Scientifically Speaking | Can the world’s most popular banana be saved?

In the US and Europe, it can be hard to find any banana other than the Cavendish. Even in India, it represents as much as 60% of bananas traded these days. Cavendish bananas face imminent threat from Panama disease caused by another strain of fungus known as Tropical Race 4
Representational Image. (Getty Images/iStockphoto) PREMIUM
Representational Image. (Getty Images/iStockphoto)
Updated on Sep 21, 2021 03:24 PM IST
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By Anirban Mahapatra

I have tasted bananas from all over the world, but I have yet to come across the wide variety of delectable bananas that I have eaten in India anywhere else. Of these varieties, my personal favourite is a small yellow banana found across South India that has the wonderful consistency of custard, a fruity aroma, and a heavenly taste. These bananas don’t ship well since they have thin skin and bruise easily. But anyone who has not tasted this banana has no idea what a banana should taste like. This is, of course an opinion, and not a fact.

Contrast the wide array of tangy and sweet bananas that are popular in parts of India with bland Cavendish bananas. At over 50% of the world’s share of bananas, the Cavendish is the most popular commercial banana variety globally. In the United States and Europe, it can be hard to find any banana other than the Cavendish. Even in India, it represents as much as 60% of bananas traded these days (though it often sells by different names).

The Cavendish is ubiquitous, but it is a mediocre fruit. It is not the highest-yielding or the best-tasting, but it travels reasonably well, and is perfectly acceptable to many people. It looks like what many people think a banana should look like, it doesn’t have seeds, and it isn’t too tangy. But perhaps the most important characteristic of the Cavendish is that it is resistant to Panama disease, caused by a fungus known as Tropical Race 1 (TR1).

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TR1 devastated Gros Michel, which was the predecessor of the Cavendish. By all accounts, Gros Michel was a delicious banana. But taste only goes so far in the face of a deadly fungal epidemic. When TR1 contaminated soils, entire fields had to be abandoned. And soon the world found a new banana to mass produce and ship globally.

The irony is that, today, Cavendish bananas also face imminent threat from Panama disease caused by another strain of fungus known as Tropical Race 4 (TR4). TR4 has been spreading across the globe for the past few decades, making its way from Taiwan to the rest of Asia and Australia, and from there to Africa. More recently, it has been found in Latin American countries, which are the primary source of bananas for Europe and North America.

Because consumers prefer seedless bananas over wild ones full of seeds, commercial bananas are propagated. Cavendish bananas, which are seedless, are nearly identical to one another. This lack of genetic variation makes it particularly susceptible to TR4.

Panama disease caused by TR4 has been colorfully called “banana Covid” by some commentators. In the face of its calamitous spread, we will need multiple approaches to save the world’s most popular banana.

There are three ways that scientists are trying to save the Cavendish banana right now. One is by creating biological exclusion zones. Since the fungus resides in the soil, the idea is to keep plots with infected plants and soil away from other plots. This approach seems to be limiting the spread of the fungus in parts of Australia right now. But overall, it remains to be seen how sustainable this approach will be. As the fungus spreads across more fields, it becomes harder to control. This approach only buys us time.

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The second approach, which is being pioneered by a research group in the Netherlands, is to get rid of soil altogether, and to grow Cavendish bananas in materials that are soil substitutes. Removing soil does remove the ability of TR4 to spread, but this does not seem to be an economically viable or scalable solution for the world’s most popular banana. Cavendish bananas are popular because they can be produced and distributed on a global scale inexpensively.

The third approach is to use gene editing to turn on a gene in the Cavendish banana that prevents TR4 from causing disease and spreading. Apparently, the Cavendish has a faulty gene required for resistance to TR4 that had to be recovered from a wild banana variety. Earlier this year, Australian researchers reported that they had successfully tested a genetically modified version of the Cavendish that appears to be resistant to TR4. While this is a welcome development, it will take years before this banana can be field tested. And in many parts of the world, this “Cavendish 2.0” banana may face barriers to acceptance after being classified as a genetically modified food-source.

As population grows and the world faces consequences of the climate crisis, food security will remain vitally important. But we must also remember that genetic monocultures are particularly susceptible to infectious diseases. We may successfully hold off TR4, but what about the next fungal scourge? Will we be back to the drawing board with Tropical Race 5? As long as most of the world is reliant on one kind of banana, the best we can hope for is an uneasy détente between plant and pathogen.

More than just differences in taste, a wide variety of bananas safeguards the future of an important crop. Plants flourish when they are maintained in a sustainable manner with genetic variations. Ensuring variation is necessary for crop security, health, and economic viability.

Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction

The views expressed are personal

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Friday, October 22, 2021