A crucial crop in new trouble

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. …