Panama disease

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.

In the 1950s, Panama disease wiped out (except in Asia) the Gros Michel banana, the dominant cultivar of bananas, inflicting enormous costs and forcing producers to switch to other, disease-resistant cultivars. However, new strains of Panama disease threaten the production of today's most popular cultivar, Cavendish.

Susceptibility

The fungus causes significant damage to banana crops due to its virulence as well as the identical genetic composition of modern banana crops. Modern breeds of banana cannot reproduce sexually because they have no seeds and the male flowers do not produce viable pollen. New banana plants develop through asexual reproduction: after the fruiting stem has matured, fruited, and been cut down, the base of the plant produces suckers, which can be cut off and planted elsewhere. This is like taking a cutting from a rose bush; the cuttings have to be moved and planted by humans. As a result, these banana plants are nearly identical genetically, and thus have the same susceptibility to the same diseases. Once a disease, fungus, or pest overcomes the plant's defenses it can quickly infect the whole cultivated area. That happened to the Gros Michel banana. Over several decades the fungus spread from Panama to neighboring countries, moving north through Costa Rica to Guatemala and south into Colombia and Ecuador. By 1960, fifty years after the disease was discovered, the Gros Michel banana was in effect wiped out.

History

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, however, a serious disease was present in the banana plantations of Central America. The problem was first diagnosed in Panama and named after it. Panama disease, which is a fungus (Fusarium oxysporum), is particularly virulent. It is transmitted through soil and water. F. oxysporum can live dormant in soil for about 30 years or until it is stimulated to germinate by a susceptible host. It usually enters through the root system and travels into the xylem vessels. As the fungus disrupts the plant's vascular system, the leaves turn yellow and begin to wilt. After the fungus finishes its cycle the plant eventually dies from dehydration.

The response

The banana industry was in a serious crisis. Only at the last minute, a new banana 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. However, the Cavendish was introduced during a time of increased urbanization and culinary sophistication. By the end of the 1970s, people were demanding a banana that could be transported great distances intact, ready to ripen, and with a consistent taste. Shortly thereafter, Malaysia entered the banana growing picture. 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. But within a few years, the new plants began to die. An unknown pathogen was working its way into the roots, discoloring leaves and choking off the plants' water supply.

It took several years for scientists to identify the problem. They found that it was Panama disease, striking a banana variety that was thought to be immune. It turns out that the Cavendish had not been immune to the affliction, only to the strain of the fungus that destroyed the Gros Michel. The version of the fungus that annihilated the Gros Michel was only found in the Western hemisphere, but the version found in Malaysian soil was different, and was deadly to the Cavendish variety. 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.

Cavendish devastation era

The blight is devastating banana crops worldwide. It has spread to Pakistan, the Philippines, and Indonesia. It is on the rise in Africa and Australia. It is yet to arrive in Latin America, but scientists believe that is only a matter of time. 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 genetic material 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 the correct amount of time, travel long distances undamaged, and be easy to grow in great quantities. Currently there is no banana that meets all of these requirements.

Banana breeding impeded by triploidy

One major impediment in breeding bananas is that, Gros Michel and Cavendish bananas are triploid and thus attempt at meiosis in the plant's ovules produces a non-viable mess. 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. But 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 in.

Experience showed them that where both meiosis steps failed, causing 3+3 + 1 = a heptaploid seedling, or when the seedling is aneuploid, results are not as good.

Second-generation breeding using those new tetraploids as both parents tends not to yield such good results, because the first generation contain the Gros Michel triploid gene set intact (plus hopefully useful features in the added 4th 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.^[1]

See also

• Black sigatoka

References

Bibliography

• Go Bananas : New Scientist, pp 38–41 20 April 2013

External links

• Musapedia page on Fusarium wilt
• Can This Fruit be Saved? (discusses the disease threat to banana crops)