Fighting Back Against Ug99 Wheat Stem Rust

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Wheat is the most commonly grown crop around the world, with more than 760 million tonnes produced each year. So, what happens when a virulent new strain of wheat stem rust threatens global crops?

Food crops are vulnerable to an array of ever-evolving pathogens. And once in a while, a new strain pops up that threatens the global food supply.

 

Ug99 wheat stem rust is one such strain. First identified in Uganda in 1998–1999, during International Maize and Wheat Improvement Center (CIMMYT) germplasm evaluations, this highly virulent strain of wheat stem rust set off alarm bells around the world. After more than 30 years of effective genetic resistance to stem rust, a new strain was discovered capable of infecting more than 80 percent of the available wheat varieties.

 

“When we began to survey how far the disease had spread within Africa, we were shocked to discover that it wasn’t only present in Uganda,” says Dr Sridhar Bhavani, head of rust pathology and molecular genetics at CIMMYT’s headquarters in Mexico. “It had already spread into multiple countries within East Africa, including Kenya, Ethiopia, Tanzania, and Rwanda.”

 

Since then, Ug99 has been detected in more than 14 different countries, spreading from East Africa and through the Middle East to Iraq in 2019. Seventeen variants of Ug99 wheat stem rust have been detected so far, which adds more complexity to the existing challenge.

Fighting Back Against Ug99 - Outlook
Fighting Back Against Ug99 - Outlook

Food crops are vulnerable to an array of ever-evolving pathogens. And once in a while, a new strain pops up that threatens the global food supply.

 

Ug99 wheat stem rust is one such strain. First identified in Uganda in 1998–1999, during International Maize and Wheat Improvement Center (CIMMYT) germplasm evaluations, this highly virulent strain of wheat stem rust set off alarm bells around the world. After more than 30 years of effective genetic resistance to stem rust, a new strain was discovered capable of infecting more than 80 percent of the available wheat varieties.

 

“When we began to survey how far the disease had spread within Africa, we were shocked to discover that it wasn’t only present in Uganda,” says Dr Sridhar Bhavani, head of rust pathology and molecular genetics at CIMMYT’s headquarters in Mexico. “It had already spread into multiple countries within East Africa, including Kenya, Ethiopia, Tanzania, and Rwanda.”

 

Since then, Ug99 has been detected in more than 14 different countries, spreading from East Africa and through the Middle East to Iraq in 2019. Seventeen variants of Ug99 wheat stem rust have been detected so far, which adds more complexity to the existing challenge.

“The disease produces billions of very minute spores, known as urediniospores,” says Dr Bhavani. “And because they are so light, these spores can easily be carried by strong wind currents to susceptible geographies thousands of kilometers away.”

 

This put many of the world’s largest wheat-growing regions at risk, as the pathogen’s wind migration pathways pointed towards Asia’s most powerful wheat producers. “At that point, we were not prepared for the consequences of the disease’s migration,” says Dr Bhavani. “Just 10 percent susceptibility of the global wheat area to wheat stem rust could have led to billions of dollars of losses—not to mention hunger and poverty in those economies that were dependent on wheat and wheat-based farming systems where devastating epidemics could have occurred, as the majority of the varieties were susceptible.”

 

And it wasn’t just a matter of migration, it was also the degree of devastation. While leaf rust and yellow rust incur yield losses of 60–70 percent at most, a severe invasion of wheat stem rust can destroy an entire crop, leaving farmers with nothing at all to harvest.

 

What can be done?

With a villain as virulent as Ug99, there are many ways to fight back—and for this pathogen in particular, it takes every available tool to gain the upper hand.

 

Fungicides are the simplest, most obvious solution—and they are effective against rust diseases—but they often aren’t always an option in the regions that need them most. “In the majority of the developing countries that grow wheat, small-scale farmers cannot afford to use fungicides—especially in most parts of Africa,” explains Dr Bhavani. “These farmers would be the biggest losers to this disease if their varieties were susceptible.”

