Ug99 stem rust remains a threat despite slow migration

Ug99 is spreading slower than scientists anticipated but it is only a matter of time before it wipes out crops in some of the world’s most important wheat-growing regions.

Ug99 is a race of stem rust first discovered in Uganda in 1999. Its discovery jarred wheat breeders around the world because it is virulent to Sr31, a stem rust resistance gene that was widely used in international wheat breeding programs.

“It was one gene that provided resistance to stem rust and it was effective everywhere,” said Agriculture Canada plant pathologist Tom Fetch.

“When that went down, all of the sudden, oh boy.”

Ug99 has since spread to 13 countries and there are 13 variants of the Ug99 race of stem rust.

Egypt is the most recent country where the Ug99 race was detected in 2014. It is now found all along the east coast of Africa.

In 2006, it crossed the Red Sea to Yemen. The following year, it was discovered in Iran.

“I think a lot of us are surprised that it hasn’t moved from Iran,” said Fetch.

“It’s a pretty short hop to get on prevailing winds towards Pakistan and India where they grow lots of wheat.”

India is the world’s second largest wheat producer behind China. Pakistan ranks ninth. Both countries grow varieties that are susceptible to Ug99.

“Those countries would have potential for some pretty serious crop losses,” said Fetch.

The Food and Agriculture Organization of the United Nations says stem rust is the most damaging wheat disease. It is capable of causing yield losses of 70 percent or more.

“The disease has the capacity to turn a healthy looking crop only weeks away from harvest into nothing more than a tangle of black stems and shriveled grains at harvest,” the FAO said in a fact sheet on the disease.

Fetch said the most plausible explanation why Ug99 has yet to appear in India and Pakistan is that it has been dry in that region prior to this year.

To get enough inoculum to cause a problem, the disease needs dewy periods at night so the spores can germinate and infect the wheat plants.

It is only a matter of time until it makes its way to those two major wheat producing countries and when it does, the damage will be devastating because small farmers can’t afford the fungicide required to keep it at bay.

Fetch said it will eventually arrive in North America. The spores will either travel on trade winds across the Pacific Ocean or they will make the trip on the pant leg of one of the 200 million international tourists that visit the Americas each year.

The good news is that Canadian breeders have spent the last 18 years building Ug99 resistance into some wheat varieties.

AC Cadillac, a line of hard red spring wheat that received registration in 1997, has proven very resilient against the disease. Fetch has been testing the line in a field nursery in Kenya since 2005 and it is still demonstrating a high level of resistance.

AAC Tenacious is a new line of spring wheat with the same dual-gene source of stem rust resistance as AC Cadillac.

There are also other lines of wheat with intermediate resistance that would stand up to the disease under moderate infection levels.

“We have made some really good progress in Canada,” said Fetch.

But the disease keeps morphing and has overcome three or four resistance genes in recent years, so there is a constant need to keep finding new sources of resistance.

That is why Fetch is pleased that the Bill & Melinda Gates Foundation has contributed $24 million to the Delivering Genetic Gain in Wheat partnership, a collaboration of more than 2,000 scientists in 23 countries.

The funding runs through Dec. 31, 2019, and will pay for monitoring the spread of the disease and for breeding activities to develop strains of wheat that are heat tolerant and resistant to rusts and other diseases.

Fetch noted there are other virulent strains of stem rust that must be monitored as well.

For instance, thousands of acres of wheat in Sicily were attacked in 2016 by a new strain of stem rust. It was one of the biggest stem rust outbreaks in Europe in more than 50 years, according to a Reuters News Agency story.


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