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Flesh-eating Cassava Virus Pushes West Across Africa

This post originally appeared on CIAT: International Center for Tropical Agriculture.

Scientists are struggling to contain a flesh-eating virus sweeping across the cassava plantations of Africa, destined for the world’s biggest producer and consumer, Nigeria.

The devastating epidemic of Cassava Brown Streak Disease (CBSD) has already made it halfway across the continent, with reports of new outbreaks in DR Congo—the world’s third largest producer—and Angola, where production has boomed in recent years. Around 300 million people in sub-Saharan Africa depend on cassava, where it is a vital staple food and the second-most important source of carbohydrate.

Root symptoms of CBSD. EARRNET picture is from IITA, Dr. E. Kanju.
Root symptoms of CBSD. (Photo credit: IITA, Dr. E. Kanju)

Little known until as recently as 2003, CBSD has suddenly become the most dangerous cassava virus in Africa, with the potential to wipe out entire harvests. An unseen killer, the disease consumes the cassava roots while they are still in the ground, without producing visible symptoms in the rest of the plant. Only at harvest do farmers discover the blighted, inedible roots which, when cut open, reveal trademark brown blotches of destroyed flesh that give the disease its name.

“Cassava is already incredibly important for Africa and is poised to play an even bigger role in the future,” said Claude Fauquet of CIAT, and a coordinator of the Global Cassava Partnership for the 21st Century (GCP21), an alliance of the world’s leading cassava scientists, donors and private industry, which will declare war on CBSD and other cassava diseases at a week-long conference at the Rockefeller Foundation Bellagio Center in Italy, which begins today. “We’re particularly concerned that the disease could spread to West Africa and particularly Nigeria—the world’s largest producer and consumer of cassava—because Nigeria would provide a gateway for an invasion of West Africa, where about 150 million people depend on the crop.”

As well as a staple food, cassava starch is used to make biofuel, paper, and even beer. Nigeria is the first country in Africa to attempt to tap the potential of cassava for industrial starch production, with increasing investment from foreign-run starch processing factories supplying the global starch industry. Nigeria hopes to mimic the success of countries in SE Asia, where a cassava-driven starch industry now generates USD$5 billion per year, employing millions of smallholder farmers and numerous small-scale processors.

Scientists have attributed the sudden boom in CBSD in Africa to an explosion in whitefly numbers—insects that carry the pathogen and infect the cassava plants while feeding on the sap, or phloem. A persistent pest, whitefly populations have rocketed in the region in recent years, which scientists have linked to rising temperatures resulting in more favourable breeding conditions.

“We used to see only three or four whiteflies per plant; now we’re seeing thousands,” said James Legg, a leading cassava expert at the International Institute of Tropical Agriculture (IITA). “You literally have a situation where human beings are competing for food—with flies.”

The spread can be accelerated by humans too—through the movement and planting of infected stem cuttings – the most common way to propagate cassava. While scientists note it would take several years for the disease to spread across the continent via whiteflies alone, the movement of cassava stakes could spark outbreaks in new areas overnight.

This is worrying news for cassava, regularly championed here as the “Rambo root” for its extraordinary ability to survive extreme temperatures and tolerate poor soils. CIAT research published last year showed it could be one of the most climate change-resilient crops that African farmers can plant if its susceptibility to pest and disease outbreaks can be effectively tackled. As well as CBSD, scientists will also focus on cassava mosaic disease, which has plagued Africa from more than a century, each year bringing about losses in the region of 50 million tonnes.

The GCP21 meeting at Bellagio, Declaring war on cassava viruses in Africa, will aim to accelerate research to find long-lasting solutions. These include selective breeding to develop cassava varieties with greater disease resistance, such as those recently released in Tanzania. It will also involve going back to South America, where cassava originated, and working with scientists to mine the cassava gene bank at CIAT in Colombia—the biggest repository of cassava samples in the world—in order to find and develop genetically superior cassava plants that are completely resistant to CBSD and other diseases.

Cassava Farmer Africa

“It’s time for the world to recalibrate its scientific priorities,” continued Fauquet. “Cassava has the greatest potential to reduce hunger and poverty in Africa—more than any other crop—but these diseases are crippling yields and destroying this potential. We need to treat CBSD and other destructive viruses like the smallpox of cassava—formidable diseases, but threats we can eradicate if everyone pulls together.”

Conference participants will also discuss developing a regional strategy to gradually, village by village, replace farmers’ existing infested cassava plants with virus-free, resistant varieties, as well as ways of controlling whiteflies that are cost effective and environmentally sustainable. They will also consider the need for new research into the potential threat African cassava producers face from the introduction of new pests and diseases from elsewhere.

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“It’s a tale of two continents,” said CIAT’s Agrobiodiversity Research Area director Joe Tohme, who jointly co-ordinates GCP21. “We’ve seen in the very recent past the devastation caused when cassava pests ‘jump’ from Latin America to Africa, so we need to be realistic and plan for the inevitability of outbreaks of new diseases in Africa by developing plants resistant to frog skin disease, for example, a major viral disease of cassava in South America which, luckily, is not present in Africa. Past events show very clearly that even though these two continents are far apart, cassava pests and diseases will somehow, eventually, find a way of getting from one to the other.”

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