LAGOS — The Bill and Melinda Gates Foundation gave a $12-million (nine-million-euro) grant Monday to a project aimed at boosting yam production and doubling the income of west African farmers of the crop.

It is the foundation's largest grant for yam production anywhere in the world, said a spokesman of the International Institute of Tropical Agriculture in Nigeria's southwest city of Ibadan.

The project's initial focus is on 200,000 smallholder farm families in Ghana and Nigeria, 90 percent of whom cultivate less than two acres, the institute said in a statement.

Yam, a tuber plant, is a staple food in many tropical countries and provides the most important source of dietary calories in Nigeria and Ghana.

But a variety of pests and diseases have dramatically reduced harvests.

"Most farmers cultivate yams mainly for household consumption, but if we can increase yields, while also improving marketing conditions, then many of these farmers should be able to earn a steady income from growing yams," institute director general Nteranya Sanginga said.