Bananas: A frightful lesson from the past

The wild banana is a giant herb with a fruit that normally contains a mass of hard pea-sized seeds that make it virtually inedible. In the remote past, hunter-gatherers sometimes discovered plants that produced tasty, seedless, soft fruit. But these seedless plants were mutated and by definition sterile. Sometime at the very end of the last ice age, early farmers in Southeast Asia started to cultivate these sterile varieties by replanting cuttings of these mutants.

These days the ‘Cavendish’ is the worlds premier commercially grown banana and that means that means that if some pest or disease emerges in one part of the world, the ‘Cavendish’ may be extinct within a few short years.

A frightening example can be found in the predecessor of the ‘Cavendish’: the ‘Gros Michel’ and until the 1950s this variety dominated the world’s commercial banana business. The ‘Gros Michel’ was discovered by French botanists in Asia in the 1820s and was richer and sweeter in taste than today’s ‘Cavendish’. The ‘Gros Michel’ fell victim to a soil fungus that produced a wilt known as Panama disease. Because all commercially grown bananas were identical ‘Gros Michaels’, they all withered worldwide and mass unemployment quickly followed.

Now, the ‘Cavendish’ is under threat too: it’s under attack from the ever evolving fungi that causes Panama disease and also one that leads to Black Sigatoka. The only way to keep these diseases at bay is heavy spraying with pesticides: forty sprayings of fungicide a year is typical, making the Cavendish the most heavily sprayed food crop in the world.

One wonders if those pesticides could have a detrimental effect on human health. It is no real surprise that women in banana-packing plants in Costa Rica suffer double the average rates of leukemia[1] and birth defects. Furthermore, a fifth of male banana workers are sterile[2], possibly as a result of exposure to dibromochloropropane, which is now banned.

Meanwhile, the latest mutation of Panama disease, called tropical race 4 (TR4) has now reached South Africa, Australia and much of Asia. Millions of banana plants have died in southern China and pesticides cannot control it. So, it is only a matter of time before the disease reaches the commercial plantations of Ecuador, Costa Rica, Honduras, and Colombia.

The problem is that we neglected the fruit’s genetic base in the wild and we risked losing the possible genes they contain. Genes that could be and undermining the future of the fruit.

Normally plant breeders would engineer resistance into a plant that is at risk but you should remember that the banana is sterile and thus that particular route is very difficult. Which begs the question: what can we do to rescue the banana? Recently scientists from Wageningen University in The Netherlands have been trying to ‘unpeel’ the genome of the banana and to their own amazement, the wild banana species for which the genome has been sequenced turned out to be highly resistant to TR4. And that fact alone may turn out to be the most important result because if these scientist can insert this resistance into the ‘tame’ banana, it may even survive.

[1] Wesseling et al: Cancer in banana plantation workers in Costa Rica in Journal of Epidemiology - 1997 
[2] Bretveld et al: Influence of pesticides on male fertility in Scandinavian Journal of Work, Environment and Health -2007