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A healthy wheat crop in Uganda, just weeks from harvest, turns into a tangle of black stems and shrivelled grains. As much as 80 per cent of the harvest is lost, a fate that destroys the farmer’s investment in the fields and damages the livelihood of the family.
Soon wheat fields in Kenya, Ethiopia and Egypt experience the same fate. Iran follows, along with India, Pakistan and Lebanon. Then countries in Asia and Europe show signs.
The culprit is wheat stem rust. A plant disease that has been known for decades, a virulent new strain, Ug99, emerged in 1999 to ravage wheat production across the globe — and was spread by the wind.
"Plant disease can be very cryptic and look like water stress or nutrient deficiencies. These tools enable someone to diagnose it effectively where it may have been missed before."
Stephen Parnell, spatial epidemiologist, University of Salford
Wheat stem rust is just one example of plant pests and diseases that farmers and agricultural experts across the globe are battling. They are a silent threat to food security, responsible for up to 40 per cent of global food crop losses, according to the Food and Agriculture Organization.
While crop pests and diseases can be spread by environmental factors, such as the wind, they also move into new places via global trade, traffic and transport. As the world prepares to feed its expected population of more than nine billion people by 2050, preventing plant disease outbreaks is becoming more urgent.
Indeed, this has been recognised on a global scale with the United Nation’s declaration of 2020 as the International Year of Plant Health. But experts say more research has to be done — and quickly — in order to prevent a global crop disease pandemic.
Evolutionary arms race
“There is a constant evolutionary battle between pathogens and their hosts,” Helen Fones, a plant pathologist from the University of Exeter, tells SciDev.Net. “Each continually evolves to overcome the latest strategy that the other has created to infect or resist infection.”
Fones’ latest research, published with colleagues in the journal Nature Food (8 June), points to this ‘evolutionary arms race’ to conclude that no interventions last forever. Fungi, such as wheat stem rust, pose the greatest threat, according to the researchers.
Highly adaptable and able to evolve rapidly, fungi have short generation times and can reach high population sizes in agricultural fields.
“This aspect of their biology makes them hard to predict,” says Fones. As fungal pathogens are introduced through global trade systems, they attack crops that lack immunity.
“In a new location there are new, naïve hosts lacking immunity, a release from competition and other opportunities. For this reason, the transported fungus often thrives,” she says.
Exacerbating this is a global food system that emphasises monoculture practices — large-scale production of single crop species — which increases the devastating impact of a crop disease if it becomes resistant to defences.
While regional crop diversity has increased over the past 50 years, researchers say large industrial farms in Asia, Europe and South America are all growing the same one crop species across thousands of hectares of land.
“At a global scale, the world’s agricultural regions are now starting to look more and more similar to one another than they did in the past,” Adam Martin, an ecologist from the University of Toronto, tells SciDev.Net.
As this trend continues, he says, many different parts of the world will be affected by the same pests and disease outbreaks. This will likely lead to “major and surprising disruptions”.
Martin says built-in back-ups in the global food system may buffer some of the negative impacts of crop loss. While a country might import a disease-ravaged crop from another country, it could source a nutritiously similar crop to replace it. But those buffers may only shelter wealthy countries.
“When the world has seen major shocks or disruptions to global food or economic systems, what we do know is that less-developed countries tend to bear the brunt of negative impacts,” he says.
Looking to the world food crisis of 2007-2008, in which staple food prices soared, sheds some light on those impacts: poverty, malnutrition, and economic and social unrest. During the crisis, protests and riots broke out across 48 countries, including Syria, Venezuela and 14 African countries.
Detect and treat
Smart technology, like disease mapping, could play a role in stopping a devastating plant pandemic in its tracks.
Mapping is especially helpful with early detection and treatment targeting, Stephen Parnell, a spatial epidemiologist at the University of Salford, says.
“Identifying plant disease is like a needle in a haystack problem. Maps are like a metal detector to show you where and how to target resources,” he says.
Some of these maps work through modelling that combines environmental risk factors, such as wind and rainfall, with other factors, such as crop spread and the proximity of a previously detected disease. Others rely on farmers and community members to detect and record disease signs using a smartphone app.
“Plant disease can be very cryptic and look like water stress or nutrient deficiencies. These tools enable someone to diagnose it effectively where it may have been missed before,” Parnell says.
One challenge, however, is the ability to gather these data in time. “Some plant diseases express quickly, and others can be infectious for a year before showing symptoms,” he says.
How to treat the disease is another challenge. Fungicides are an effective first-line defence, but only for so long, says Fones.
“Fungi are constantly evolving and that includes evolving resistance to new fungicides,” she says. Worryingly, she says, the same fungicides used on farms are also used in human medicine.
Azoles, for example, are the most widely-used class of plant fungicides, while being among the frontline anti-fungal drugs for humans. Research suggests this dual role is promoting azole-resistance, with one study in the Netherlands showing 100 per cent mortality among patients suffering related fungal infections.
The conflict between the use of fungicides in agricultural settings and their clinical effectiveness highlights the limitations of anti-fungal interventions, researchers say.
Still, Fones says not using fungicides is not an option. "Without fungicides we might expect to lose 30 to 50 per cent of the wheat harvest in the UK in a bad year. For crops that don't have resistance and rely only on fungicides for protection, it could be 100 per cent,” she says.
“Therefore, we really need new, agriculture-specific fungal control methods.”
Protecting the plant
Some biochemists argue that the most effective protection against a pandemic is to avoid plants getting sick in the first place.
“Modern plant science has produced more targeted, efficient tools for crop breeding, allowing us to alter the genomes of crops,” Diana Horvath, president of the 2Blades Foundation, tells SciDev.Net.
“Now we can extend a plant’s strong, existing resistance by enabling it to detect new pathogens.”
2Blades is a non-profit supporting genetic solutions for crop disease resistance. Horvath says its scientists can engineer seeds with gene stacks, where multiple resistance genes and modes of action make it much harder for a pathogen to infect the plant.
This multiple resistance approach for fighting infections is already seen in the biomedical world, she says, with AIDS ‘triple cocktail’ treatments and triple antibiotic ointments.“With resistance genes, we have shown that we can prevent most of the damage caused by major diseases of major crops, such as wheat stem rust, and protect not only the individual plant, but also its progeny – its seeds and tubers,” Horvath says.
Global agriculture’s best weapon, then, may be strengthening plant pathogen research. But Horvath says plant and agricultural research is chronically underfunded, receiving just a tenth of the investment given to biomedical research.
This may be further affected by the COVID-19 pandemic. “We’ve seen a lot of global development organisations switch priority to COVID-19. You can see why that’s important, but the plant pandemic scenario hasn’t gone away,” she says.
What a crop disease pandemic will look like, and how the world will fight it, is difficult to predict. But, the University of Toronto’s Martin says, “both science and economics suggest we’re going to find out sooner rather than later.”