Something is killing Ramadhani Juma’s cassava crop. “Maybe it’s too much water,” he says, fingering clusters of withered yellow leaves on a six-foot-high plant. “Or too much sun.” Juma works a small plot, barely more than an acre, near the town of Bagamoyo, on the Indian Ocean about 40 miles north of Dar es Salaam, Tanzania. On a rainy March morning, trailed by two of his four young sons, he’s talking with a technician from the big city, 28-year-old Deogratius Mark of the Mikocheni Agricultural Research Institute. Mark tells Juma his problem is neither sun nor rain. The real cassava killers, far too small to see, are viruses