Mark breaks off some wet leaves; a few whiteflies dart away. The pinhead-size flies, he explains, transmit two viruses. One ravages cassava leaves, and a second, called brown streak virus, destroys the starchy, edible root—a catastrophe that usually isn’t discovered until harvest time. Juma is typical of the farmers Mark meets—most have never heard of the viral diseases. “Can you imagine how he’ll feel if I tell him he has to uproot all these plants?” Mark says quietly.