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[NAIROBI] Rising soil temperatures are increasing the spread of a deadly, parasitic weed that significantly reduces crop yields in Sub-Saharan Africa, Striga, according to scientists.

The noxious weed, also known as witch-weed, usually thrives in the warm and humid tropics but is now spreading to cooler and wetter highlands as a result of warmer soils driven by global warming and low soil fertility, which provides the right conditions for Striga to thrive.

This spread has threatened the livelihoods of around 100 million people, with more than four million hectares of maize crops infected. In general, Striga reduces maize and cowpea yields by up to 80 per cent in Sub-Saharan Africa.

Some farmers are now abandoning maize cultivation for cassava as the parasitic plant colonises areas at attitudes more than 1,500 metres above sea level, said Mel Oluoch, head of the Integrated Striga Management in Africa programme at the International Institute of Research in Tropical Agriculture (IITA), during a farmers' field day in Western Kenya last month (17 January) organised by ISMA.

"Striga currently remains the biggest threat to maize production, particularly in the East Africa region where corn is the staple for millions of inhabitants," said Oluoch, adding that the weed devastates plants by attaching itself at the root base, starving the host for nutrients.

"More worrying is that the weed is spreading fast to areas not traditionally known to be infected, owing to climate change-related impacts and the resulting rise in soil temperatures coupled with low soil fertility across most of Africa," Oluoch told SciDev.Net and added that they have been observing the weed for more than two years.

With Striga-related losses estimated at US$1 billion per year, a number of research bodies in Africa, including the IITA, the International Centre for Insect Physiology and Ecology (ICIPE), International Wheat and Maize Improvement Center (CIMMYT) and the African Agriculture Technology Foundation, launched in 2009 a concerted effort to tame the weed.*

They are using biological methods such as crop rotation and intercropping — a low cost technology where farmers plant a 'break crop', the one not susceptible to Striga, such as nappier and cassava — to boost soil fertility and halt multiplication of the weed.

These methods are being piloted in western Kenya.

Also being piloted are chemical options, including coating planting seeds with chemicals to kill off the weed at the germination stage, but progress has been slowed by high costs of coated seed which is limiting their availability, according to ICIPE scientist Jimmy Pittchar.

Owing to the weed's stubborn nature, Olouch said, it can only be contained by a combination of technologies and control measures over a period of no less than ten years.

"It advisable farmers use a host of methods in managing the weed to give farmers options best suited to them, in cost and environmental terms," says Pittchar.

This article has been produced by SciDev.Net's Sub-Saharan Africa desk.

*11 February 2013 This article was updated to say that the Striga-related losses are estimated to be at US$1 billion per year, not US$8 billion as stated before.