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Devastating New Stem Rust Advances Beyond Africa

“It’s probably not a matter of if but when.”

–TOM FETCH, AAFC

“Behold, seven heads of grain came up on one stalk, healthy and good. And behold, seven heads of grain, thin and blasted with the east wind, sprang up after them. And the thin heads of grain swallowed up the seven healthy and full ears.” Genesis 41:5-7

Scholars believe the Bible story describes stem rust emerging from ancient Africa to destroy wheat crops and spread famine throughout the known world.

Now, 3,500 years later, Africa may be poised to do it again.

A new strain of stem rust called Ug99 is spreading through Africa and advancing on southern Asia. Scientists fear it’s just a matter of time before the plant disease reaches the wheat fields of Pakistan and India, where over 15 per cent of the world’s wheat is grown. Beyond that, potentially, the wheat fields of the world lie in its path.

What’s particularly worrisome is that much of the world’s wheat has little or no resistance to Ug99, making it a major threat to the global food system.

“This is the most virulent stem rust that has been detected since scientific plant pathology emerged 90 years ago,” said Richard Ward, senior associate director with the Durable Rust Resistance in Wheat project at Cornell University.

“Basically, our best estimate is that 90 per cent of the (world’s) wheat plants have no inherent resistance. So it is biblical.”

ON THE MOVE

Discovered in Uganda in 1999 (hence its name), Ug99 has been confirmed in five countries: Ethiopia, Kenya, Yeman, Sudan and Iran, said Ward.

The threat of the disease travelling further afield is “absolutely real,” he said from his office in Ithaca, New York.

While not yet a global pandemic, Ug99 appears headed for southern Asia, where wheat is a staple crop for 1.4 billion people, most of them very poor.

Wind models employed by monitoring systems show the rust strain could soon enter central Asia, Afghanistan and the Caucasus.

At that rate, Ug99 will eventually reach China, the world’s largest wheat producer. Then beyond, perhaps borne on trade winds to North America. Or even on the bodies of travellers, which is how the SARS epidemic jumped from Hong Kong to Canada six years ago.

“It’s probably not a matter of if but when,” said Tom Fetch, an Agriculture and Agri-Food Canada cereal stem rust research scientist in Winnipeg. “We’re in a race against time to get something out before it gets here.”

Fetch and Ward attended the Borlaug Global Rust Initiative 2009 Technical Workshop in Ciudad Obregon, Mexico March 17-19. The workshop is named after Norman Borlaug, a U. S. plant scientist who won the Nobel Peace Prize in 1970 for launching the Green Revolution. The 95-year-old Borlaug gave the keynote address.

PROGRESS

The workshop heard that scientists have made progress over the last four years in developing resistant varieties. Wheat lines combining resistance with quality characteristics are under multiplication in seven countries, Ward said.

But plant breeding is a slow, painstaking process which involves selecting for a desired trait, incorporating it into a plant and then “back-crossing” to get the proper agronomic characteristics for a commercial variety.

All the while, Ug99 is trying to stay one jump ahead of the process.

Researchers in Kenya were recently alarmed to learn that Ug99 had mutated to overcome two major stem rust resistance genes called Sr24 and Sr36 which had previously been effective against the original form of the disease.

There are about half a dozen known resistant genes which can still be used, said Fetch. But the fear is that a mutating Ug99 could pick them off one by one, as it did with Sr24 and Sr36.

That fear is heightened by the presence in central Asia of a shrub called the barberry. It’s a host plant which allows rust to reproduce on it in a sexual combination that can lead to diversity or mutation. Without the barberry, propagation is strictly asexual.

An eradication program in North America during 1917-18 helped reduce the evolution of rust pathogens by eliminating the barberry host.

But the barberry is so widespread in central Asia that eradicating it is virtually impossible. Its presence makes Ug99’s further mutation probable, said Ward.

RESISTANT VARIETIES

Here in Canada, two Hard Red Spring wheat varieties – Peace and AC Cadillac – have resistance to Ug99. But they make up only a small percentage of the wheat grown on the Prairies.

What’s needed to develop a resistant variety from scratch is to incorporate two or more of the still-resistant genes – a process called gene stacking – to ensure long-term resistance. That takes even longer than regular breeding, said Fetch.

Even if Ug99 does arrive in North America, wheat growers can combat it with fungicides, which are usually effective on rust. But that option isn’t open to farmers in less developed countries, where cost and availability often make such products inaccessible. Even if fungicides were available, farmers in those countries may be unaccustomed to them. Improperly using these chemicals could result in human health disasters, warned Ward.

This leaves resistant varieties as the best option and puts researchers in a race to develop them before Ug99 goes global.

Ward gives scientists five to seven years “if we’re lucky” to replace existing wheat varieties with resistant ones.

Fortunately, there’s still time, he said.

“It’s not a meteorite about to hit the planet,” said Ward. “This time we can see Katrina offshore and we’re going to react as fast as we can.”

More information is available at www.globalrust.org.[email protected]

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