The Peanut Plague

A toxic fungus infects crops eaten across the developing world. Scientists are engineering a solution.

By Jori Lewis|Friday, November 10, 2017
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A Senegalese peanut farmer holds a handful of harvest. Researchers hope to wipe out aflatoxin, a toxic fungus that can grow on the country’s main crop.
Jacques Pavlovsky/Sygma/Getty Images

More than 10,000 years ago, somewhere in the Andean foothills between Argentina and Bolivia, two wild legume species mixed, probably with the help of some pollinating bees. Their offspring was atypical — a freak of nature that couldn’t remix with its wild ancestors and cousins. The freak plant continued to evolve, first on its own, and then by selection as farmers domesticated it for its tasty seeds that grew, not from its branches like most beans and peas, but beneath the soil. Merchants carried it throughout South America and eventually to the Caribbean islands. From there, Spanish clerics and conquistadors took the first peanuts to Europe and then on to Asia and Africa. The world learned to love the humble peanut.

The transplant came to West Africa early, but it’s only been two centuries since farmers began growing it commercially. Since then, Senegal, which is roughly the size of South Dakota, has regularly been one of the top 10 peanut producers in the world. And farmers grow it under untrustworthy rainfalls, with little fertilizer and mostly the pest control that nature provides.

When it does rain, the capital city of Dakar empties out as tailors and taxi drivers, bureaucrats and teachers head to their villages to plant.

For generations, Fatou Binetou Diop and her family have grown these South American transplants on their land in Méckhé (pronounced “may hay”). The town springs up from the dunes after a two-hour drive from Dakar. “People here say that peanuts are gold,” Diop says. “Because with peanuts, you can get a lot of things.”

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Senegalese women sort peanuts.
Seyllou/AFP/Getty Images

Méckhé owes its early growth, however moderate, to the peanut. A railroad, built in the late 19th century to collect peanuts for shipment to France, stopped in Méckhé, making it a boomtown.

Then, as now, farmers sold their harvest to middlemen, big vegetable oil companies and exporters, although today those exporters are likely to be Chinese, not French. Village residents eat peanuts, too, of course — pressed into oil, roasted and salted, candied in sugar or ground for use in a bevy of savory sauces.

But this cash crop also causes sickness. The legumes are susceptible to aflatoxins, a highly carcinogenic family of molds that grows on many crops. At high levels, the fungal poison can cause acute liver damage and death. Rare outbreaks of aflatoxin poisoning in India and Kenya have killed hundreds. And public health experts believe that as many as 500 million poor people in sub-Saharan Africa, Asia and Latin America are being slowly poisoned by long-term cumulative exposure to aflatoxins, which can stunt a child’s growth, suppress the immune system and lead to liver damage or cancer. But aflatoxins’ impact is neither uniform nor immediately visible, which makes it hard to fight.

Ndiakhate Fall, secretary-general of the farmers’ association in Méckhé, says most of his organization’s 5,000 members are skeptical of aflatoxin’s dangers.

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On the street in Méckhé, a Senegalese farming town.
Ibrahima Thiam

They’ve eaten peanuts for generations and haven’t seen anyone die from it yet.

“You know, in our country dying even at 20 or 25 years old, for us it is just destiny,” Fall says. “And we say that it was God that decided that it should be this way. We don’t ask questions.”

For decades, though, scientists have been asking questions. They’re trying to breed a peanut resistant to the deadly toxin. They have had little success, but now a new breeding initiative is trying to crack the code of the peanut’s past to help the plant face the future — toxin-free.

Toxic Combination

Two common types of fungi that live in the air and soil, Aspergillus flavus and Aspergillus parasiticus, produce aflatoxin. The contamination can develop almost anywhere heat and humidity combine.

Burrowing insects help the fungus get into the shell to infect the peanut seeds. The mold can start growing in the soil, or in the field post-harvest, or even once the nuts have been shelled and packed. But once the mold starts, there’s no turning it back. In the U.S., one moldy “hot” peanut can cause inspectors to toss a whole tractor-trailer of nuts.

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Peanuts grow underground in contact with the soil, leaving them vulnerable to aflatoxin, a fungal poison. Pictured here: Aspergillus flavus.
Renee Arias

Regulations in the U.S. and Europe that set maximum levels of aflatoxin in food keep tainted products out of the food system, even though farmers might end up throwing out portions of their harvest as a result. Experts estimate that aflatoxin-related crop losses cost American farmers about $500 million each year. In the developing world, though, even where such regulations exist, they are rarely enforced. Instead, tainted corn and peanuts permeate the local markets and regularly find their way onto the plates of the unwary.

What is mostly a crop problem in the U.S. has become a serious health problem elsewhere, especially in Africa.

