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All wheat varieties will have to be replaced

Wheat stem rust devastates crops
(United States Department of Agriculture)

Winds carried ash clouds from a volcano in Chile thousands of kilometres across the Atlantic Ocean to affect flights in South Africa on 19 June, so it is possible that the spores of the variants of a deadly mutant fungus, Ug99, a wheat stem rust that surfaced in South Africa in 2009, could travel to Australia - one of the world's four main wheat exporters - in the same way.

Dave Hodson, who heads the Global Cereal Rust Monitoring System at the Food and Agriculture Organization (FAO), noted that this has happened three times before - the last time in 1973. Spores of the fungus travelled from South Africa to Australia in 1969, causing outbreaks almost four years later that destroyed hundreds of thousands of dollars worth of wheat.

The fungus, which causes rust-coloured patches on the infected parts of the plant, is spread by spores that can survive harsh winters. They germinate in warmer conditions and are usually transported by the wind - but sometimes even on clothing - over long distances and across continents.

Countries like the US are using models to study air currents and rainfall to track how the spores of the fungus might travel from Africa, where the newest outbreaks have been reported.

The fungus has begun mutating rapidly in the last few years, earning it the title of the "polio of agriculture". The new mutations or "races" of this feared disease, which can destroy entire fields of wheat, 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.

The new mutants of Ug99 have been found in 11 countries, all in Africa and the Middle East: Uganda, Kenya, Ethiopia, Sudan, Yemen, Iran, Tanzania, South Africa, Zimbabwe, Mozambique and Eritrea.

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The newest mutation, or race, of Ug99 was discovered in 2009 in South Africa. In 2007 another was found in the wheat-growing belt of South Africa's Western Cape Province. 

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.

All wheat has to be replaced

Up to 90 percent of wheat varieties in the world are susceptible to Ug99 and its variants, and all of them will eventually have to be replaced by new “super” varieties that are resistant to the deadly pathogen, said Ronnie Coffman, head of the Durable Rust Resistance in Wheat Project at Cornell University and Ravi Singh, a senior scientist in plant genetics and pathology at the Mexico-based International Maize and Wheat Improvement Centre (CIMMYT).

The scientists spoke to IRIN after a global meeting on the disease in mid-June in St Paul-Minneapolis in the US, organized by the Borlaug Global Rust Initiative.

Getting countries to replace their existing wheat varieties with new rust-resistant strains will take a lot of investment and political commitment, Coffman and Singh acknowledged.

The potent pathogen poses a bigger threat to developing countries. About half of the 600 million tons of wheat produced every year is grown in these countries, where cost generally keeps the use of fungicide low; capacity and technology to monitor the movement of the fungus are usually inadequate, and "many have not been replacing their wheat varieties with hardier ones for as long as 10 years," said Singh.

A global effort was launched to protect the world's most consumed cereal from the variants of Ug99. Five years later, in 2010, scientists said they were closer to producing super varieties of wheat that will resist the lethal fungus and improve yields by as much as 15 percent. But it will take another few years for the varieties to be tested in local conditions in various countries.

''Up to 90 percent of wheat varieties in the world are susceptible to Ug99 and its variants, and all of them will eventually have to be replaced by new “super” varieties that are resistant to the deadly pathogen''

The new super wheat varieties have several minor rust-resistant genes pooled together. It is more difficult for the fungus to attack and break down the pooled genes, giving them an edge over single rust-resistant genes.

The US Agency for International Development (USAID) has pledged some funding to help Ethiopia replace its older varieties with the new super wheat that is resistant to stem rust, said Coffman. "We need more initiatives like these."

In the meantime, fungicides are an option. "You don't have to replace all the varieties with the super variety if you use fungicide regularly," said Zak Pretorius, professor of plant pathology at the University of the Free State, in South Africa, who characterized the new variants of Ug99 discovered there.

Farmers in South Africa and rich countries such as the US and Canada can afford fungicides, unlike those in poor countries. The cost of applying fungicide pushes up production costs by about 40 percent in Kenya, where wheat is grown mostly by small-scale farmers.


With relatively poor defences against the fungus in most developing countries, the need for surveillance is critical. South Africa probably has the know-how to track the movement of the pathogen, said Pretorius, "but we have not got around to organizing it".

There are other challenges too, such as the unrest in the Middle East, which has affected monitoring and containing the disease in countries like Yemen. Rust-resistant wheat varieties distributed to Yemen recently perished in quarantine, said CIMMYT's Singh.

FAO's Hodson said he was studying wind movements and other environmental factors to track the fungus. "But we do need better surveillance systems in place in countries, for which you need funds." He noted that the number of countries contributing information to Ug99 surveillance has grown from two in 2007 to 20 today.

Donors and the private sector have stepped forward to help, said Cornell's Coffman. Denmark set up a Global Rust Reference Centre at its Aarhus University in 2008, funded by the Bill & Melinda Gates Foundation and the UK Department for International Development (DFID) to help countries and institutions across the world with research.

Yellow rust and climate change

Yellow rust, another fungal infection, is an even greater threat to wheat crops at the moment, the St Paul-Minneapolis conference heard. "It caused up to 40 percent of crop losses in Syria in 2010," said Hodson. It also caused heavy losses in Ethiopia, Tajikistan and Iran in 2010.

The rust appears to be adapting to warmer conditions and moving into areas where it has not been recorded before. Singh and Hodson said warmer winters seemed to be allowing time for the pathogen to develop.

"Our super wheat varieties are also resistant to yellow rust, so it makes a sound economic case to replace all varieties,” said Singh. “You get more for the price of one."


This article was produced by IRIN News while it was part of the United Nations Office for the Coordination of Humanitarian Affairs. Please send queries on copyright or liability to the UN. For more information: https://shop.un.org/rights-permissions

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