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The world’s bananas are under attack

Popular Science logo Popular Science 14/09/2017 Kate Baggaley

bananas © Flickr user MICOLO J Thanx bananas Bananas are threatened by a deadly fungus, among other ailments.

Bananas are serious business. Around the world, roughly 145 million tons of bananas are grown each year. In some areas, they are a staple food; in the East African highlands, people consume them several times a day. In these areas, annual banana consumption can go as high as 400 pounds per capita.

Yet bananas are also being threatened by a number of diseases, and could suffer more as climate change encourages those pathogens to spread. Banana fields around the world are under siege from a deadly fungus, while a bacterial disease is devastating plantations in East Africa. These most troubling ailments cause any banana plant they touch to wilt and die.

Researchers worry that these fruits are in deep trouble. But don’t panic yet—bananas are far from doomed. Scientists are working on ways to contain the pestilences, and create new varieties that have a better shot at surviving. “It’s not completely hopeless,” says Charles Staver, an agricultural scientist at Bioversity International who is based in Montpelier, France.

A plague of plagues

The kind of banana found in most global supermarkets is called the Cavendish. Many consider it to be blander and less tasty than the variety we used to dine on, the Gros Michel. Unfortunately, most of these succulent bananas were wiped out in the mid-20th Century by an ailment called Panama disease (although they are still grown locally today).

The Cavendish now accounts for 99 percent of banana exports, a position it climbed to thanks to its resilience against the fungus that causes Panama disease. But there are still many other types of bananas out there, including cooking bananas that aren’t eaten raw. In parts of Uganda, Tanzania, Rwanda, Burundi and Eastern Congo, people farm bananas that aren’t grown anyplace else, Staver says.

Unfortunately, a new and virulent strain of the fungus behind Panama disease is now on the loose. Cavendish bananas are very vulnerable to it; scientists aren’t sure yet how many other varieties are susceptible. Known as Tropical race 4, it’s the most critical menace to bananas globally.

This illness, caused by the Fusarium fungus, was first spotted in Taiwan in 1967. In recent years, however, it has spread alarmingly. The fungus has swept through Asia and Africa, with devastating impacts on the Chinese banana industry, Staver says. Australia has particularly rigorous quarantine procedures, but the disease has appeared on several farms there as well.

The fungus invades a plant by infecting its roots and then moving up through the xylem, the tissue that transports water and nutrients. With its xylem blocked up, the plant wilts and dies. Fusarium is sneaky, too—the spores can linger in the soil for decades, waiting to infect new banana plants. If unchecked, Tropical Race 4 could kill 80 percent of the bananas grown each year, researchers have predicted.

Panama disease © Charles Staver Panama disease For all its ferocity, Tropical Race 4 is not the only disease battering bananas. Banana Xanthomonas wilt, a lethal bacterial infection, causes bananas to rot and seep “cream to pale yellow bacterial ooze” as the plant dies. Like Fusarium, the bacteria clog the plant’s vascular tissue so it can’t get nutrients. This bacterial wilt was contained in Ethiopia until 2001, but has now spread across East Africa, striking Uganda and DR Congo particularly hard.

About one third of the world’s bananas are grown in Africa, and roughly 40 percent of these come from East Africa. In these countries, where most of the fruits are grown by smallholder farmers, bacterial wilt is the most pressing concern. Carried by insects and infected tools, the bacteria spread more quickly than the fungus. And as far as researchers know, bacterial wilt can infect—and invariably kill—any type of banana.

“As soon as one plant gets infected in the field, in few months time the whole field gets wiped out,” says Leena Tripathi, a plant biotechnologist at the International Institute of Tropical Agriculture, Nairobi, Kenya. “It’s devastating the livelihoods of millions of farmers who depend on bananas.”

Yet another hazard is banana bunchy top virus, which is spread by infected planting materials and by banana aphids as they snack on the plants. It’s common in parts of Africa and Asia. This disease also affects all types of bananas, but some can survive it better than others. It turns leaves stunted and “bunched” in appearance.