“Every time we measure aflatoxin exposure in humans, in children and adults [in Africa], there’s always very high positive rates,” says Yun Yun Gong, a food toxicologist at Queen’s University Belfast. She’s tracked aflatoxin exposure around the world by looking at specific blood biomarkers in humans. In most developed countries, the rates are low to nonexistent.

But in underdeveloped countries, especially in some parts of Africa, aflatoxin exposure rates go up to more than 90 percent. “Senegal is perhaps one of the highest risk populations we have measured,” she says.

Sorting out infected peanuts just after harvest can keep aflatoxin from spreading; specific planting and farming practices can also help. And for more than four decades, researchers have been searching for the key: an aflatoxin-resistant peanut seed. They’re counting on the genes of the peanut’s wild ancestors to unlock the plant’s next evolutionary stage.

Return to the Wild

Most farmers in Senegal avoid their fields at noon, when the sun is high and the dry season temperatures soar north of 100 degrees Fahrenheit, says Daniel Foncéka, a scientist with the French Agricultural Research Center for International Development. He runs the drought adaptation program for the Senegalese Agricultural Research Agency (ISRA). But a field is exactly where he finds himself, far from shade as the wind blows dust and hot air at his face.

He is here to check on the peanuts. Stretching out before him are rows of peanut plants with tiny yellow buds.

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Peanut pickers work in Seine de Saloium, Senegal.
Zoonar GmbH/Alamy Stock Photo

The research station in Nioro du Rip sits deep in the heart of Senegal’s peanut basin, one of a handful of ISRA stations that test new techniques. And this is where Foncéka continues the research he started as a doctoral student almost 10 years ago. He’s trying to expand the cultivated peanut’s gene pool with genes from its wild cousins. “The diversity of peanuts is limited,” Foncéka says. “But the wild species are very resistant to many diseases.” They are also more resistant to aflatoxin.

Years ago, Foncéka’s team at the Regional Center for Studies on Plant Drought Resistance crossed Fleur 11, a common peanut variety in Senegal, with a hybrid of the peanut plant’s ancestors, A. duranensis and A. ipaënsis. That is not a cross that happens easily in nature. But in the comfort of a laboratory and a greenhouse, a plant breeder can create a wild peanut hybrid capable of breeding with the cultivated peanut.

From there, they created a population of more than 100 peanut plants that incorporated different parts of the ancestors’ genomes. This was the first step in a painstaking process to try to identify how the wild species’ genes affect traits like disease resistance or size, and to try to correlate those changes to specific parts of their genomes.

In the latest phase, which brings Foncéka to Nioro on this hot, dry day, the research team is taking what they learned one step further. They have crossed two Fleur 11 varieties that mix different bits of the ancestors’ genes that control peanut size to see if they can create a larger peanut — one that grows well even in the face of the prolonged dry spells. They also will test the plants’ aflatoxin levels.

Stressing the Peanuts

Another approach, besides creating hybrids, is to study exactly how peanuts naturally resist the toxin. Hot conditions near the growth cycle’s end spur Aspergillus growth and aflatoxin contamination. ISRA researcher Issa Faye says abundant rains in 2015, for example, made a difference. “We evaluated lots of fungi, and there was not a lot of [aflatoxin] contamination,” he says. “Now, when you have pockets of drought, at the end of the growing cycle, the contamination is much higher.”

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Ibrahima Thiam

Scientists do not completely understand the complex plant-soil-fungus-toxin interaction that infects some plants with aflatoxin and spares others. What they do know is that a stressed-out plant, like a stressed-out human, is more susceptible to disease and fungal contamination.

“Those levels of stress might vary within even a single seed,” says Peggy Ozias-Akins, a plant geneticist and peanut expert at the University of Georgia. “There might be certain cells that would be under more stress than other cells. Or certainly within a certain plant, there might be some pods that are more stressed than other pods. So it’s a very nonuniform type of response.”

Ozias-Akins says that, over the centuries, humans chose peanut seeds for particular characteristics — bigger seeds or easier-to-open shells or faster growth. But they lost some things in the process, like genes for disease resistance that still occur in wild peanut ancestors. Her lab is collaborating with Foncéka and Faye, through a project with the Peanut and Mycotoxin Innovation Lab financed by the U.S. Agency for International Development. They hope to identify the genes and mechanisms responsible for aflatoxin resistance.

The GMO Non-Solution

Creating an aflatoxin-resistant seed is not straightforward. Resistance could mean resurrecting the peanut ancestors’ drought tolerance; or it could be a plant with a knack for repelling bugs; or it could be found by hacking the plant’s immune system through mechanisms scientists are just beginning to understand.

