Can Science Save Man's Favorite Banana From Extinction?

The Cavendish banana is under threat of a rapidly spreading fungus that's now reached Jordan. Science proposes to circumvent the deadly disease with high-tech.

Yair Israeli

Bananas are among the most popular fruits in the world. Yet scientists are racing to rescue man's favorite variety, the Cavendish, from the threat of extinction. Fusarium banana wilt, also known as Panama disease, first erupted in Southeast Asia, but has since spread to Africa, Australia - and now to Jordan, a mere hop away from Israel's plantations.

Wilt is attacking the very species people like most – the sweet Cavendish variety. This monocrop dominates the global banana trade, a market estimated at $8.9 billion a year, according to figures by the United Nations' Food and Agriculture Organization. Bananas, it also says, are the eighth most important food crop in the world, and the fourth most important in the developing nations. In a call for action last month, the FAO pegged a strain of the wilt “one of the world’s most destructive banana diseases.”

The FAO recommends precautionary measures to prevent further spread of the disease, such as raising awareness, quarantining infected sites and preventing infected soil and planting materials from being moved, mostly because not much else can be done. There is no effective treatment against wilt, let alone the Tropical Race 4 strain of the disease.

Heavy infection right next door

TR4 was discovered in Southeast Asia in the 1990s, as leaves yellowed and wilted, and whole plantations died. In less than a decade it had spread to Australia, China, Indonesia, Malaysia, the Philippines and Taiwan. Most recently it was reported right here in the Middle East.

Asia accounts for 15 percent of all banana exports and Africa for another 4 percent. The fear is that TR4 will reach South America and the Caribbean, which together account for 81 percent of the global banana trade.

A banana plantation completely destroyed within five months of infection. Photo by Yair Israeli

In Jordan, bananas are grown along the Jordan River - the natural border between Israel and Hashemite Kingdom. “This brings us right into the picture,” says Dr. Stanley Freemen, a researcher at Israel's Volcani Center, who is closely monitoring the spread of TR4. The Jordan Valley is one of three areas in Israel where bananas are cultivated.

Though TR4 has yet to be identified in Israel or the Palestinian Authority, Freeman says the proximity of Jordanian and Israeli plantation is unsettling, especially since it seems the disease reached Jordan as early as 2006. According to his figures, 80 percent of banana plantations in Jordan could already be infected.

“We are deeply concerned about the disease possibly spreading from Jordan to Israel, and into the Palestinian Authority,” he says.

In the West Bank, bananas are grown near Jericho. But lack of free access to the area prevents from researchers such as Freeman to assess the situation there. “I’m trying to get permission to go,” he says.

Asexual life: A recipe for disaster

Fusarium is a soil-borne fungus that attacks the roots. Once the soil is infested, it can't be replanted with Cavendish for up to three decades.

How exactly TR4 reached Jordan is a mystery. One possibility is that infected plant material was inadvertently moved there with soil from an infected area in Africa or Asia. Short-distance infections, however, are easier to explain: Bananas propagate by shoots (called pups) that grow from the base of the mother plant. Severing the pups and moving them from field to field is an easy, cheap way of cultivation.

The downside of this traditional method is that when infected plants are moved, TR4 moves with them.

Unlike other plants that reproduce sexually, Cavendish propagation is asexual - the pups are clones of the parent plant. Gene mixing in sexual beings can potentially create resistant strains, but Cavendish clones have exactly the same genes and they've virtually been this way for the past 120 million years. In short, its asexual life makes the Cavendish species more vulnerable to disease.

This wouldn't be the first time wilt has all but wiped out the global banana industry. In the 1950s, the Race 1 variation decimated Gros Michel banana plantations in Central and South America. Gros Michel was then replaced by a cultivar resistant to Race 1 - the Cavendish.

Now the fear is that history will repeat itself.

In Israel, all banana cultivation is done through tissue culture plants in a controlled and sterile environment. Rahan Meristem, a company located in northern Israel, supplies seedlings grown in labs to banana plantations, both local and global, “including in Arab countries in the Middle East,” says Dr. Eli Khayat, Rahan’s science director. The company also develops resistant banana varieties, though not yet to TR4.

Cultivation by tissue culture costs more than replanting pups, but when dealing with large plantations, the traditional method can't keep up anyway.

Another advantage of tissue-cultured plants: they don't transfer soil-borne pathogens such as Panama disease.

Finding resistant strains

Attempts to locate a Cavendish cultivar resistant to TR4 have met with limited success. Khayat, who also teaches at the Hebrew University’s Faculty of Agriculture, hopes genetic engineering can offer a solution. “There are a number of resistant banana variants in nature,” he says. “They'd never be used for food, but the fact that such resistance exists begs the conclusion that it’s possible.”

However, even if a TR4-resistant variety is developed, "resistance to one strain doesn’t promise resistance to another strain,” Khayat points out. Convincing the public that genetically modified bananas are safe would also be an issue, though Khayat expects opposition would die out if there were no alternatives.

But if TR4 reaches Central and South America before a resistant Cavendish variety is found, the world's favorite banana could disappear, says Khayat, at least in the medium-term.

It bears noting that the Gros Michel is not extinct, merely extremely rare. And if the Cavendish is decimated or wiped out, lacking alternatives, people in developing nations who depend on bananas will make do with some less popular variety. As for the West, Khayat surmises, as long as there are Cavendishes to be had, even if they cost more - people won't settle for less than the sweet banana they grew to love.