 

Even in developed countries, farmers can’t rely solely on fungicides, as the pathogen develops a natural resistance over time.

 

Dr Bhavani notes that, “Europe is also battling a huge challenge with yellow rust, for example, where multiple applications of fungicides are being advocated, but it is still unable to control rust completely.”

 

According to Dr Bhavani, the long-term solution for diseases such as Ug99 wheat stem rust lies in introducing genetics less susceptible to the pathogen—and then providing the world with new, more resilient wheat varieties that continue to grow and thrive.

 

The first step involves identifying genes that can provide resistance to Ug99. “When Ug99 was first detected, we had over 50 genes that were characterized to be resistant for stem rust—but this unique strain was able to overcome more than 50 percent of the genes that were catalogued at the time. In addition to that, Ug99 was able to overcome multiple resistance genes, so the pathogen was mutating and evolving virulence for whatever gene we were trying to deploy.”

Fighting Back Against Ug99 - Outlook
Fighting Back Against Ug99 - Outlook

“The disease produces billions of very minute spores, known as urediniospores,” says Dr Bhavani. “And because they are so light, these spores can easily be carried by strong wind currents to susceptible geographies thousands of kilometers away.”

 

This put many of the world’s largest wheat-growing regions at risk, as the pathogen’s wind migration pathways pointed towards Asia’s most powerful wheat producers. “At that point, we were not prepared for the consequences of the disease’s migration,” says Dr Bhavani. “Just 10 percent susceptibility of the global wheat area to wheat stem rust could have led to billions of dollars of losses—not to mention hunger and poverty in those economies that were dependent on wheat and wheat-based farming systems where devastating epidemics could have occurred, as the majority of the varieties were susceptible.”

 

And it wasn’t just a matter of migration, it was also the degree of devastation. While leaf rust and yellow rust incur yield losses of 60–70 percent at most, a severe invasion of wheat stem rust can destroy an entire crop, leaving farmers with nothing at all to harvest.

 

What can be done?

With a villain as virulent as Ug99, there are many ways to fight back—and for this pathogen in particular, it takes every available tool to gain the upper hand.

 

Fungicides are the simplest, most obvious solution—and they are effective against rust diseases—but they often aren’t always an option in the regions that need them most. “In the majority of the developing countries that grow wheat, small-scale farmers cannot afford to use fungicides—especially in most parts of Africa,” explains Dr Bhavani. “These farmers would be the biggest losers to this disease if their varieties were susceptible.”

 

Even in developed countries, farmers can’t rely solely on fungicides, as the pathogen develops a natural resistance over time.

 

Dr Bhavani notes that, “Europe is also battling a huge challenge with yellow rust, for example, where multiple applications of fungicides are being advocated, but it is still unable to control rust completely.”

 

According to Dr Bhavani, the long-term solution for diseases such as Ug99 wheat stem rust lies in introducing genetics less susceptible to the pathogen—and then providing the world with new, more resilient wheat varieties that continue to grow and thrive.

 

The first step involves identifying genes that can provide resistance to Ug99. “When Ug99 was first detected, we had over 50 genes that were characterized to be resistant for stem rust—but this unique strain was able to overcome more than 50 percent of the genes that were catalogued at the time. In addition to that, Ug99 was able to overcome multiple resistance genes, so the pathogen was mutating and evolving virulence for whatever gene we were trying to deploy.”

The result? A series of boom-and-bust cycles, where the newly deployed wheat varieties provided several years of resistance and relief before the pathogen evolved to threaten the wheat once again.

 

Longer-term success lies in combining multiple resistance genes within a single wheat variety. “When you incorporate a combination of genes—known as complex adult plant resistance (APR) genes—into a variety, you can enhance its resilience,” says Dr Bhavani. “Even if one of the genes gets broken down, the other genes will still confer effective resistance.”

 

With this in mind, CIMMYT has released more than 200 new resistant wheat varieties over the past decade. By acting swiftly and deploying more-resilient varieties at the source of the disease—East Africa—they were able to reduce the inoculum load and prevent the spread of the devastating pathogen.