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Peanuts are bagged for shipping.
Hemis/Alamy Stock Photo

At the U.S. Department of Agriculture’s National Peanut Research Laboratory in Georgia, plant pathologist Renee Arias has been working on a technique in her lab to target and control aflatoxin at the cellular level. Her team takes tiny pieces of the Aspergillus genes that instruct the fungus on how to make aflatoxin, and they insert them into the peanut.

“When the plant reads that [Aspergillus fungus] and doesn’t know where it’s coming from, it just says, ‘Oh, this is dangerous,’ ” explains Arias. And it will destroy the genes in the fungus, effectively immunizing the plant against aflatoxin. The initial results have been encouraging; the technique has reduced aflatoxin contamination by 74 to 100 percent.

But the resulting peanut would be transgenic — it would have genes from multiple species — which means it would carry a label indicating genetic modification. And that’s a problem, according to Arias. Even if these new peanuts could withstand the Aspergillus fungus’ toxins, it is not clear if they could survive the toxic attitudes toward GMOs in some countries. In Africa, only three out of the continent’s 54 countries allow farmers to grow GMO crops commercially. Senegal still restricts GMOs, although the government’s national biotechnology committee is working to potentially loosen restrictions on these kinds of crops, which scientists maintain are safe.

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Senegalese children gather nuts.
Georges Gobet/AFP/Getty images

Change does not come fast, though, and Arias says that she does not want to fine-tune a technology that will be too hard to commercialize.

“Until everyone embraces transgenics — which, personally, I’ve been working on these things for over 20 years, and I don’t see any problem with using biotechnology — we are also exploring alternatives,” says Arias.

Her lab is looking at non-transgenic ways of provoking a similar response. One option: stimulating anti-microbial substances called phytoalexins that healthy peanut plants produce to stop or slow the growth of fungi. She is mum on details, though. “We don’t want to get scooped on that,” she says. The research is still in its early stages.

Sowing a Fungus

The tail end of the long dry season finds the Senegalese countryside in anticipation. The rainy season might debut sometime in June with a teasing rain or a fast-moving squall that tears across the country and turns the sand into mud. But the real rains start in July, as storm after storm churns and sweeps across the open plains, rinsing the dust from the air, before spinning out into the open waters of the Atlantic Ocean. Periodic rains will take hold: Grass will sprout, trees will flower and farmers will sow their peanut seeds and pray for the rain to keep coming and for the crickets to stay away.

And yet, almost as often as not, the rains do stop, not just for a day or two, but for several days on end. Heat and humidity — prime contamination conditions — run rampant, and so the risk of aflatoxin developing remains high.

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A truck is topped out with peanut bags slated for export.
Doelan Yann/Getty Images

That’s one of the reasons why Lamine Senghor, a plant pathologist with the Senegalese Ministry of Agriculture’s Vegetation Protection Unit (DPV), says that although an aflatoxin-resistant seed could be useful, he is tired of waiting. “They have been doing this research at ISRA for a long, long time, and they have never solved the problem,” says Senghor. “We cannot wait five years or 10 years.”

Instead, he and the DPV have embraced another solution. They are taking the lead on the research and rollout of a soil treatment called Aflasafe. When farmers spread Aflasafe on their fields, they are introducing a strain of Aspergillus flavus that does not produce aflatoxin.

“The fungus will spread out in the entire environment, and go and occupy all the food sources that would have been normally occupied by [toxin-producing] Aspergillus flavus,” says Ranajit Bandyopadhyay. He’s a senior plant pathologist with the International Institute of Tropical Agriculture (IITA) in Nigeria and one of the developers of Aflasafe. He likens it to a probiotic for the soil — even if a mold should develop on the peanuts before harvest, that mold will not be capable of producing aflatoxin. IITA field trials show that the product can reduce aflatoxin by about 80 percent under certain conditions.

That sounds like a perfect solution, but other researchers caution that such bio-control techniques are not panaceas. Research on similar products in the U.S. has shown that if there is an intense dry spell, the safe Aspergillus strain may not be able to outcompete the toxin-producing forms.

And Aflasafe faces a basic problem in Senegal. Will small farmers, who are barely able or willing to pay for fertilizer or pesticide, pay for a product to avoid aflatoxin contamination when few people even believe it’s real?

Back in Méckhé, Ndiakhate Fall, who helps run the local farmers’ organization, weighs in.

“It will be difficult,” he says. But his members might invest in biological controls and aflatoxin-resistant seeds if they knew they could sell their peanuts at a premium. Maybe.

Méckhé’s peanut boomtown days are over; the peanut train is defunct, and many peanut farmers have gone, too, because the dry seasons are often too long and the rains too short. “Some come back during the rainy season, but others don’t,” says Fall. Only the very old or the very young are left — along with the scientists here and around the world, all working toward solutions. And waiting.

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