National Geographic : 2014 Oct
40 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 cas- sava killers, far too small to see, are viruses. Mark breaks off some wet leaves; a few white- flies dart away. The pinhead-size flies, he ex- plains, 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. Juma is wearing torn blue shorts and a faded green T-shirt with “Would you like to buy a vowel?” printed on the front. He listens care- fully to Mark’s diagnosis. Then he unshoulders his heavy hoe and starts digging. His oldest son, who is ten, nibbles a cassava leaf. Uncovering a cassava root, Juma splits it open with one swing of his hoe. He sighs—the creamy white flesh is streaked with brown, rotting starch. To save enough of the crop to sell and to feed his family, Juma will have to harvest a month early. I ask how important cassava is to him. “Mihogo ni kila kitu,” he replies in Swahili. “Cassava is everything.” Most Tanzanians are subsistence farmers. In Africa small family farms grow more than 90 percent of all crops, and cassava is a staple for more than 250 million people. It grows even in marginal soils, and it tolerates heat waves and droughts. It would be the perfect crop for 21st- century Africa—were it not for the whitefly, whose range is expanding as the climate warms. The same viruses that have invaded Juma’s field have already spread throughout East Africa. Before leaving Bagamoyo, we meet one of Juma’s neighbors, Shija Kagembe. His cassava fields have fared no better. He listens silently as Mark tells him what the viruses have done. “How can you help us?” he asks. Answering that question will be one of the greatest challenges of this century. Climate change and population growth will make life increasingly precarious for Juma, Kagembe, and other small farmers in the developing world—and for the people they feed. For most of the 20th century humanity managed to stay ahead in the Malthusian race between popula- tion growth and food supply. Will we be able to The genes of all living things on Earth—including the sunflower, a valuable oil crop—consist of varying sequences of four chemical compounds: adenine, thymine, cytosine, and guanine, abbreviated as A, T, C, and G. By identifying genes and manipulating them, scientists hope to create new crops that will help us face the challenges of global warming and population growth.