This week, the Bill & Melinda Gates Foundation announced a $24 million grant to Cornell University, funding a global wheat research group, known as Delivering Genetic Gain in Wheat. The aim is to develop new lines of wheat that are both heat-tolerant and disease-resistant. David Hodson, senior scientist with the International Maize and Wheat Improvement Center (CIMMYT), based in Addis Ababa, Ethiopia, is part of the research group. He recently sat down with FERN editor-in-chief Sam Fromartz in Washington to discuss the re-emergence of stem-rust disease, a virulent fungal pathogen that attacks wheat plants and causes devastating crop losses, especially in poorer countries. Hodson specializes in pathogen surveillance and runs the website RustTracker.org.
Can you give us the landscape of stem rust now? How far has the disease spread?
We had the initial detection in Uganda in 1998-99, which is where the name Ug99 came from. This original race has now grown to 13, so essentially it’s a very closely related family of stem-rust races. We’ve been able to identify the new races that have occurred, most likely through mutations, and we’ve also been able to track the migration of these races. We’re looking at a distribution now across 13 confirmed countries. From Egypt all the way down to South Africa, every wheat-growing country in eastern and southern Africa has confirmed an Ug99 race.
Is that the wheat-growing region in Africa?
Ethiopia is the biggest producer in sub-Saharan Africa, followed by South Africa. But Egypt is a big producer and also one of the biggest importers of wheat. Wheat is everything in Egypt. And the movement of the disease into Egypt was just confirmed. Using samples from 2014, three Ug99 races were identified. That event is potentially quite significant, not only for Egyptian wheat production, but for the potential of the disease to now spread into the Mediterranean basin.
So potentially, winds can take the disease across the Mediterranean?
Yes, from Egypt the most likely path would be eastward into Israel, Lebanon, Syria, Turkey – all those areas. But if the disease gets established there, it could circulate around the Mediterranean, helped by winds or human migration. We could potentially see a much wider dispersal. We’ve got it in Africa, north to south. We’ve also got Yemen and Iran as confirmed.
Stem rust has really re-emerged as a disease of concern. We’re looking at big, important wheat-producing areas that are extremely susceptible, and the rust races we have now are much worse than in the past because they have overcome many resistance genes. And a lot of those genes were used widely throughout the world.
You said ‘re-emerged.’ Had the disease been licked?
In the first half of the 20th century it was the most destructive wheat disease in the world. And then basically largely due to resistance efforts led by Dr. Norman Borlaug (during the Green Revolution), the disease was overcome. The genes that he bred into wheat lasted 40 years. So stem rust was a huge success story. It was a forgotten disease.
Just before Ug99 appeared, there were probably less than 10 people worldwide who were working on stem rust. That knowledge was almost lost completely because it just wasn’t an issue. Then Ug99 appeared and it managed to overcome the resistance genes that had been used successfully all around the world.
And once the disease gets a toehold, it mutates?
That’s what we’re seeing now. If the races in Africa were to go into Australia, Europe, the U.S., they would have the potential to really cause a lot of damage and the concern would be that none of the wheat farmers today would have had any experience dealing with stem rust. There’s a whole generation of farmers that have never seen the disease. Even in the advanced farming areas, you could certainly see the potential to do damage.
How damaging has it been in Africa?
We’ve seen the biggest problems in East Africa. In Kenya, in 2007, we had a mutation that overcame a resistance gene called SR-24 [stem rust resistance gene No. 24]. The main variety in Kenya was protected by that single gene, so when the mutation occurred, there was an epidemic. In Ethiopia, since 2014, we saw another race of stem rust come in, which is not related to Ug99, and that took out the main wheat variety. So we had stem rust on 30,000-50,000 hectares and 100-percent losses in the hardest-hit areas. It amounts to about 2-3 percent of the wheat crop, but for the farmers who bore the brunt of it, it was a big hit. Then they got hit the following year as well, because they continued to grow the susceptible variety.
Because they saved seed?
Yes, there aren’t too many choices. So farmers in some areas lost their crop two years running. The fallback defense is fungicides, but there are a lot of issues: one is cost, another is accessibility — the farmers have to be able to get the right fungicide at the right time, which isn’t always possible. Farmers may also lack knowledge about applying fungicides.
Why was East Africa so vulnerable to the disease?
East Africa was the one place in the world where stem rust managed to hold on. The environments are so diverse, and you have continual cropping of wheat. It’s an obligate pathogen, so it needs green, living tissue to survive. In East Africa, you’ll have a wheat crop ready to harvest, and in the next field, one that’s emerging. It’s called a “green bridge.” It can jump from one crop right to the next, so you have a continual cycle that favors the pathogen.
Could the wars in Syria, or Yemen, act as a catalyst for the disease?
Well, the situation in Syria, Yemen, and Iraq is basically a black hole in terms of surveillance. And with it moving to Egypt, the most likely place it would go next is to Israel, Lebanon, and Syria. If it moves into conflict areas, we have basically zero knowledge of what’s going on.
And migration of the disease is likely?
It will most likely happen, it’s just a matter of when. That’s a message: we can’t be complacent. We’ve made a lot of progress but rust mutates so quickly. And these new variants are taking out some very important genes on a global scale. A lot of varieties around the world have suddenly become susceptible.
Is the worry that it will eventually spread into the breadbasket of South Asia?
South Asia has always been a big concern, going back to when Ug99 was detected. It hasn’t happened yet, but it’s such a big breadbasket and so many people are dependent on wheat. In India and Pakistan and other key countries, there are some 60-70 new varieties that have differing levels of resistance. We’ve seen these new varieties being released, so we’re in a better situation, but it is fluid because the pathogen is changing. And it’s probably changing faster than the breeders can keep up with it.
What impact has climate change had on the disease?
This is really the big unknown. It’s almost certainly going to influence pests and diseases, but we really don’t have a good handle on how that’s going to happen. But you can envisage new areas becoming suitable for the pathogen. If there’s warming, then areas that were previously sub-optimal for stem rust might become more hospitable. If there’s moisture with that warming, you can see new areas opening up to the disease.