2017年7月29日雅思阅读考试回忆及解析

　　B. In some ways, the banana today resembles the potato before blight brought famine to Ireland a century and a half ago. But ‘it holds a lesson for other crops, too’, says Emile Frison, top banana at the International Network for the Improvement of Banana and Plantain in Montpellier, France. ‘The state of the banana’, Frison warns, ‘can teach a broader lesson the increasing standardization of food crops round the world is threatening their ability to adapt and survive.’

　　C. The first Stone Age plant breeders cultivated these sterile freaks by replanting cuttings from their stems. And the descendants of those original cuttings are the bananas we still eat today. Each is a virtual clone, almost devoid of genetic diversity. And that uniformity makes it ripe for disease like no other crop on Earth. Traditional varieties of sexually reproducing crops have always had a much broader genetic base, and the genes will recombine in new arrangement in each generation. This gives them much greater flexibility in evolving responses to disease – and far more genetic resources to draw on in the face of an attack. But that advantage is fading fast, as growers increasingly plant the same few, high-yielding varieties. Plant breeders work feverishly to maintain resistance in these standardized crops. Should these efforts falter, yields of even the most productive crop could swiftly crash. ‘When some pest or disease comes along, severe epidemics can occur,’ says Geoff Hawtin, director of the Rome-based International Plant Genetic Resources Institute.

　　D. The banana is an excellent case in point. Until the 1950s, one variety, the Gros Michel, dominated the world’s commercial banana business. Found by French botanists in Asian the 1820s, the Gros Michel was by all accounts a fine banana, richer and sweeter than today’s standard banana and without the latter’s bitter aftertaste when green. But it was vulnerable to a soil fungus that produced a wilt known as Panama disease. ‘Once the fungus gets into the soil it remains there for many years. There is nothing farmers can do. Even chemical spraying won’t get rid of it,’ says Rodomiro Ortiz, director of the Inter-national Institute for Tropical Agriculture in Ibadan, Nigeria. So plantation owners played a running game, abandoning infested fields and moving so ‘clean’ land – until they ran out of clean land in the 1950s and had to abandon the Gros Michel. Its successor and still the reigning commercial king, is the Cavendish banana, a 19th century British discovery from southern China. The Cavendish is resistant to Panama disease and, as a result, it literally saved the international banana industry. During the 1960s, it replaced the Gros Michel on supermarket shelves. If you buy a banana today, it is almost certainly a Cavendish. But even so, it is a minority in the world’s banana crop.