September 13, 2011

Panama Disease Threatens Common Banana

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Bananas are America’s favorite fresh fruit. Every year Americans eat more of them than apples and oranges combined. But unlike apples, for which there are Granny Smiths, Red Delicious, Fuji, among others, and oranges, which are members of the citrus family, there is only one banana variation readily found in American markets: the Cavendish.

The Cavendish is the seedless, yellow “dessert type” banana that Americans slice into their morning cereal and adorn their banana splits with. Of the thousands of banana cultivars, or variations, available worldwide the Cavendish is by far the most common. As a result of the vegetative growing techniques that large corporations use to produce the Cavendish, each of the over 100 billion commercial bananas sold annually is a genetically identical clone of one another.  This lack of natural diversity, though beneficial for international marketing, has left the Cavendish vulnerable to species wide disaster—and unfortunately for the billions of people who enjoy them, a tropical threat known as Panama disease is currently devastating banana plantations throughout South Asia and Australia, and threatens to spread to the Americas.

Panama Disease, also called Fusarium wilt, is caused by the fungus Fusarium oxysporum. The fungus originates in the soil and travels up the plant’s vascular system, essentially rotting it from the inside out. A January edition of The New Yorker refers to the disease as the “H.I.V of banana plantations” because it can be easily transmitted from plantation to plantation through contact with infested soil.

“The problem with Fusarium wilt is that it is caused by a fungus infecting the roots of the banana plant, eventually moving into the vascular system and basically plugging it up so that the plant can’t get sufficient water and minerals,” said Dr. Alice Churchill, plant pathology and microbiology. “This causes the leaves to wilt and turn yellow, resulting in reduced photosynthesis and eventual death of the plant,” she said. Churchill’s own research is with a similar fungus caused disease that affects banana plant leaves called Black Sigatoka, also known as “black leaf streak.”

Black Sigatoka, while serious, is successfully controllable with chemical sprayings, unlike Panama disease. Currently placing quarantines around infected areas in South Asia and Australia is one of the few protective methods that scientists can take to prevent the Fusarium wilt from spreading. Though many question the possibility of a banana apocalypse, according to Churchill, such an event is likely to occur because it has happened once before.

“By the end of the first half of the last century, a strain of the Panama disease pathogen, known as Race One, had basically wiped out the ‘Gros Michel’ cultivar, which was the commercial banana grown at that time in Central America,” she said. The Gros Michel was the dessert banana that grandparents enjoyed as kids; it was apparently bigger, hardier, and tastier than the Cavendish variety that is eaten now. But the reason why people no longer make banana cream pie out of Gros Michel today is because by the 1960s the cultivar was rendered virtually extinct by the Race One form of Panama disease. This was the time when the Cavendish replaced it as the globally produced commercial banana because it was found to be resistant to Race One.  But now a new strain of the disease, Tropical Race Four, has appeared in South Asia—and this time the Cavendish is not immune.

“If disseminated widely, Tropical Race Four would affect approximately 85 percent of banana production worldwide,” warns Churchill. “So if, or when, it comes to this part of the world, not only will it kill Cavendish banana, which is typically what most of us eat, in the developed countries, it will destroy other cultivars as well, including many types of cooking bananas.” She explained that almost 90% of bananas worldwide are grown for local consumption and over half a billion people in places like Africa and Asia depend on them as a staple food.

“In the United States we eat on average 33 pounds of bananas per year, but in Uganda and other countries in east Africa they eat almost 550 pounds of bananas in some form each year” she said. “This disease is a problem, not only because of its potential impact on the price and availability of our favorite fruit, but also because it’s a life changing event for the people in developing countries who rely on bananas as a staple food and incomes. Those affected by Fusarium wilt lose both their livelihoods and an important source of nutrition.”

Most banana scientists agree that it’s only a matter of time before tropical Race Four of Panama disease makes it to this part of the world. “All it takes is one person with [infested] soil on their boots to inadvertently introduce it into Central or South America, the source of bananas for North America.”

To defend against the upcoming outbreak, some scientists have turned toward genetic modification in order to build a better banana. There are efforts to sequence the genome of resistant banana cultivars, identify the genes that give them resistance and then transfer that gene into the susceptible Cavendish bananas. Another possible solution to the banana blight would be the commercial production of a different banana cultivar, one that is resistant to Tropical Race Four, which would replace the Cavendish. But the prospects for this alternative are low because many of the current resistant cultivars look and taste much different from the Cavendish and may not find acceptance in the global market.

“My guess is that for the future of the banana, we’ll have something new, but I think it’s likely going to require genetic engineering,” predicted Churchill. “It’s going to require people having a better understanding of why genetic engineering may be the only means for continued export production of this sterile crop and where we may need to compromise to have continued easy access to America’s favorite fruit. Otherwise, bananas probably won’t be as readily available on our tables as they are now.”

Original Author: Nicholas St. Fleur