2017年7月29日雅思阅读考试回忆及解析

　　E. Half a billion people in Asia and Africa depend on bananas. Bananas provide the largest source of calories and are eaten daily. Its name is synonymous with food. But the day of reckoning may be coming for the Cavendish and its indigenous kin. Another fungal disease, black Sigatoka, has become a global epidemic since its first appearance in Fiji in 1963. Left to itself, black Sigatoka – which causes brown wounds on leaves and pre-mature fruit ripening –cuts fruit yields by 50 to 70 per cent and reduces the productive lifetime of banana plants from 30 years to as little as 2 or 3. Commercial growers keep Sigatoka at bay by a massive chemical assault. Forty sprayings of fungicide a year is typical. But despite the fungicides, diseases such as black Sigatoka are getting more and more difficult to control. ‘As soon as you bring in a new fungicide, they develop resistance,’ says Frison. ‘One thing we can be sure of is that the Sigatoka won’t lose in this battle.’ Poor farmers, who cannot afford chemicals, have it even worse. They can do little more than watch their plants die. ‘Most of the banana fields in Amazonia have already been destroyed by the disease,’ says Luadir Gasparotto, Brazil’s leading banana pathologist with the government research agency EMBRAPA. Production is likely to fall by 70 percent as the disease spreads, he predicts. The only option will be to find a new variety.

　　F. But how? Almost all edible varieties are susceptible to the diseases, so growers cannot simply change to a different banana. With most crops, such a threat would unleash an army of breeders, scouring the world for resistant relatives whose traits they can breed into commercial varieties. Not so with the banana. Because all edible varieties are sterile, bringing in new genetic traits to help cope with pests and diseases is nearly impossible. Nearly, but not totally. Very rarely, a sterile banana will experience a genetic accident that allows an almost normal seed to develop, giving breeders a tiny window for improvement. Breeders at the Honduran Foundation of Agricultural Research have tried to exploit this to create disease-resistant varieties. Further backcrossing with wild bananas yielded a new seedless banana resistant to both black Sigatoka and Panama disease.

　　G. Neither Western supermarket consumers nor peasant growers like the new hybrid. Some accuse it of tasting more like an apple than a banana. Not surprisingly, the majority of plant breeders have till now turned their backs on the banana and got to work on easier plants. And commercial banana companies are now washing their hands of the whole breading effort, preferring to fund a search for new fungicides instead. ‘We supported a breeding programme for 40 years, but it wasn’t able to develop an alternative to Cavendish. It was very expensive and we got nothing back, ’ says Ronald Romero, head of research at Chiquita, one of the Big Three companies that dominate the international banana trade.