Photos: Owen / Mainstream
Bananas may soon be only a memory for consumers around the globe. The arrival of a deadly fungal banana disease in the Americas led Colombia to declare a state of emergency in August, according the Colombian Ministry of Agriculture.
Bananas are the most popular fruit in the United States with annual banana consumption more than doubling that of apples and oranges combined, according to U. N. statistics. But the United States favorite fruit snack may disappear entirely because no treatment for the disease has been found; fungicides are ineffective. The fungal disease has caused widespread destruction on banana plantations in Asia, Africa, and Australia, and its arrival in Colombia signals a similar fate for banana production all across the Americas.
The fungal disease has spread across about 420 acres of land in La Guajira, a province in northeastern Colombia. Bananas are Colombia’s third-largest agricultural export, and police, military and government resources are being deployed to contain the disease and prevent its spread into surrounding areas. Four farms in La Guijira have been quarantined, but scientists suspect that the fungus has already spread into a wider area.
The difficulty with containing the fungus, known as TR4, is that it can survive in the form of spores in the soil for up to several decades. Not only does this make it extremely hard to eradicate, but also makes its spread hard to contain. Infected soil stuck on work boots, tools, equipment tires, or garbage can be a vector for spreading the fungus. “Soil is very difficult to contain. Who knows how many cars and people have entered that [infected] farm and carried [the fungus] elsewhere?” said Fernando Garcia-Bastidas, a plant pathologist who led the TR4 testing confirming TR4’s presence in Colombia, in an interview on nature.com.
The difficulty in combatting the disease lies in TR4’s resistance to fungicide treatment. This resistance has led producers in Asia and Africa, where banana producers have been battling TR4 for years, to adopt the practice of moving production into fresh, unaffected areas when the TR4 infection renders a current banana plantation unprofitable.
In Colombia, workers have been uprooting and removing infected plants, and using chemicals to kill plants within a 60-foot radius surrounding any confirmed infection. The soil is then covered with a plastic sheet that raises the soil temperature and helps prevent the fungus from spreading.
The eradication and containment efforts are only a temporary solution, however. “We will continue to work towards stopping this disease from spreading to the rest of Colombia, but eventually we have to transition to other varieties of banana that will resist this disease,” said Colombian Agricultural Minister Andres Valencia in an Associated Press interview.(Columbian article)
This is not the first time the global banana industry has faced a devastating epidemic. In the 1950s the Gros Michel, which was the main commercially produced banana variety at that time, was decimated by a similar fungal disease to TR4. The Gros Michel was then replaced by the Cavendish banana variety which now forms the vast majority of commercially marketed bananas.
The Gros Michel and Cavendish are both suited to commercial production because they ripen slowly and ship well, key qualities for a product in international markets. The Cavendish banana currently comprises 99% of global banana shipments, and there is no obvious variety for the next transition as the TR4 infection renders Cavendish production no longer viable.
The Cavendish cannot be bred for TR4 resistance because it is a sterile variety and reproduced by cloning. Thus, two options are being explored in preparation for the continued spread of TR4: genetic modification of the Cavendish and the marketing of new, different varieties of banana in place of the Cavendish. But, marketing a new variety would present challenges in shipping, and consumers would have to be sold on a banana with different qualities of texture and taste than the Cavendish.
Plant geneticists are currently working on two different options for genetically altering the Cavendish for TR4 resistance: transgenic bananas and gene edited bananas. “Transgenic [research] is taking a gene from one organism and putting in another gene,” said Bryan Benz, Associate Professor of Science at Umpqua Community College. “They are looking for a resistant banana variety that they can take the gene from… and then transplant it [the gene] into the Cavendish to make it resistant to the fungus.”
Biotechnologist James Dale and his team have conducted several experiments in making a transgenic banana. The most important involved taking a gene from a wild banana variety that confers resistance to TR4 infection and inserting it into the Cavendish genome.
The team is currently conducting a larger field trial for their transgenic Cavendish and reports early results showing resistance to TR4 infection. The transgenic bananas generated mild interest at first, “But then Colombia declared a national emergency and now the amount of interest is through the roof,” said Dale in an interview on nature.com.
Gene editing involves using technology called CRISPR to modify a genome without inserting genetic material from another organism. “If they [researchers] know there is a mutation in a certain area, they can cut that mutated section out and then replace it with the correct sequence,” said Benz. “[researchers] can also remove genes using CRISPR.”
Dale’s Australian team is working on a project to turn on a dormant gene in the Cavendish genome that might confer TR4 resistance to the Cavendish, and they are not alone. Another team lead by Leena Tripathi from the International Institute of Tropical Agriculture in Nairobi, is using CRISPR to try and suppress specific genes in the Cavendish genome that contribute to TR4 infection vulnerability.
Research groups in the Philippines and U. K. are also working on editing the Cavendish genome to enhance TR4 resistance. However, it will be years before a genetically modified banana will be available for commercial production, according to an article published on nature.com.(also Nature CRISPR article)
The banana industry’s current policy is to slow the spread of TR4 to buy time for developing new alternatives for the doomed Cavendish, but the future remains uncertain.
“These epidemics develop slowly, so the [spread of TR4] will take some time,” said Randy Ploetz, a University of Florida plant pathologist in an interview for nature.com.(Alarm article)“But eventually, it will no longer be possible to produce Cavendish for international trade.”