July 2012
Source: CIMMYT newsletter
Ug99 slips across international borders, mowing down resistance and leaving a trail of death and destruction. Scientists work against the clock to stop this ruthless killer—sometimes known as the Red Menace—before it’s too late…
Ug99 might sound like a rogue agent in the latest thriller, but in fact it is the codename for a virulent strain of stem rust, a fungal disease that attacks wheat. Although this story has no spies or explosions, the stakes are high—stem rust can leave entire fields rotten and broken. Protecting the world from Ug99 will prevent devastating crop losses and hunger for the poor.
“Why should the global community be concerned about this disease?” asks Kenya-based CIMMYT wheat breeder Sridhar Bhavani (photo). “As predicted based on wind patterns, Ug99 spores have migrated from Africa to Iran, and it’s likely that they may reach the breadbaskets of India and China. An epidemic in such regions where susceptible wheat varieties are still grown would affect national economies.”
An international research effort underpinned by African scientists and technicians is now supplying new wheat varieties that are resistant to Ug99—both for African countries where outbreaks already occur and wheat farming regions that lie in the disease’s path. Like people who have been vaccinated against an illness, these plants stay healthy, even when Ug99 passes through.
Return of an old foe
Though unknown to many today, stem rust was once widely feared. The ancient Romans sought protection from the rust god Robigus with the sacrifice of red-colored animals. Two millennia later, between 1917 and 1935, the disease repeatedly destroyed more than 20% of the US wheat harvest. It was a clear priority for Nobel laureate and CIMMYT founding father Norman Borlaug, who dedicated much of his career to breeding high-yielding, rust-resistant wheats that spread across the world and rescued many from the brink of starvation.
By the 1970s, stem rust seemed to be extinct, but in fact it was quietly biding its time in farmers’ fields. We now know that most modern wheat varieties owed their resistance to a single gene they hold in common, called Sr31. Over the decades in East Africa the fungus that causes stem rust has evolved a way around it, resulting in Ug99. When tested, up to 80% of the world’s wheat was found to be susceptible.
The disease can travel long distances on the wind, and is now present in various Eastern and Southern Africa countries, as well as Sudan, Yemen, and Iran. As Borlaug said, “rust never sleeps”, and neither does evolution. Seven different variants belonging to the Ug99 lineage have already been identified, some of which are able to defeat important new resistance genes, Sr24 and Sr36, that scientists had hoped would buy time for the world’s wheat crops. Each mutation poses a new challenge to the formidable team of pathologists, breeders, field and laboratory technicians, and many other experts who are working together to find and deploy sources of resistance. It is a truly global effort, but the next generation of productive, healthy wheats will come, in large part, from Africa.
From problem to solutions
Kenya was the first country to be attacked by Ug99 after its emergence in Uganda, confirmed in 1999. There have been several epidemics, as most of the varieties planted in Kenya were susceptible and wheat is grown continuously throughout the year, so more mature crops can infect younger plants. Most farmers do not recognize the early symptoms of rust or know to destroy infected crops to contain spores. “Wheat farming is still a gamble for small-scale famers, as they lack inputs and knowledge of the disease and its control,” says Bhavani. “They can only hope that the crop survives and produces some grain by escaping the disease.”
However, the presence of Ug99 in Kenya has been turned into an opportunity that is benefitting the world, thanks to a close partnership between the Kenya Agricultural Research Institute (KARI), the international Borlaug Global Rust Initiative (BGRI), and CIMMYT. Each year, specialists at KARI’s Njoro field station now test as many as 50,000 wheat lines, from research organizations and national breeding programs of more than 25 countries.
The wheat lines in question are screened under artificial epidemics created by deliberately growing rows of susceptible varieties and inoculating these with the disease, so that they then produce large quantities of infectious rust spores. Teams of researchers work long days examining thousands of plants for symptoms and scoring them for resistance. With this information, wheat breeders can decide which materials to use to develop varieties that resist Ug99, and test their latest products.
Resistance in farmers’ hands
Resistant wheat varieties are now available to farmers in several countries, with more due for release in 2012. With funding from the Bill & Melinda Gates foundation, DIFD, and USAID, private and public sector seed producers—including forward-thinking farmers—in Afghanistan, Bangladesh, Ethiopia, Egypt, Nepal, and Pakistan are multiplying the new seed. Even sown on as little as 5% of the total wheat area, in an emergency the resistant wheat harvest could be used as seed, allowing complete replacement of susceptible varieties within a year should Ug99 appear. Most of these countries are set to pass the 5% mark in the 2012-2013 growing season.
