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This is the VOA Special English Agriculture Report.

About 800 million people in Africa, Asia and South America eat cassava. The plant is a major source of food energy and a major food security crop. It can survive in poor soil and without much water. Also, the root can stay in the ground for as long as three years, so it can be harvested as needed.

But in East Africa the plant is under attack. Cassava brown streak disease is a more destructive form of cassava mosaic. The mosaic has been active in East Africa for about 100 years. It limits plant growth. But brown streak can destroy a crop. The virus was identified in Uganda in 2004 and has spread fast in areas extending from Lake Victoria.

So far, brown streak has not jumped to Nigeria, the world's largest producer of cassava. But it threatens more than 30 million tons a year of production in East Africa. In some areas of Uganda, rates of brown streak reached more than 85 percent in 2005 and 2008.

Claude Fauquet is a scientist at the Donald Danforth Plant Science Center in Saint Louis, Missouri. He says a few varieties of cassava can resist brown streak, but these are not the kinds Africans like. He is working to develop disease-resistant plants, but he says it will probably take five years.

Loss of cassava crops could lead to hunger. And hunger can lead to migration and conflict. About 50 million dollars is coming from the Gates Foundation, the Monsanto Fund and the United States Agency for International Development. But Claude Fauquet says much more is needed to fight the disease.

Brown streak can be hard to identify in the field. Irregular yellow spots may appear on lower leaves. But farmers sometimes do not find the disease until they cut open a cassava. If there is only a small amount of rot, the dead material can be cut away. But if the disease has progressed, the whole root is ruined.

Scientists partly blame whiteflies for spreading the disease from plant to plant. Brown streak also spreads if farmers sell or give away cuttings of infected plants.

Cassava has many food uses but the plant is not safe to eat unless it is specially prepared. It must be processed through methods like boiling, grinding or fermenting. A substance that can produce deadly levels of cyanide when eaten must be removed.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Karen Legget.

cassava: a type of flour made from the thick roots of a tropical plant 木薯粉

cassava mosaic: 木薯花叶

cyanide: a highly poisonous chemical compound 氰化物

（来源：VOA 编辑：陈丹妮）