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. Modern supercrops will be a big help. Mark tells Juma his problem is neither sun nor rain. The real cassava killers, far too small to see, are viruses.
Mark breaks off some wet leaves; a few whiteflies dart away. The pinhead-size flies, he explains, transmit two viruses. Juma is typical of the farmers Mark meets—most have never heard of the viral diseases. 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. 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 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.
