- The world's most popular edible variety is about to be wiped out by a fungal invader. Again.
- We've already lost Gros Michel bananas, which were the world's favorite until the 1960s.
- The solution? Possibly genetic editing, but more likely a greater availability of exotic varieties.
They're certainly among the most convenient fruits. Bananas are compact, tidy, bundles of potassium and deliciousness. And when we says "bananas," what we really mean are Cavendish bananas, an edible, cultivated subgroup of the fruit. Ninety-nine percent of the bananas sold in the world are seedless Cavendish bananas, although in the wild there are over a thousand strains of bananas, many of which are unsuitable for eating. Unfortunately, Cavendish bananas are about to go extinct.
Cavendish clones and T4
Cavendish bananas are pretty much genetically identical — they're all sterile clones from the fruit of a single English tree, grown in 1834 by William Cavendish, the 6th Duke of Devonshire, in his greenhouse. As such, they're all vulnerable to the same threats. What's killing them now is a soil-borne fungus, Fusarium oxysporum f.sp. cubense (Foc), also known as Fusarium Wilt Tropical Race 4 (TR4). It kills a bananas by infecting its root and vascular system, rendering it unable to take in critical minerals and water.
TR4 first began ruining Cavendish bananas in Malaysia and Indonesia around 1990, and has since made its way though Southeast Asia and to the Middle East and Africa. Last year, it reached the world's main source of bananas, Latin America, prompting Colombia to declare a "phytosanitary emergency."
Growers are doing what they can to beat back T4's advance — including using clean planting materials so as to avoid spreading soil contamination — and Australia has shown some success in slowing down the assault. However, these are stopgap efforts that are ultimately unlikely to save the Cavendish.
Not Fusarium oxysporum’s first rodeo
This is not the banana industry's first encounter with this fungus. Up to the 1960s, the world's most popular edible banana was the Gros Michel, or "Big Mike," variety. To meet worldwide demand, growers got into the Gros Michel monoculture business big-time, with thousands of tropical-forest hectares converted into massive plantations growing these bananas.
What spelled doom for the Gros Michel banana was, yes, Fusarium oxysporum — the disease it caused was known as "Fusarium Wilt," or "Panama Wilt." It was the T1 version of today's T4, and it largely wiped out the Gros Michel banana, nearly taking the entire banana industry down with it. (You can still find a Gros Michel banana, but it's not easy.)
The Cavendish didn't quite have Gros Michel's rich taste but it wasn't vulnerable to T1, and so it took the place of the Gros Michel as the world's main edible banana.
Not the first rodeo for the Cavendish
The Cavendish is also susceptible to another fungal invader via a disease called "Black Sigatoka." This fungus, Pseudocercospora fijiensis, destroys the plants' leaves, producing cell death that damages the plants' ability to photosynthesize. If left uncontrolled, crop yields can be reduced by 35 to 50 percent.
Growers are fighting back with continual leaf trimming and the liberal use of fungicides — more than 50 applications of the toxic chemicals may be required each year to bring Black Sigatoka under control. This is, of course, harmful to workers managing the crops and to the environment, and makes growing Cavendish bananas less profitable. If this weren't bad enough, repeated applications of fungicides end up strengthening the fungus and making it even harder to control by selecting for mutations that can withstand the chemicals.
Fixing the Cavendish?
There is one field full of healthy Cavendish bananas and T4, however. It's in the town of Humpty Doo in Australia. In 2012, researchers led by James Dale of Queensland University of Technology inserted a gene from a wild banana into their Cavendish bananas and that did the trick. "That trial ran for three years, and had four lines, which showed very high resistance," said Dale. "Now we've planted a much larger trial with new lines and seen exactly the same results. We've essentially got Cavendish that are highly resistant or immune to TR4."
Others are looking into re-jiggering the banana's own genome to switch off its vulnerability to the fungus.
Hervé Vanderschuren at KU Leuven in Belgium is seeing if he can use CRISPR-cas9 to activate a gene similar to the one inserted into the Humpty Doo crop: "You can take this gene and transfer it to Cavendish. Another option is to say, okay, in the Cavendish I see the same gene, but it doesn't have the right mutations. Let's manipulate the gene to one that confers resistance. It's a kind of speed evolution."
Leena Tripathi of the International Institute of Tropical Agriculture has been thinking more subtractively. She has knocked out the genes that T4 affects: "It's easier to do that than turn on a resistance gene." She actually has a batch of T4-resistant Cavendish bananas ready for testing, but since there's no T4 in Kenya where she works, they'll have to be sent abroad to be tested.
Still, would you eat a genetically modified banana? Many wouldn't, and it's therefore unlikely that genetically modified Cavendish bananas will reach the worldwide market in time, especially considering the extended testing for safety that various governments would require.
Even so, scientists see genetic modification of bananas as being an avenue worthy of pursuit for the long term. Says Tripathi, "I hope we can bring back the Gros Michel. I like it better than Cavendish."
Hope for the banana-lover
Some are already looking beyond the Cavendish to the development of even better bananas.
Dale is part of the team developing the so-called "golden banana," a fruit packed with Vitamin A that could help reduce world hunger. We've written about this intriguing project before.
In addition, Dale says the world is full of banana varieties, and he envisions a world in which we'll be able to select from among the most delicious. In particular, he's looking forward to the appearance in supermarkets of Peru's popular Isla banana, as well as Blue Java bananas that taste like ice cream. "I think more bananas will be available," he says. "In 10 years' time, it's going to be a very different market."
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