Scientists are not aiming for total coverage of one or two resistant varieties, but to continue releasing new and even better-yielding varieties to enhance productivity and genetic diversity. With their modern understanding of how resistance works, breeders are working hard to make sure they do not rely on one all-or-nothing gene. Instead they are using harder-to-spot “slow-rusting” genes, which confer a type of “adult plant resistance” that cannot be easily evaluated in seedlings in the greenhouse, but must instead be evaluated in adult plants in the field. Individually, the effects of slow rusting genes are minor, slowing down the disease rather than bestowing total resistance. However, four or five such genes together can result in near immunity. Just as a combination lock with one or two numbers would not last long against a burglar, but adding more numbers makes it harder to crack, smart breeding creates plants with enough minor genes to stop rust in its tracks. Though trickier to breed for, this additive resistance is much less likely to be hit hard by new mutations of the disease—these are like the burglar finding out one number in the code, but having no idea of the rest.
Kenya is reaping the benefits of the Njoro partnership, as KARI develops high-yielding, Ug99-resistant wheats for the Kenyan market. Two such varieties—Robin and Eagle 10—have already been released, and KARI has produced and sold more than 200 tons of their seed. Another three varieties are in advanced testing for imminent release. Kenya currently produces only a third of its annual wheat requirement of 900,000 tons, with farmers reluctant to take the risk of losing their entire crop to rust. However, Bhavani is optimistic that “once resistant varieties are in farmers’ hands, they will be more confident in growing wheat.”
Cultivating capacity
It is not just seed that flows through Njoro—the station has become a hub for capacity-building. Each year, KARI staff like Bernard Otuko travel to Mexico to attend CIMMYT’s wheat improvement and pathology course. After that experience in 2009, he is now completing a master’s degree in wheat breeding and supervises trials at Njoro, where what he learned about seedling selection and disease identification has been especially important. “I gained a lot of knowledge and interaction with other breeders and field supervisors,” he says. “We exchanged information on lessons learned, what we could improve on, and what we are doing right. It’s also a good opportunity to know the sources of the materials and how our work here fits into the larger picture of how new varieties are developed.”
In 2012 Njoro will host its fourth annual stem rust training course, with experts from Kenya, CIMMYT, and around the world gathering to share knowledge with other researchers. This rust know-how is not just for scientists: the Njoro station also holds training days for farmers, covering early detection of stem rust, prevention methods, and resistant varieties available.
Together, scientists from Africa and throughout the world have risen to the challenge of Ug99. Thanks to their expertise, hard work, and unstoppable partnerships, farmers now hold in their hands a gift from Njoro: seed that will give a healthy crop no matter when the wind brings its deadly cargo. By staying one step ahead of this destructive disease and preventing further, graver epidemics, we will ensure food security and the sustainable production of wheat worldwide.
Photo: D. Hansen/University of Minnesota
Beating double trouble in Ethiopia
The title of public enemy number one is a close-run competition in the wheat-growing world. While Ug99 poses a huge potential threat, its cousin stripe rust (also known as yellow rust) currently causes the most damage. Just like stem rust, in recent years it has got tougher. New, highly-aggressive strains have emerged, able to overcome a key resistance gene, Yr27, that many widely-grown varieties rely on. It has caused major epidemics with losses of up to 40% in countries including Azerbaijan, Ethiopia, Iraq, Syria, Tajikistan, and Uzbekistan.
As in many locations, in Ethiopia, where CIMMYT works closely with the Ethiopian Institute of Agricultural Research (EIAR), resistance to one disease simply is not enough. Stem rust is the chief problem in the short-season wheat cycle (February–May), whereas in the main season (June–November) stripe rust is the biggest menace, causing a major epidemic in 2010 and serious losses in 2011.
The tide is turning. “A significant proportion of farmers is shifting to new varieties that are resistant, or moderately resistant, to yellow rust,” says CIMMYT’s Ethiopian wheat breeder Bekele Abeyo. Two CIMMYT lines were released as varieties Kakaba and Danda’a in 2011. “Danda’a is doing well and some farmers have reached yields of almost 10 tons per hectare,” he says. “There has been a very important program on seed multiplication—demand is high as farmers are interested in the new varieties. Danda’a and Kakaba together are probably now grown on about 100,000 hectares”.
Crucially, these new varieties are also resistant to Ug99 and give high yields. Around 5,000 advanced wheat lines are screened for both rusts each year on three EIAR research stations, with a special focus on durum wheat. The next challenge for breeders is Septoria tritici blotch, a different fungal disease that is causing infections in CIMMYT lines. In the race to protect the world’s wheat yields from ever-evolving disease, there’s no time to sit back and relax.
For more information on stripe rust in Ethiopia, see CIMMYT’s 2011 news story “Resistant wheats and Ethiopian farmers battle deadly fungus.”
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