 

“We released quite a few varieties that carry complex APRs—and they are still holding after 10 years,” says Dr Bhavani, “suggesting that this approach is the way forward when it comes to building complex resistance and enhancing a variety’s sustainability.”

 

What’s to come?

Right now, it seems like science has solved the problem, but this pathogen is not done with us yet. Ug99 wheat stem rust will continue to evolve, and ongoing research is required if we want to stay ahead of the game.

 

“Developing and producing a new variety can take six to eight years,” says Dr Bhavani. “After that, it can take just as long—if not longer—to deploy a released variety into farmers’ fields. And in that time, the pathogen is not sitting idle. It’s trying to mutate—and we have had experiences where a variety gets defeated before it even reaches the farmers.”

Fighting Back Against Ug99 - Outlook
Fighting Back Against Ug99 - Outlook

The result? A series of boom-and-bust cycles, where the newly deployed wheat varieties provided several years of resistance and relief before the pathogen evolved to threaten the wheat once again.

 

Longer-term success lies in combining multiple resistance genes within a single wheat variety. “When you incorporate a combination of genes—known as complex adult plant resistance (APR) genes—into a variety, you can enhance its resilience,” says Dr Bhavani. “Even if one of the genes gets broken down, the other genes will still confer effective resistance.”

 

With this in mind, CIMMYT has released more than 200 new resistant wheat varieties over the past decade. By acting swiftly and deploying more-resilient varieties at the source of the disease—East Africa—they were able to reduce the inoculum load and prevent the spread of the devastating pathogen.

 

“We released quite a few varieties that carry complex APRs—and they are still holding after 10 years,” says Dr Bhavani, “suggesting that this approach is the way forward when it comes to building complex resistance and enhancing a variety’s sustainability.”

 

What’s to come?

Right now, it seems like science has solved the problem, but this pathogen is not done with us yet. Ug99 wheat stem rust will continue to evolve, and ongoing research is required if we want to stay ahead of the game.

 

“Developing and producing a new variety can take six to eight years,” says Dr Bhavani. “After that, it can take just as long—if not longer—to deploy a released variety into farmers’ fields. And in that time, the pathogen is not sitting idle. It’s trying to mutate—and we have had experiences where a variety gets defeated before it even reaches the farmers.”

Hope is not lost. New solutions are always on the horizon, and it’s only a matter of time before gene editing tools and technologies such as CRISPR will enhance our ability to fight back. There is also much to be done in terms of informing farmers about the importance of this disease. Teaching them how to scout for it early on, adopt resistant varieties, and ensure that they know how to protect those varieties and prevent the spread of Ug99 wheat stem rust is critical.

 

“All of these solutions go hand in hand,” concludes Dr. Bhavani. “Surveillance is needed to understand the pathogen virulence diversity and spread. Gene discovery is needed to enhance breeders’ toolboxes with diverse genes for rust resistance. And then a deployment strategy is needed to ensure that farmers have everything they need to keep the disease at bay.”

Fighting Back Against Ug99 - Outlook
Fighting Back Against Ug99 - Outlook

Hope is not lost. New solutions are always on the horizon, and it’s only a matter of time before gene editing tools and technologies such as CRISPR will enhance our ability to fight back. There is also much to be done in terms of informing farmers about the importance of this disease. Teaching them how to scout for it early on, adopt resistant varieties, and ensure that they know how to protect those varieties and prevent the spread of Ug99 wheat stem rust is critical.

 

“All of these solutions go hand in hand,” concludes Dr. Bhavani. “Surveillance is needed to understand the pathogen virulence diversity and spread. Gene discovery is needed to enhance breeders’ toolboxes with diverse genes for rust resistance. And then a deployment strategy is needed to ensure that farmers have everything they need to keep the disease at bay.”

Sources:

  • Background interview: André Du Toit
  • Expert interview: Dr. Sridhar Bhavani
  • Fetch et al., 2021, Stem rust: Its history in Kenya and research to combat a global wheat threat, Canadian Journal of Plant Pathology