Seed World

Reacting to Rust Germination March 2012



Reacting to Rust

A fungus under control for 50 years is back and ravaging wheat crops in Africa, the Middle East and Asia—and it could be headed this way.

The disease is called wheat steam rust—once it infects a field, it corrodes the stalks, turning them shades of brown and red that give the disease its name. Farmers can do little but harvest what’s left, sometimes losing 60 per cent of their crop.

The disease has been in remission since the 1960s, but in 1999 a new strain known as Ug99 appeared in Uganda and has now spread across 12 countries, including South Africa, Yemen, Iran, India, Bangladesh and Nepal. Scientists believe that if it reaches North America, it could start in Southern Texas and go all the way to the Canadian plains. In fact, crop scientists fear that Ug99 stem rust could wipe out more than 90 per cent of worldwide wheat crops—with statistics like that it is clear that developing new varieties with increased disease resistance is a top priority. As a result, the race is on to develop new wheat varieties that are resistant to Ug99 before it reaches North America.

Scientists have identified a mere half-dozen genes that are immediately useful for protecting wheat from Ug99. Incorporating them into crops using conventional breeding techniques is a nine to 12 year process that has only just begun. And that process will have to be repeated for each of the thousands of wheat varieties in the marketplace today.

“All the seed needs to change in the next few years,” said Ronnie Coffman, a plant breeder who heads the Durable Rust Resistance in Wheat project, in a New York Times article. “It’s really an enormous undertaking.”

Working Together

The challenge has sparked a unique, co-operative global response named the Borlaug Global Rust Initiative and established by the International Maize and Wheat Improvement Center, the International Center for Agricultural Research in the Dry Areas, the Food and Agriculture Organization of the United Nations, the United States Department of Agriculture’s Agricultural Research Service and Cornell University.

In addition, earlier this year the Bill and Melinda Gates Foundation, together with the Department for International Development in the United Kingdom, awarded more money—US $40 million—to Cornell University to continue combating this ancient disease by developing wheat varieties that are resistant to Ug99 and new, emerging strains of stem rust disease.

Since 2008, when the Durable Rust Resistance in Wheat project at Cornell was first funded by the foundation, researchers have distributed new stem rust resistant wheat varieties for testing and evaluation to more than 125 sites in 40 countries. They have strengthened rust screening nurseries in Kenya and Ethiopia and distributed nearly five tons of Ug99-resistant seed for planting in seven countries that are at high risk for food insecurity, including Ethiopia, Kenya, Egypt, Pakistan, Afghanistan, Bangladesh and Nepal.

Canada’s Role

In Canada, scientists are also conducting research to combat Ug99. In 2009, Agriculture and Agri-Food Canada invested $13 million into a rust breeding initiative across the country. Tom Fetch, plant pathologist at Agriculture and Agri-Food Canada’s Winnipeg research centre, is the program’s lead, and he says Canadian wheat growers are better off than their counterparts in most parts of the world.

Research thus far has turned up only a handful of promising resistance genes, but two lines in Canada have shown good resistance: AC Cadillac and Peace. The lines are being used in current Canadian breeding programs, and F4 lines are being screened in the rust plots in Kenya through a partnership between the International Maize and Wheat Improvement Center and the Kenya Agricultural Research Institute, which allows wheat and barley lines from breeders around the world to be screened for Ug99 resistance.

“After six years of testing in Kenya, these lines remain highly resistant and we have now found out why,” says Fetch. The varieties both have one “major” stem rust gene in combination with the LR34 gene, a combination containing high resistance. “Now that we’ve figured out the genetics, these two genes are a good starting point for plant breeders,” says Fetch. “Two genes are ideal. What happened with Ug99 was that a lot of wheat germplasm in developing countries relied on just one gene—SR31.”

However, Fetch says Canadian breeders never used the SR31 gene due to quality issues. In fact, it has now been discovered that the presence of the major stem rust gene in AC Cadillac and Peace, that give both varieties high resistance to Ug99, are actually linked closely to a gene for bunt disease resistance and were likely bred into the varieties by chance due to the bunt resistance gene. “It’s actually an amazing breeding story,” says Fetch. “We’re just lucky to have brought this major stem rust gene along into Canadian varieties and have now found out by chance that it is resistant to Ug99.”

Fetch says the goal now is to find other genes that are also resistant. Last winter in Kenya, his team screened some lines containing the SR22 and SR26 genes found in older Canadian germplasm and found lines with either the 22 or 26 genes, which are effective against Ug99 as well. “The goal is to use those in combination with AC Cadillac to create a multi-gene stack of resistance,” he explains.

Fetch’s team in Winnipeg is also working on finding new genes by looking at wild species and moving them into common wheat varieties.

The AAFC program is comprised of 18 scientists across the country, including two scientists in Alberta. Rob Graf and Harpinder Randhawa, both working at the Lethbridge Research Centre, are looking at incorporating effective genes into their winter wheat and soft white spring wheat programs, respectively.

While the big international focus has been on wheat, barley is also susceptible to the same strains of stem rust. Fetch says that of the six genes described for stem rust resistance, only Rpg4 was resistant to Ug99. “To my knowledge, none of the barley varieties in Canada or the United States carry that resistant gene,” says Fetch. “But barley usually seems to escape most of the stem rust infections in Canada because of its early maturity, so that is somewhat encouraging in the short term.”

Is it Coming to Canada?

Ug99 was first detected in Uganda in 1999 and since then has spread to at least 12 other countries including South Africa. Tom Fetch, plant pathologist at Agriculture and Agri-Food Canada’s Winnipeg research centre, says its recent movement into South Africa is critical because now it can continue to spread by wind in one of two directions:
1) Directly to Australia across the Indian Ocean, a wind pattern that has spread other diseases in the past. “As a result, Australia is now on high alert and has been ramping up its breeding efforts,” says Fetch.
2) Across the Atlantic Ocean to Brazil. “Scientists who study wind patterns have developed a model showing how the disease might enter the Western Hemisphere by moving from South Africa to, specifically, Brazil,” says Fetch. As a result, he is in the process of setting up a monitoring system in Brazil as part of the Borlaug Global Rust Initiative’s tracking project.

The Challenge Continues

The disease shows no signs of slowing down; in fact, new strains of Ug99 have now been discovered. Fetch says seven different strains of Ug99, and possibly eight, have now been identified. “The disease is changing. We need to continue to monitor its movement to know which genes are still effective,” he says.

Leading wheat experts from Australia, Asia, Africa, Europe and the Americas, who recently gathered in St. Petersburg, Russia for a global wheat event organized by the Borlaug Global Rust Initiative, said the evolving pathogen may pose an even greater threat to global wheat production than the original Ug99.

“With the new mutations we are seeing, countries cannot afford to wait until rust ‘bites’ them,” said Ravi Singh, distinguished senior scientist in plant genetics and pathology with the Mexico-based CIMMYT. “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. Already, most of the varieties planted in the wheat fields of the world are vulnerable to the original form of Ug99. 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.”

Concern over this disease has also created a major feat—global partnership and co-operation, and the sharing of genetics in the worldwide battle to stop the disease. “The Global Rust Initiative has really brought people from all around the world together,” says Fetch, “to refocus and reenergize efforts on rust research.”

Julie McNabb