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Global Wheat Crop Threatened by Fungus: A Q&A with Han Joachim Braun

A new strain of a devastating fungus could impact wheat crops the world over--and scientists are scrambling to nip it in the bud
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©ISTOCKPHOTO.COM/TOMAS BERCIC

In 1999 agricultural researchers discovered in Uganda a new variety of stem rust—a fungus that infects wheat plants and wiped out 40 percent of U.S. wheat harvests in the 1950s. Millions of spores have spread from Uganda to neighboring Kenya and beyond to Ethiopia, Sudan and Yemen, wiping out as much as 80 percent of a country's harvest. In fact, the only thing that has stopped the rust from devastating the breadbaskets of China, India and Ukraine has been several years of drought in Iran.

The world should hope for three more dry years in that region, because that's how long it will take to breed enough seeds of wheat strains that are resistant to the fungus, according to Hans Joachim Braun, director of the global wheat program at leading agricultural research institute, the International Maize and Wheat Improvement Center (CIMMYT) in Mexico. An international symposium on the agricultural threat was held this week in Ciudad Obregon, Mexico, and ScientificAmerican.com spoke with Braun, who attended, to glean the latest developments on efforts to defeat the fungus.

[An edited transcript of the interview follows.]

What is the problem we are facing?
Ten years ago we identified a new stem rust race in Uganda—that's why it's called Ug99. More than 90 percent of the world's wheat varieties are susceptible to it. Clearly, this represents a major threat to production because, historically, stem rust was the most important wheat disease.

In the late 1950s stem rust was the first disease for which agricultural scientists developed resistant wheat strains. Resistance was so good that for 50 years, we didn't worry. Norman Borlaug [1970 Nobel Peace Prize–winner and developer of resistant wheat] saw the susceptibility to Ug99 and he rang the alert bell. The Global Rust Initiative was established then to fight stem rust on a global level.

Some 300 to 350 people involved in wheat breeding, and particularly rust resistance, gathered this week [at the international symposium] to discuss the latest progress in developing varieties resistant to stem rust.

How big is the problem?
Stem rust has been confirmed in Uganda, Kenya, Ethiopia, Yemen, Sudan and Iran. Historically, central and eastern Africa is a big center for new rust races. We know that it spreads very fast from East Africa to Asia, southern Africa, even Australia.

From our monitoring system, the rust from eastern Africa is already moving into Asia. It is already established in Iran. Then it can travel via Afghanistan to Pakistan into India and then China. That happened before in 1986.

Back then, a yellow rust race was discovered in East Africa. Within six years, it moved from there to India and caused more than a billion dollars in wheat losses, mainly in Turkey, Syria, Iran and Pakistan.

We have what is called a Rustmapper, which continuously monitors wind movements. At any time, we can look up where there is an outbreak of rust disease and check the direction in which the wind is blowing. That allows us to predict where to look for rust movement.

Based on wind direction, we've seen spores move in 72 hours from Yemen via Pakistan to India and up to China. At any point, if it rains and the spores come down, we could have a new epidemic.

Remember, there are billions of spores produced once a susceptible variety is infected, so it multiplies very fast in the right environment. They go into the air, are carried by the wind and, in a short period, infect large areas. Last year, we were lucky that Iran had a very severe drought so the rust would not be multiplying. I am sorry for Iranian farmers but it really protected the world from Ug99.

How was the yellow rust stopped?
We identified new sources of resistance and replaced susceptible varieties with new varieties. In the case of Ug99, this process is slower because resistance can only be tested in Kenya and Ethiopia.

Every wheat breeder who knows that stem rust could be a problem is sending breed lines to Kenya and Ethiopia for screening. Some 20,000 to 30,000 lines have been tested for response to this rust. The world really has to thank these two countries for making their resources available. Otherwise, we could not screen for resistance to this disease and the world would just have to wait for this disease to arrive.

What  has the testing found so far?

We have found wheat that is resistant to the fungus. This [symposium] brings rust researchers together so they can exchange what new genes are available to fight the infection. Most of the rust-resistant genes are not effective anymore.

Generally, rust resistance is based on one gene. Either the plant is resistant or susceptible. A spore lands on the surface of the leaf. The spore germinates. The plant responds [if it's resistant]. The cells that the spore tries to open kill themselves and the spore cannot grow and also dies. This is a typical resistance reaction.

But it only takes one mutation on the rust side, and then it can overcome this resistance. Such resistance genes typically last for four or five years. Then nature produces a mutant that can overcome this resistance.

CIMMYT has developed another approach where we try to bring together four or five minor genes. None of these individual genes provide total resistance or immunity but each reduces the infection by 15 to 20 percent. If you bring four or five genes like that together, you can bring the level of resistance very high. Then a mutation that overcomes one gene doesn't cause as much of a problem.

That worked with leaf rust. We are now producing lines for stem rust that combine such minor genes. We have identified such lines in Kenya, and USAID [U.S. Agency for International Development] has provided funds for seed production in countries such as Ethiopia, Egypt, Afghanistan, Pakistan and India. The multiplication of resistant varieties is well underway.

How vulnerable are countries like Australia, Canada, the U.S. and Ukraine, which provide the bulk of the world's wheat?
They are as vulnerable as the rest of the world. Seventy-five percent of the varieties in the U.S. are susceptible. Historically, stem rust was the major disease in Australia, U.S., Canada and Ukraine. Their [temperate] environments allow the rust to move fast.

Imagine a tourist in a wheat field in Kenya. Millions of spores cling to his trousers and shoes. He doesn't wash them and goes into a wheat field that is ready to be infected. You only need a few spores.

The most dangerous transmission is through people. A very virulent yellow rust was introduced to Australia by a farmer who visited Europe.

Will farmers plant these new strains?

We must provide farmers with varieties that are better than what they currently grow. Farmers haven't seen stem rust for 50 years so they will just ignore [the threat]. We have to have strains with 10 percent higher yield, otherwise they won't change.

Will these new strains offer benefits for other problems, like drought?

We cannot develop a cultivar only for one specific trait. They have to have a package. That package includes drought tolerance, yield, ability to withstand nutrient deficiency, and resistance to a wide spectrum of disease.

I am concerned about investment in wheat research. The private sector has limited interest in investing in wheat. Most wheat research is done by public institutions. But we could have similar progress in wheat like genetically modified cotton, canola and soy. Five or six big international companies invest more than a billion dollars each year in maize research. That's twice as much as the budget of all [public sector] international agricultural research centers.

Transgenic wheat [which incorporates modified or imported genes] would be interesting. But we're not allowed to use it. No country has released a genetically modified wheat. If we could use genetic modification, that would be a new road to address production constraints in wheat.

Will the new cultivars be susceptible to some new form of rust?
If we have only major genes the [new strains] will not last very long, that's why [CIMMYT] is pushing this minor-gene, or durable-gene, resistance concept. Bring together four or five genes [where] each has a smaller effect. [Combine] all five [of these] genes, each of which provides 20 percent resistance, and you may have zero infection. This resistance will last much longer than [that] based on only one gene.

Have farmers started planting the new strains?
We did an emergency multiplication last summertime. We made 3.5 tons of a total of 12 resistant lines. This seed has been sent to the six countries I mentioned earlier, plus Turkey. The multiplication is going well. The lines will produce all together several hundred tons of seed in these countries. Larger areas will be planted this and next fall, then the year after that [the seed will] also be given to farmers. What we really need, what we really pray for, is another three to four years where the environment is not conducive to a stem rust epidemic. Then we would have sufficient varieties out there to avoid disaster.

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