MONOCULTURES are a worry. Efficient, certainly; resilient, no. If something goes wrong, the whole crop can be disastrously lost. In practice, many of the plants usually thought of as monocultures—soyabeans, say, or potatoes—are grown as single varieties only in a few places. Round the world, there are plenty of different versions. But there is one genuinely global monoculture. It is a crop in which the dominant variety has been wiped out by disease within living memory. And it is facing new threats to its survival. It is the humble banana.

There are over 1,000 varieties of wild banana in the world. But 95% of banana exports come from a single cultivated variety, the Cavendish. They are basically clones, that is, genetically identical plants. This means they do not have seeds and are nicer to eat. It also means that if one plant is at risk, they all are.

Monoculture has produced a vast industry. The Cavendish can survive weeks in a ship’s hold, unlike most varieties. Because the fruit is roughly the same size and shape, shipping and processing can be standardised, scaled up and made super-efficient. The result is that the banana has become the most valuable fruit in the world (botanically, it is actually a herb, but never mind). Exports rose from 11.5m tonnes in 2001 to 16.5m in 2012, worth $7 billion. Americans eat more bananas than oranges and apples put together. Satisfying that taste is a big business. Three firms—Dole, Chiquita and Del Monte—account for half the world’s exports. (Update: on Match 10th Chiquita merged with another distributor, Fyffes, to become the largest of the companies by volume, see here).

But what if the plant were attacked by a disease it cannot fight off? That isn’t a hypothetical question. In 1963, a disease called Black Sigatoka, which destroys the leaves of the plant (see picture), was discovered in Fiji and spread quickly. By 1975, it was in the plantations of Central America. Black Sigatoka can reduce the fruit yield by half. The Cavendish has no resistance to it.

Caused by a fungus called Mycosphaerella musicola, the disease can be controlled only with doses of fungicide—large ones. As many as 50 sprays are needed in a season. That can cost $1,000 a hectare, or a fifth of the value of the crop. Now, there are signs that the fungus is getting more resistant: growers report that they are having to increase applications by up to a quarter in some places.

Recently hope has begun to rise for a different way to combat the disease: bombarding the plant with gamma rays. The UN’s Food and Agriculture Organisation and the International Atomic Energy Agency (yes, really) have been using this technique, called irradiation, to generate random mutations. Three of the mutant bananas have shown resistance to Black Sigatoka—in the laboratory, at least. Now they are being tested in the fields, with some initial success.

Supposing they work, however, and are widely adopted, that would not be the end of the disease threat. For another potentially catastrophic problem has reappeared: Panama disease. This attacks the roots of the plant. It is potentially a bigger threat than Black Sigatoka because it lives in the soil for decades and once the soil is infected, it almost impossible to eradicate. Panama disease wiped out the previous top banana, a variety called Gros Michel, or “Big Mike”, in the 1960s. The song, “Yes, we have no bananas” commemorates shortages brought on by the early onset of Panama disease. The Cavendish triumphed because it was resistant to the strain of the fungus, Fusarium oxysporum, that killed off the Gros Michel.

But not, it turns out, to all the strains. Recently, a virulent strain called Foc Tropical Race 4 (TR4) has started to spread from banana plantations in Asia and Australia. It recently made its first landing in Africa (in Mozambique) and the Middle East (in Jordan). Though it has not yet appeared in Latin America, the biggest banana-producing continent, that seems only a matter of time.

At the moment, the only way to cope with the disease is to quarantine infected fields when the problem first appears and to ensure that all seedings of new plants are clean. But once the fungus gets established, the disease is almost impossible to stop.

So the race is on to find a resistant banana. One of the wild specimens may fit the bill. Or one of the cultivated species that aren't exported might do. (The Cavendish monopolises the export business but there are other varieties grown for domestic consumption). The trouble is they tend to be pungent, or too delicate to survive shipping. So plant breeders will have to find a resistant variety and combine its traits with the consumer-friendly characteristics of the Cavendish—and that takes time. Genetic modification might be quicker.

Musa acuminata malaccensis, a wild Asian banana that is the precursor of the edible kind, is thought to resist Panama disease. After its genome was sequenced in 2012, scientists inserted genes from it into the Cavendish and grew the hybrid successfully in infected ground in Australia. But this is still at the experimental stage.

Even if plant breeders can fix both Panama disease and Black Sigatoka, though, the presence of two such catastrophic diseases is a wake-up call. The banana business has thrived on standardisation and large scale. But as Fazil Dusunceli of the Food and Agriculture Organisation puts it, “the more diversity we have, the less risk we have.” So companies face a profound choice about their business. Do they diversify into other varieties, which may have more resistance but will be harder and more expensive to grow? Or do they stick with the reliable Cavendish, which consumers like, is cheap to process but much harder to protect from disease?