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June 13, 2010

Genetic mutations increase global risk of deadly wheat stem rust

by Joe DeCapua

Scientists are warning of a new threat to the world’s wheat crops. They say there are four new genetic mutations of Ug99, a strain of a wheat pathogen known as stem rust.

The mutations of the reddish-brown fungus called races are overcoming existing genetic resistance that was developed to safeguard wheat crops. Stem rust can spread very quickly by the release of spores.

Wheat experts from around the world met recently in St. Petersburg, Russia. One of them is Dr. Hans-Joachim Braun, director of the Global Wheat Program at the Mexican-based International Maize and Wheat Improvement Center.

“The concern,” he says, “is that stem rust is historically the most important disease of wheat. And that’s why it got a lot of attention some 50 years ago when breeders successfully developed stem rust resistant varieties,” he says.

Then, in 1999, Ug99 was discovered in Uganda and later in Kenya, Ethiopia, Sudan, Yemen and Iran. It’s blamed for an 80 percent loss of Kenyan Farmers wheat during several seasons.

“We didn’t have a stem rust epidemic for nearly 50 years, from 1953 until the late 90s, when a new stem rust race was identified in Uganda. And it turned out that this new stem rust race was virulent on more than 90 percent of all currently commercially grown cultivars,” he says.

A cultivar is a plant deliberately selected for its characteristics, such as color, crop yield or disease resistance.

Scientists knew that if Ug99 spread, it could have devastating effects on grain baskets around the world.

Efforts to breed disease resistant wheat were very successful, but that success may have dulled awareness of the threat of stem rust. But Braun says scientists knew that mutations could happen.

“For 45 years we had very, very little problems. And so in most countries, breeding for stem rust resistance was de-emphasized. And when Ug99 occurred there were less than 10 stem rust specialists still working and active around the world,” he says. “It was basically complacency, but not so much on the scientists’ side. They were aware it could happen, but it was really an issue of funding.There are now very good monitoring systems in place to make sure that if there is an outbreak somewhere that we can react very fast,” Braun says.

Breeders are trying to stay a step ahead of stem rust mutations by growing more disease resistant varieties of wheat. Samples are sent to Kenya for evaluation.

“Annually, about…45,000 potentially new wheat varieties are sent to Kenya. And then the scientists go to Kenya and evaluate the material and identify the lines which are resistant,” he says.

Scientists are also looking for plant resistance genes that could be bread into varieties of wheat.

“At present, breeding for stem rust resistance is a top priority for many, many breeding programs around the world,” says Braun.

While stem rust is not harmful to humans, Braun says there could be “a major disaster” if a stem rust epidemic were to break out in a major wheat growing area of the world, such as in the Punjab region of Pakistan and India.

Scientists say the evolving pathogen may pose an even greater threat to global wheat production than the original Ug99.


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