再Banana disease, March 1st 2014 P58 (バナナ)
When banana started to be widely exported in the 1870s, they were an exotic treat. But by the 1950s the fruit (botanically, a herb, but never mind) was a favourite of millions far from the tropics. Then Panama disease struck. The soil fungus swept through Central and South America, killing banana plants in its paths. By the 1960s Gros Michel (Big Mike), the variety accounting for virtually all exports, was close to extinction. The export industry approached collapse. But in the nick of time growers identified a resistant commercial variety, called Cavendish. Compared with Gros Michel, it was small and bland. Gros Michels could be flung into train carriages and ships' holds；Cavendishes had to be packed in cardboard and shipped in pricy refrigerated containers. But there was no other alternative. Soon Cavendish replaced Gros Michel as the world's top banana：the variety now accounts for 95% of all exports. Bananas are now the world's most valuable fruit. Exports rose from 11.9m tonnes in 2001 to 16.5m in 2012. Americans eat more bananas than apples and oranges together. But once more the export industry is fighting to survive and this time, on two fronts. First, Black Sigatoka, a disease which blackens leaves and can halve yields, is showing resistance to the fungicide used to combat it. It is normally controlled by spraying almost weekly, which increases growers' costs considerably. Now growers in some places are having to increase dosage substantially, suggesting that spraying could soon become not just pricey, but ineffective. Second, Foc Tropical Race 4, a strain of Panama disease that attacks the Cavendish, has struck in several countries. Central and South America, which produce four-fifths of exports, have so far escaped. But ゛it's not a question whether it will occur there,゛says Gert Kema, a plant pathologist at Wageningen University in the Netherlands. ゛It's a question of when.゛ The race is now on to find a banana that is both resistant to the two diseases and commercially viable. Consumers might turn their noses up at pungent varieties. Thin-skinned ones would not survive weeks in ship's hold. A candidate may be hidden in the Laboratory of Tropical Crop Improvement in Leuven, Belgium, which houses a big collection of specimens of bananas and plantains (close relatives that must be cooked before eating). Modifying the Cavendish is another, perhaps quicker, approach. Scientists at the UN and the International Atomic Energy Agency have bombered plants with gamma rays；three of the resulting mutants have shown resistance in the laboratory to Black Sigatoka. And Musa acuminata malaccensis, a wild Asian fruit that is the precursor of edible bananas, is thought to be resistant to Panama disease. A hybrid Cavendish containing some of its genes has grown well in infected ground. But in both cases, field tests are needed. Cavendish, like other cultivated banana varieties, is seedless and propagated by cuttings. That produce clones, which is efficient for exporters, since the fruit are all similar in shape and size. But it also means a single disease can threaten the entire crop - and the non-tropical world's banana supply. If the export industry is to have a long-term future, it needs to diversify.
