A virus that attacks the cassava plant - a staple food in many poor countries - is threatening to become an epidemic in parts of Africa, the United Nations has warned.

Cassava brown streak virus (CBSV) - which was initially identified by Professor Gary Foster and colleagues at the University of Bristol in England - caused Cassava brown steak disease (CBSD), which can lead to losses of up to 100 percent of the root harvest, while reducing the crop's market value owing to necrotic lesions.

According to the University of Bristol, CBSD has become an extremely serious constraint to cassava production in East Africa as well as a threat to cassava production throughout Africa.

The U.N.'s Food and Agriculture Organization (FAO) warned the situation is urgent and is asking for more funding for surveillance of the virus.

None of the cassava varieties currently being distributed to farmers seem to be tolerant to the effects of CBSD. We urgently need to get information on the extent and severity of the outbreak, and to support investments to identify disease-tolerant varieties and coping strategies for farmers, said Jan Helsen, leader of FAO's European Union-funded Regional Cassava Initiative in Eastern and Central Africa.

CBSD has been classified as one of the seven most dangerous plant diseases in the world for the impact it can have on food and economic security across Africa.

Helsen added: The disease manifests itself in different ways depending on local conditions. In some cases it shows symptoms only on the roots. An apparently healthy plant may be found to have spoiled roots only when harvested, with obvious consequences for food security.

Bristol scholars noted that cassava is crucial to the health of Africans, providing much-needed carbohydrates for around 200 million people on the continent.

Cassava is an excellent crop for poor farmers as it can be cultivated year round and has flexibility in its harvesting times, providing food in periods when other food staples are not available, Bristol said in a statement.

Its ability to better withstand drought and grow in poorer soils than other staples is also contributing to cassava replacing maize as a primary food crop.

FAO said the virus first appeared in Uganda in 2006, but has recently been detected in Burundi and the Democratic Republic of Congo.

Mike Robson, a plant production and protection officer with the FAO, told BBC: The main ways of controlling [the virus] are to try and control the movement of planting material. Cassava is propagated from cuttings and if you move a cutting that has the infection you're effectively moving it to a new area.

He added: The other thing that farmers can do if they suspect they have the disease is to harvest early. They will get smaller roots of cassava but they will be less affected by the disease - it shows up late in the production cycle.