for the fungus to attack and break down [the pooled genes]."
Singh said efforts were also being directed toward producing high-yielding rust-resistant varieties of wheat to motivate farmers to plant them. "Many farmers in developing countries are not aware of, or reluctant to use, the rust-resistant varieties."