The Science of Bananas (Why are Bananas Vulnerable?)

Why are bananas so susceptible to disease and at risk of extinction?

So we’ve been hammering on about the ‘Bananageddon’ that Cavendish bananas are about face. But what is the Cavendish, and why should we worry? Why are bananas headed for extinction?  

In the developed world, we have access to only a limited number of banana varieties. The predominant variety is the Cavendish banana, the most commonly consumed fruit in the world. It replaced the Gros Michel variety, the former favourite banana, when it went extinct in the 80s. Now, the Cavendish is heading for the same fate.  

A big part of the problem is the way bananas are grown. And this problem is not limited to banana production. The bananas which occupy the shelves of our supermarkets come from large, commercial plantations, which grow bananas in a ‘monoculture system’. This is an intensive method of farming where a single type of crop, usually the same variety, is grown shoulder to shoulder over huge expanses of land. In Costa Rica, 420 sq km of land is covered with banana monoculture. That’s 60 football pitches of banana plant after banana plant. In Ecuador, it can even take several hours on public transport to leave areas dedicated to banana production. Monoculture is said to increase crop yield, or the value of your land, because you can produce more crop per unit area.  

A banana monoculture

However, monoculture translates to boom and bust. This tight concentration of a single crop, though profitable, is a beacon of attraction for pests. Eventually and inevitably, pests swarm farms, impacting plant health, and therefore fruit quantity and quality. Some pests cause disease, and with so many crops in such a small space, disease spreads rapidly. This is the challenge faced by bananas. And that challenge is of a fungal kind.   

The two diseases currently attacking banana crops around the world are known as Black Sigatoka and Panama disease. Both are caused by fungi species which grow inside banana organs, draining away their nutrients or reducing their ability to photosynthesise. These fungal parasites slowly starve the banana to death. Black Sigatoka causes fruit losses of up to 50%, and Panama disease is responsible for wiping out the Gros Michel, the banana formerly eaten by your grandparents and now extinct. Panama disease has already caused mass banana die-offs in Asia, and it’s only a matter of time before the disease reaches Central America too, the final banana stronghold.  

The fruit of a banana plant with Panama disease. http://horticulture.com.au/panama-disease-and-the-banana-plant-protection-program/

Okay, are you worried yet? Unfortunately, there’s a little more I need to share. All of this is made worse by the biology of the banana crop. All bananas considered to belong within the Cavendish variety descend from a reproductive cross between just two individuals, from two wild species. One with a fleshy but distasteful fruit, and the other with a fruit containing huge seeds. The outcome of this cross? A very tasty, carbohydrate rich, yellow, seedless fruit, perfect for eating. But also sterile, which is almost always the case when individuals from two species are crossed. This means that Cavendish bananas are unable to produce offspring sexually; male pollen never finds its way to the female ova. Instead, we have gained more bananas from regenerating the same individual over and over and over. This can be done by taking a cutting of the young stem, and replanting it, which eventually gives rise to a whole new banana plant. 

If you cut down a banana plant at the stem when the fruit are ripe, a new one will grow in its place. This allows farmers to skip sexual reproduction, and regenerate plants from the same individuals over and over. http://www.hutterites.org/nigeria/fruits-of-the-land/

Skipping sex makes bananas vulnerable. Sexual reproduction produces individuals which have inherited 50% of their genes from one parent, and 50% from a second. But the 50% passed on is always random. This means that siblings, despite sharing mothers, never inherit the same 50% of their mothers genes. This is also true for the genes they inherit from their father. This shuffling allows a population to contain variety in the genes possessed by each individual, and this genetic variation is required for a species to adapt to change. For obvious reasons, it is hugely beneficial for a species to be able to evolve so that it is more resistant to a disease. But the bananas we eat all regrew from a single individual, and therefore contain exactly the same genes as their ‘parent’ plant. For this reason they can be thought of as clones. A population of clones has no genetic variation, as all individuals are identical, and therefore cannot evolve resistance to disease. And let’s not forget that because of monoculture, such diseases speed through the population because crops are planted so close together.  

If in doubt about how damaging pesticides really are, here is an example of common warning and advisory signs on pesticide packaging. We know that banana workers aren’t receiving proper protection, and that pesticides find their way into our natural ecosystems. https://www.intechopen.com/books/herbicides-physiology-of-action-and-safety/safety-measures-for-handlers-workers-against-herbicide-intoxication-risk

It is almost a scientific miracle that the Cavendish group have lasted this long. Almost. This miracle can be explained by heavy pesticide use. Worldwide, 2.5 million kilograms of pesticides were used in 2009. 85% of this can be linked to banana production. These pesticides keep bananas alive. However, Black Sigatoka is highly resistant, and no pesticide is known to be able to halt Panama disease. Regardless, commercial plantation owners respond by continuing to increase the amount of pesticide they dump on their land. This will lead to increased resistance of the fungi to the poisons, as the fungi contain high levels of genetic variation and can evolve rapidly. The Cavendish cannot evolve resistance to the fungi. Fungi that are becoming more harmful with every drop of pesticide we administer. Eventually the fungi will win. We’re sitting on a Cavendish extinction. 

Unless we do something differently.  

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REFERENCES 

http://willsull.net/la370/resources/Module-2/TheSterileBanana.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3606705/ 

http://journals.plos.org/plosone/articleid=10.1371/journal.pone.0120384#pone.0120384.ref015 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2946087/ 

https://www.damninteresting.com/the-unfortunate-sex-life-of-the-banana/

Mollie Gupta

Assistant Creative Director

Mollie is an aspiring conservation biologist, and is optimistic that we can find ways to preserve the world’s incredible biodiversity whilst ensuring that peoples’ livelihoods prosper. Her previous research has focused on human interactions with ecosystems at the landscape level, including through forest restoration in Brazil and the introduction of an invasive newt species in the UK. Mollie’s childhood was spent stalking her friends pets, sifting through rockpools, and watching tadpoles grow into frogs in her makeshift tank. Having grown up in London, Mollie is at home in the hustle and bustle of busy city life, however she is always happiest by far when she is surrounded by trees and the quiet rustle of birds and squirrels. Apart from tree-hugging, you often find Mollie eating cake, doodling on scraps of paper or playing basketball. She hopes that Bananageddon will empower people to make small changes in their lives which will help contribute to a healthier world for everyone.