Second helpings of Cassava pudding

FRANCIS MUTUNGI, a farmer in dry eastern Kenya, stands engulfed by his cassava plants, the bright green leaves spreading out like six-fingered hands to touch his shoulders. Some have been dug up; their roots (the edible bit of the plant) lie like giant beached starfish.

Luckily, he planted a variety resistant to a disease that is ravaging Africa’s second-most-important crop in terms of calories. A neighbour’s cassava is barely knee-high; its leaves are blighted, its roots shrivelled and rotting.

Cassava (also known as tapioca or manioc) is the world’s fourth-largest calorie provider. It grows in hot, arid regions where other crops will not thrive. It resists drought and climate change better than cereal crops like rice. It has to be processed within a couple of days of harvesting, and is vulnerable to huge post-harvest losses (40% in some places). So it is not much eaten by city dwellers.

But it has one amazing plus. It will keep in the ground for years. That makes it “the perfect food-security crop”, says Claude Fauquet, of the Danforth Plant Science Centre in St Louis, Missouri, which researches Africa’s overlooked crops. You plant it alongside maize or beans. If those fail, eat the cassava; if they thrive, save it for another year. (Less happily, with no need to stop for the harvest, wars in cassava-growing lands can be continuous.)

Now two viruses—cassava mosaic disease and brown streak disease—are destroying up to half the crop in areas they infest. In Africa, where the diseases are rampant, farmers get about 8.5 tonnes a hectare; in Vietnam, where they are unknown, the yield is 17 tonnes; in South India, 26 tonnes. The result is that cassava remains a subsistence crop in Africa, but in Asia is something on which you can build a business (it can also be used to make clothing, paper and biofuels). Mr Fauquet reckons a yield of 20 tonnes a hectare means money in the bank; 25 tonnes means a house and private school for the children. But below 12 tonnes, it traps a farmer in poverty. The diseases make a vast difference.

One problem is that viruses cannot be killed by fungicides and other agrochemicals. Another is that planting stem cuttings, the usual way of propagating cassava, passes down disease more readily than propagation by seeds. Mr Fauquet has managed to breed disease-resistant plants in his laboratory. But field trials have only just begun for mosaic disease and do not start for brown streak until later this year. He thinks genetic modification of this humble crop, so far from the world of advanced science, will provide the most comprehensive defence and be the best way to boost yields.