Banana bunchy top virus has been around for decades, but only recently are we seeing more serious losses and recognizing how much of a threat it is; it now seems to be accelerating, Staver says. This is particularly worrisome through the Congo Basin. In this area, farmers could lose their banana crops and have to switch to others like cassava. “They’re not going to starve to death, but they’re going to lose a major component of their diet,” he says.

In parts of Central Africa, farms can be devastated by bunchy top virus, and lack support from research organizations and other services to help them cope. “Villages end up without bananas,” Staver says. “They have no support from anywhere, nobody’s paying attention.”

Another major disease is black sigatoka, a leaf-attacking fungal ailment found everywhere. Bananas are also harassed by pests like nematodes and weevils. Climate change is expected to impact bananas, too.

The good news is that bananas are pretty hardy, at least when it comes to this last threat; we grow them in subtropical and tropical conditions around the world, ranging from South Africa to Turkey. If temperatures rise, bananas should still thrive. But as extreme events like storms and droughts become more intense, individual plantations will likely get clobbered, Staver says. And warmer temperatures could encourage some of the pathogens that infect bananas to grow more aggressively, Tripathi explains, so disease might spread faster.

Hope for bananas

But there is some heartening news for bananas: The new strain of Panama disease has yet to reach Latin America. And scientists in Taiwan have managed to grow Cavendish bananas that are somewhat resilient against the ailment, continuing to produce fruit longer in the presence of the fungus. These cultivars are grown commercially in the Philippines, Staver says.

We could also screen the bacteria and fungi that grow naturally on bananas and identify ones that can protect the plant from infection. Then we could gift the plants with more of these helpful microbes. This has worked for strawberries and tomatoes in the past; the microbes are commercially available. Researchers have identified some promising bacteria for the bananas, too, Staver says, but this work is very preliminary.

Other researchers are modifying bananas to withstand these deadly foes, although this technique too is only in the first stages of testing. Tripathi and her colleagues are genetically engineering bananas that can withstand bacterial wilt and nematodes. Because all varieties of banana are vulnerable to bacterial wilt, conventional breeding can’t be used to develop resistant varieties. They have identified two genes in sweet peppers that are related to disease resistance. They boost the so-called hypersensitive response, which plants use to fight off disease. When the plant detects a pathogen, it kills its own cells that have come in contact with the germ. These patches of dead cells form a physical barrier that the pathogen can’t penetrate, preventing it from spreading further in the plant. The team has tested bananas that carry a pepper gene in carefully isolated fields in Uganda and found them completely resistant to bacterial wilt, Tripathi says. None of the plants even showed symptoms of illness. If these bananas had been regular, non-edited plants, each of them would have died.

banana plants © Leena Tripathi banana plants After working with two banana cultivars called the Sukali Ndiizi and Nakinyika, the scientists are now working on transferring the technology to the Cavendish and other bananas. In future, they plan to engineer bananas that carry both helpful pepper genes. Their goal is to create bananas that smallholder farmers can plant and that will be the same price as non-edited bananas, and look and taste the same. They hope to have the transgenic fruit on the market by 2022.

In the meantime, there are ways to help restrict the diseases from spreading, such as plucking the male flowers, which attract disease-carrying insects, early in a banana plant’s development. Farmers can also clean tools between uses in the fields and use clean planting materials. And the germs that cause bacterial wilt can’t survive in the soil for very long, so some farmers have been able to replant their crops, Tripathi says.

Another, drastic option would be to switch the bananas we depend on once again, moving from the Cavendish to a new cultivar. But we’re not at the point of needing to worry about that yet, Staver says. We’re not about to lose our bananas, or even just the imperiled Cavendish, to any of these diseases. “It’s a huge threat, but its not immediate,” he says. “Is it likely that bananas are going to go up in price in the supermarket in the next couple years? Probably not.”

Tropical Race 4 has been present in Taiwan for decades, yet Cavendish bananas are still grown there, Staver points out, albeit fewer than before the new strain of the fungus appeared.So bananas are facing very serious threats that must be addressed, and small farms in particular are in danger of having their crops devastated. But bananas are not doomed, or about to disappear from the face of the Earth. “Oh, I don’t believe that one, that the banana will go extinct,” Tripathi says. “That will never happen.”

Related: How to Make Banana-Bacon-Nut Toast (Provided by Cooking Light)

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