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|Common names||Panama disease of banana
vascular wilt of banana
|Causal agents||Fusarium oxysporum|
Panama disease is a plant disease of the roots of banana plants. It is a type of Fusarium wilt, caused by the fungal pathogen Fusarium oxysporum. The pathogen is resistant to fungicide and cannot be controlled chemically.
During the 1950s, Panama disease wiped out most commercial Gros Michel banana production. The Gros Michel banana was the dominant cultivar of bananas, and the blight inflicted enormous costs and forced producers to switch to other, disease-resistant cultivars. New strains of Panama disease currently threaten the production of today's most popular cultivar, Cavendish.
Causes and symptoms
The disease is caused by the Fusarium oxysporum fungus. The fungus enters the plant’s roots and spreads through the plant's xylem vessels. The disease causes significant damage. The fungus disrupts the plant's vascular system, which causes the leaves to turn yellow and wilt, and the plant eventually dies from dehydration.
Modern banana plants are reproduced asexually, through replanting the plant's basal shoot that grows after the original plant has been cut down. The fruit contains no seeds, nor does the male flower produce pollen suitable for pollination, prohibiting sexual reproduction. This causes all bananas of a single breed to be nearly genetically identical. The fungus is able to spread easily from plant to plant as the individual plants' defenses are near identical.
The fungus is transmitted through soil and water, and can be spread by people, by dirt on shoes, tires on trucks, shipping containers or other infected equipment, rain, floods and run-off water. It can live dormant in soil for about 30 years.
Gros Michel devastation era
Gros Michel was the only type of banana eaten in the United States from the late 19th century until after World War II. From the beginning,[when?] however, a serious disease was present in the banana plantations of Central America. The problem was first diagnosed in Panama after which it was named. Over several decades, the fungus spread from Panama to neighboring countries, moving north through Costa Rica to Guatemala and south into Colombia and Ecuador.
The banana industry was in a serious crisis, so a new banana thought to be immune to Panama disease was found and adopted, the Cavendish. In a few years, the devastated plantations resumed business as usual, and the transition went smoothly in the American market. Shortly thereafter, Malaysia entered the banana-growing business. Cavendish banana plantations were new to that country in the 1980s, but they rapidly expanded to meet the demand. Thousands of acres of rain forests and former palm oil plantations were shifted to banana production. Within a few years, though, the new plants began to die. It took several years to identify the problem. They found it to be Panama disease, although the Cavendish was then thought to be immune, but was immune only to the strain of the fungus that destroyed the Gros Michel. The version that annihilated the Gros Michel was found in only the Western Hemisphere, but the version found in Malaysian soil was different, and the Cavendish is susceptible to it. It killed and spread faster, inspiring more panic than its earlier counterpart in Panama. The newly discovered strain of F. oxysporum was named tropical race 4(TR4).
Cavendish devastation era
The pathogen affects banana crops worldwide. It has spread to Pakistan, the Philippines, and Indonesia. It is on the rise in Africa and Australia. It has yet to arrive in Latin America, but no barrier to its spread there has yet been identified. Scientists are trying to modify the banana plant to make it resist Panama disease and many other serious banana afflictions ranging from fungal, bacterial, and viral infections to burrowing worms and beetles. Researchers are combing remote jungles searching for new wild bananas. Hybrid bananas are being created in the hope of generating a new variety with strong resistance to diseases. Genetic engineers are adding genes from altogether different fruits, vegetables, and even fish. Some believe the best hope for a more resilient banana is through genetic engineering. However, the resulting fruit also needs to taste good, ripen in a predictable amount of time, travel long distances undamaged, and be easy to grow in great quantities. Currently, no breed or hybrid meets all of these criteria.
Presently the TR4 disease is not in the Americas or western Africa. Researchers are attempting to develop a disease-resistant banana.
In March 2015 Latin America growers met to create a regional defense effort and planned to meet again in September or October of that year. No specific regional measures are in place. Ecuador growers requested the government to fumigate all containers.
In Queensland, a farm in Tully, 800 miles north of Brisbane, was quarantined and some plants were destroyed after TR4 was detected on March 3, 2015. After an initial shutdown of the infected farm, truckloads of fruit left in April with harvesting allowed to resume under strict biosecurity arrangements. The government says it is not feasible to eradicate the fungus. Researchers like Wageningen’s Kema say the disease will continue to spread, despite efforts to contain it, as long as susceptible varieties are being grown.
Banana breeding impeded by triploidy
One major impediment in breeding bananas is polyploidy; Gros Michel and Cavendish bananas are triploid and thus attempts at meiosis in the plant's ovules cannot produce a viable gamete. Only rarely does the first reduction division in meiosis in the plants' flowers tidily fail completely, resulting in a euploid triploid ovule, which can be fertilized by normal haploid pollen from a diploid banana variety; a whole stem of bananas would contain only a few seeds and sometimes none. As a result, the resulting new banana variety is tetraploid, and thus contains seeds, and the market for bananas is not accustomed to bananas with seeds.
Second-generation breeding using those new tetraploids as both parents has tended not to yield such good results, because the first generation contains the Gros Michel triploid gene set intact (plus possibly useful features in the added fourth chromosome set), but in the second generation, the Gros Michel gene set has been broken up by meiosis.
The Honduras Foundation for Agricultural Research cultivates several varieties of the Gros Michel. They have succeeded in producing a few seeds by hand-pollinating the flowers with pollen from diploid seeded bananas.
- Black sigatoka (a leaf-spot disease of banana plants caused by the ascomycete fungus Mycosphaerella fijiensis (Morelet))
- Burton, Reg (2015-03-04). "Panama disease threatens NQ bananas". Fairfax Media.
- Fonseca Rendeiro, Mark (2012-06-06). "Science Ignored – How Bananas Took Over the World". United Academics.
- Sedgman, Phoebe. "There Might Be No Saving the World's Top Banana". Bloomberg.com. Retrieved 2015-06-06.
- Carla Helfferich (1990). "Battling for Bananas". Alaska Science Forum. Retrieved 2008-06-02.
- Go Bananas : New Scientist, pp 38–41 20 April 2013