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|Common names||Panama disease|
Fusarium wilt of banana
Vascular wilt of banana
|Causal agents||Fusarium oxysporum f.sp. cubense|
|Vectors||water, soil residues, replanting of suckers, farming tools and transport, leaf trash|
|Distribution||Indonesia, China, Malaysia, Australia, the Philippines, Jordan, Vietnam, Laos, Pakistan, Lebanon, Mozambique, Oman|
Panama disease (or Fusarium wilt) is a plant disease that infects banana plants (Musa spp.). It is a wilting disease caused by the fungus Fusarium oxysporum f. sp. cubense (Foc). The pathogen is resistant to fungicides and its control is limited to phytosanitary measures.
During the 1950s, an outbreak of Panama disease almost wiped out the commercial Gros Michel banana production. The Gros Michel banana was the dominant cultivar of bananas, and Fusarium wilt inflicted enormous costs and forced producers to switch to other, disease-resistant cultivars. Currently, a new outbreak of Panama disease caused by the strain Tropical Race 4 (TR4) threatens the production of the Cavendish banana, today's most popular cultivar.
Not all banana producing countries have been affected by the new outbreak of Panama disease (TR4) yet. Tropical Race 4 has first been identified in Taiwan and from there rapidly spread through Southeast Asia, conquering Indonesia, China, Malaysia, Australia and the Philippines. Originally thought to be limited to Asia, the disease was then suddenly identified in Jordan in 2013. Foc TR4 expansion then continued to Vietnam and Laos, as well as to the Middle East being reported in Pakistan and Lebanon. In 2015, the disease then spread to Africa, being informally announced in Mozambique and Oman. In August 2019, TR4 arrived in Colombia, a country in Latin America, the region comprising the world's biggest banana exporters.
Two external symptoms help characterize Panama disease of banana:
- Yellow leaf syndrome, the yellowing of the border of the leaves which eventually leads to bending of the petiole.
- Green leaf syndrome, which occurs in certain cultivars, marked by the persistence of the green color of the leaves followed by the bending of the petiole as in yellow leaf syndrome. Internally, the disease is characterized by a vascular discoloration. This begins in the roots and rhizomes with a yellowing that proceeds to a reddish-brown color in the pseudostem, as the pathogen blocks the plant's nutrient and water transport.
- With proceeding infection, the banana pseudostem can split and eventually, the whole plant collapses.
External symptoms often get confused with the symptoms of bacterial wilt of banana, but ways to differentiate between the two diseases include:
- Fusarium wilt proceeds from older to younger leaves, but bacterial wilt is the opposite.
- Fusarium wilt has no symptoms on the growing buds or suckers, no exudates visible within the plant, and no symptoms in the fruit. Bacterial wilt can be characterized by distorted or necrotic buds, bacterial ooze within the plant, and fruit rot and necrosis.
Once a banana plant is infected, recovery is rare, but if it does occur, any new emerging suckers will already be infected and can propagate disease if planted.
Classification and host range
Fusarium oxysporum f. sp. cubense (Foc) is a member of the Fusarium oxysporum species complex, a group of ascomycete fungi with morphological similarities. Based on their different host species, the plant pathogenic fungi of this species complex are divided into approximately 150 special forms (formae specialis, f.sp.). Fusarium oxysprorum f.sp. cubense mainly infects banana (Musa) species. The special form cubense has been subdivided into four different races, that each attack a different group of banana genotypes.
- Race 1 was involved in the 1960s Panama disease outbreak which destroyed much of the Gros Michel banana plantations in Central America. In addition to Gros Michel, Race 1 also attacks other members of the banana AAB genomic group, including Abacá, Maqueño, the Silk subgroup, the Pome subgroup, Pisang Awak, Ducasse, and Lady Finger. Cavendish cultivars are resistant to Race 1.
- Race 2 infects cooking bananas with ABB genome and the Bluggoe subgroup.
- Race 3 infecting Heliconia spp. is no longer considered pathogenic to bananas, but included into the Fusarium oxysporum f.sp. heliconiae.
- Race 4 is the causal agent of the current Panama disease outbreak since it is pathogenic to the currently used Cavendish cultivars (AAA genome). Race 4 is further subdivided into Tropical Race 4 (TR4) and Subtropical Race 4 (STR4). The latter only infects Cavendish and Race 1 and 2 susceptibles under abiotic stress.
