That event changed the history of all English-speaking countries.

Compared with amber waves of grain or the blond tresses of a field of ripe corn, cassava is an inglorious workhorse of a crop, a few spindly red stems sprouting from a clutch of brown tubers. It is filling but not very nutritious; it even contains trace amounts of cyanide, which must be removed by grinding and fermenting.

Image UNWANTED Even pigs won’t eat cassava riddled with necrotic brown lumps that was dug up by Lynet Nalugo, top, from a field in Mukono, Uganda. Credit... Tyler Hicks/The New York Times

But subsistence farmers depend on it because it’s “very drought-tolerant and very bad-management-tolerant,” said Edward Charles, a team leader for the Great Lakes Cassava Initiative, a six-country consortium based in Kenya and supported by the Gates Foundation. For example, he said, even when farmers are too weak from malaria to weed, their crops survive.

Also, the tubers can be left underground for up to three years, so if drought kills a corn or bean crop, the farmer’s family can still fend off starvation. But the plant falls prey to more than 20 pests and diseases.

Dr. Fauquet fears brown streak will cross the Congo Basin to Nigeria, the world’s biggest grower, because farmers sell cuttings to one another and border controls are nonexistent or can be evaded with bribes.

He is optimistic it will not cross the ocean into Thailand, Brazil, Indonesia or China because there is no world trade in the cuttings and few direct flights to Asia or South America. (Whiteflies, which are thought to spread the virus, have been known to stow aboard planes.)

However, he noted, mosaic virus did spread to India from Africa somehow. And Dai Peters, the Cassava Initiative’s director, noted that a mealybug that damages Brazilian cassavas has leapfrogged the globe to infect Thai fields, too.

Even if the brown streak virus is contained in Africa, Dr. Fauquet said, donors may eventually be forced to spend billions of dollars on food aid to prevent starving populations from going on the move, which could set off ethnic fighting.