By Isaiah Esipisu
NAIROBI (Thomson Reuters Foundation) – The future of cassava, one of the most climate-resilient crops in Africa, may be under threat because rising temperatures have led to a dramatic increase in the number of whiteflies, tiny insects that spread the deadly cassava brown steak virus.
Previously seen as a major problem, but one confined to eastern and central Africa, the virus is spreading, alarming scientists who say new outbreaks suggest the disease is heading west towards the world’s largest cassava producer – Nigeria.
So far, according to James Legg, a plant virologist at the International Institute for Tropical Agriculture (IITA), new countries such as Angola, Gabon and Central African Republic have been hit by the virus. Legg has done extensive research on the disease and its causes in East Africa.
Scientists have already successfully tackled an equally lethal virus – cassava mosaic disease - transmitted by the same whitefly, Bemisia tabaci. This was done by developing a variety that is resistant to the virus.
But by a cruel twist of nature, both the improved and the unaltered varieties have succumbed to a ‘new’ pandemic of cassava brown streak virus, and scientists have yet to develop a resistant variety.
The biggest worry now is that as the whitefly population hugely increases, the insects could spread across the entire African continent.
“A key question that still needs to be fully answered is ‘Why are there so many more whiteflies now than there used to be on cassava?’” Legg told Thomson Reuters Foundation in a phone interview. Whiteflies are known to do well at high temperatures and a growing suspicion is that the gradual process of climate change is favouring an increase in their numbers, he said.
The insects also seem to have a close partnership with the viruses they transmit, and some evidence has shown they flourish on plants infected by the disease, leading to a mutually beneficial relationship, Legg said.
A RESILIENT CROP FAILS
For years, cassava has been the crop of choice for food security when other crops have failed because of changes in climatic conditions, said Japheth Akhati, a small-scale farmer in western Kenya. But Akhati was forced to abandon the plant three years ago, after the one-hectare plot where he grew the improved cassava variety succumbed to brown streak disease two seasons in a row.
“It was so devastating to count losses from a crop that had never failed me before,” said the 38-year-old father of two, after losing his entire cassava crop twice. His neighbours in Esilongo village have also abandoned cassava.
When the virus attacks, the leaves of infected plants can look healthy even as the roots are being ravaged underground. In some cases, yellow patches appear on leaves of infected plants while the stem turns brown and the tubers decay. Infections can claim 100 percent of a harvest - sometimes without the farmer’s knowledge.
Scientists from three major agricultural networks – the Global Cassava Partnership, the International Centre for Tropical Agriculture, and the International Institute for Tropical Agriculture - are now working together to combat the disease, and to try to stop the virus from spreading further, particularly to Nigeria which produces 50 million tonnes of cassava a year.
An attack by the virus would be a huge economic blow for a country increasingly reliant on the crop for industrial purposes as well as for food. Nigeria produces starch from cassava to meet increasing global demand by the paper and textile industries, and even plywood manufacturers.
Ruth Amata, a research scientist from the Kenya Agricultural Research Institute, says there is clear evidence that using infected plants to create new seedlings for sale perpetuates the spread of diseases like the brown streak virus.
To prevent this, Kenyan scientists are working on a programme that will enable seed merchants to produce clean cassava planting materials. Trials are under way, with support from the Bio-Innovate programme hosted by the International Livestock Research Institute.
To grow cassava, stems of the plant are chopped into small pieces about 10 cm (4 inches) long which are planted as seedlings. The Bio-Innovate scientists are researching ways in which merchants can disinfect stems before selling the pieces to farmers. A similar practice of ensuring disease-free planting material has already succeeded in Kenya’s tea industry, where merchants sell certified seedlings.
Apart from its industrial uses, cassava has been identified as a future food crop for many African communities as climate shifts threaten other staples including wheat, maize and rice in some areas.
Thousands of farmers in semi-arid eastern Kenya have already abandoned maize in favour of cassava, sorghum and millet because the three crops have – until now - proved resilient to all forms of climatic conditions.
“We have learned how to cook and eat both cassava leaves and tubers, make flour from dried tubers for domestic and commercial consumption, and make snacks and crisps from the tubers,” Jemima Mweni, a mother of seven from Makueni County in eastern Kenya, told Thomson Reuters Foundation.
“Cassava is much too important a crop for Africa’s future to allow it to be overpowered by the actions of a tiny insect,” said Legg.
Isaiah Esipisu is a freelance journalist interested in agricultural and environmental issues. He can be reached through email@example.com
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