Modern commercially farmed banana plants are reproduced asexually, by replanting the plant's basal shoot that grows after the original plant has been cut down. Being triploid, the fruit contains no seeds, and the male flower does not produce pollen suitable for pollination, prohibiting sexual reproduction. This causes all bananas of a single breed to be nearly genetically identical. The fungus easily spreads from plant to plant because the individual plants' defenses are nearly identical.
The disease is dispersed by spores or infected material that travel in surface water or farming activities. One of the biggest issues in spreading the disease is the means by which new banana plants are planted. Suckers are taken from one plant and clonally propagated to grow new trees. About 30 to 40% of suckers from a diseased plant are infected and not all show symptoms, so the chance of growing a new, already infected plant is fairly high. Finally, the disease is known to infect certain weeds without showing symptoms, meaning it can survive in the absence of banana plants and remain undetected in a place where bananas are planted later.
FOC is thought to persist only asexually, as no sexual phase (teleomorph) has been observed. Recombination events may occur via somatic hybridisation and the parasexual cycle.
This means that the survival and dispersal of the disease relies on purely asexual spores and structures. The disease survives in chlamydospores which are released as the plant dies and can survive in the soil for up to 30 years. When the environment is ideal and there are host roots available (fungus is attracted to root exudates), these chlamydospores will germinate and hyphae will penetrate the roots, initiating infection. There is an increase in the number of symptomatic plants when inflorescences emerge and the highest disease incidence occurs right before harvest. Once infected, microconidia are produced and proliferate within the vessels of the plant's vascular system. Macroconidia are another asexual spore that tends to be found on the surface of plants killed by Panama disease. Infection is systemic, moving through the vascular system and causing yellowing and buckling that starts in older leaves and progresses to younger leaves until the entire plant dies.
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. While it took several years to find, the cause was ultimately attributed back to the Panama disease. Although the Cavendish was then thought to be immune, it was immune only to the strain of the fungus that destroyed the Gros Michel. The version that annihilated the Gros Michel was found only in 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).
TR4 devastation era
Tropical Race 4 (TR4) was discovered in Taiwan in 1989. In July 2013, members of OIRSA, a Latin American regional organisation for plant and animal health, produced a contingency plan specific to TR4 for its nine member countries (Belize, Costa Rica, Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nicaragua and Panama), the plan is only available in Spanish. 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.
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 nematodes 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. 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 cultivar or hybrid meets all of these criteria.
In Queensland, a farm in Tully, 1500 km 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. The disease was again detected in Tully in July 2017, prompting Biosecurity Queensland to impose quarantine conditions.
Spread to Colombia
In August 2019, authorities in Colombia declared a national emergency after confirming that the Panama disease had reached Latin America. "Once you see it, it is too late, and it has likely already spread outside that zone without recognition," said one expert quoted by National Geographic.
Currently, fungicides and other chemical and biological control agents have proven fairly unsuccessful, or only successful in vitro or in greenhouses, in the face of Panama disease of bananas. The most commonly used practices include mostly sanitation and quarantine practices to prevent the spread of Panama disease out of infected fields. However, the most effective tool against Panama disease is the development of banana plants resistant to Fusarium oxysporum f. sp. cubense. Unfortunately, the clonal reproduction of banana has led to a consequential lack of other varieties. Efforts are being made to produce resistant varieties, but with bananas being triploids which do not produce seeds, this is not an easy task. Creating clones from tissue cultures, rather than suckers, has proven somewhat successful in breeding resistant varieties, however, these tend to have decreased success in stress-tolerance, yield, or other beneficial traits necessary for commercial varieties. Nevertheless, these efforts are leading to the best control measure for Panama disease of banana.
Recently, an R gene (RGA2) was transformed into Cavendish bananas which showed disease resistance to Fusarium wilt tropical race 4. One specific transformed line, which consisted of 8 plants, showed resistance in the field for all of them. Unfortunately, the field trial lasted only 3 years and the plants exhibited a yield drag. 
Banana breeding impeded by triploidy
One major impediment to 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; 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 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.
- List of banana and plantain diseases
- Black sigatoka (a leaf-spot disease of banana plants caused by the ascomycete fungus Mycosphaerella fijiensis (Morelet))
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