South Africa: Another Strain of Deadly Wheat Fungus

Photo: David Gough/IRIN
Wheat

Johannesburg — As the world's supply of staple grains grows tight, scientists are learning about the discovery in South Africa of yet another deadly variant of Ug99 stem rust, a virulent fungal disease that can devastate wheat crops within weeks.

The world's top wheat scientists are gathering in Beijing, China, for a global symposium on the rapidly mutating disease, organized by the Borlaug Global Rust Initiative (BGRI). The discovery of the new variant brings the total number of new forms of Ug99 in South Africa to four, scientists at the forum told IRIN.

"There are two possibilities for the [surfacing of] Ug99 variants in South Africa," said Zacharias Pretorius, professor of plant pathology at the University of the Free State, in South Africa. "Firstly, migration of the fungal spores by wind movement from countries to the north of us, where we have detected similar races [mutations]. Secondly, I believe that at least one of the four variants developed locally through mutation."

Fortunately, Pretorius said, the wheat race that seems particularly susceptible to the new Ug99 variant is not very popular among consumers. Still, the emergence of this new rust variant is an indication of how virulent the fungus remains.

It also places the wheat fields in Australia, one of the world's major producers, under threat. Dave Hodson, a scientist with the Mexico-based International Maize and Wheat Improvement Centre (CIMMYT), points out that stem rust spores have travelled from South Africa to Australia three times before - the last time in 1973.

When spores of the fungus travelled from South Africa to Australia in 1969, it caused outbreaks that destroyed hundreds of thousands of dollars of wheat.

The polio of agriculture

Rising prices of staple grains have added momentum to efforts to control such outbreaks [of stem rust].

Wheat stem rust, also known as wheat black rust, is a windborne disease that can destroy entire fields of crops. The pathogen enters the stems of the plant and destroys its vascular tissue. There are three types of rust that can harm wheat, but stem rust is the most feared, according to the BGRI.

There have been several epidemics in East Africa, especially in Kenya, which was the first country to be attacked by the original strain of Ug99 after it was discovered in Uganda in 1999. Most farmers do not recognize the early symptoms of rust or know to destroy infected crops to eliminate spores, according CIMMYT. Epidemics in Kenya in 2007 led to 15-30 percent losses of the wheat crop.

The fungus has begun mutating rapidly over the last few years, earning it the epithet "the polio of agriculture". The new mutations, or "races", of this disease have acquired the ability to defeat two of the most important stem-rust-resistant genes, which are widely used in the world's wheat breeding programmes.

The new strain has also been reported in Zimbabwe.

"The right amount of humidity and temperature in the wheat growing areas in South Africa and Zimbabwe throughout the year have allowed the virulent strain to thrive," said Cobus le Roux, general manager of the crop division in the Agricultural Research Council in South Africa.

Researchers have also begun to re-examine the role of barberry, a woody plant commonly found in the wheat-growing areas, said Ronnie Coffman, vice chair of BGRI and principal investigator and director of the Durable Rust Resistance in Wheat Project at Cornell University.

Barberry was once eradicated in Europe and North America because it serves as a host for stem rust, allowing the rust to survive and propagate between wheat-growing seasons. "Researchers have found barberry growing in the [East African] hotspots for new strains of stem and yellow rust, are now investigating a possible link between the plant and the increased virulence of emerging rust diseases," said Coffman.

An ongoing battle

While fungicides remain an effective way to check stem rust, said Pretorius, they require well-timed applications as well as good crop coverage, especially in the lower parts of the stems of the wheat plant.

Scientists have been developing disease-resistant varieties of wheat to safeguard food supplies. Ravi Singh, a scientist at CIMMYT, has been involved in developing several minor rust-resistant genes, which are pooled together to counter the infection, giving them an edge over single rust-resistant genes.

The best way to overcome the disease is to replace vulnerable wheat varieties with some of the 20 new stem-rust-resistant varieties available. But getting the new seeds to farmers has been a problem.

"We covered 10 to 15 percent of the wheat crops this year, but access to all the farmers is a problem," said Bedada Girma, leader of the Stem Rust Task Force in the Ethiopian Institute of Agricultural Research.

In 2010, Ethiopia's wheat crops were affected by yellow rust, another fungal infection. But rural areas in Ethiopia do not have the agricultural supply shops needed to distribute new seeds. Most famers in many African countries buy their seeds from other farmers.

"We need to invest in improving our infrastructure and establish distribution centres in villages," Bedada said.

Rising prices add urgency

Rising prices of staple grains have added momentum to efforts to control such outbreaks.

The price of wheat has not climbed down since the 2007-2008 food price spike, Bedada added.

But BGRI is optimistic. "Even if [the resistant seeds are] sown on as little as five percent of the total wheat area, the resistant wheat harvest could be used as seed, allowing complete replacement of susceptible varieties within a year should Ug99 appear. Most countries are set to pass the five percent mark in the 2012-2013 growing season," said BGRI in a statement.

CIMMYT says scientists are not aiming for total coverage with just one or two resistant varieties. They plan to continue releasing new and even better-yielding varieties for productivity and genetic diversity.

But in the meantime, the "vast wheat-growing region that stretches across North Africa and Central Asia all the way to the gateway to China - the world's largest wheat-growing nation -is still vulnerable to the fungus," said Coffman.

To counter possible rust outbreaks, scientists announced the launch of a global rust tracker, which can monitor 42 million hectares of wheat in 27 developing countries

CIMMYT's Hodson and his colleagues in Beijing are developing "risk maps" that can assist researchers in countries in the path of virulent strains of stem rust and yellow rust. The maps will help to assess the severity of the threat and prepare a response. Developers hope to add data on wind speed and direction into the programme at a later stage, turning it into an early warning tool for the fungus, Hodson told IRIN.

[This report does not necessarily reflect the views of the United Nations]

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