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. The pathogen is resistant to fungicides, and its control is limited to phytosanitary measures.Ploetz, R. C. (2015). "Fusarium Wilt of Banana." Phytopathology 105(12): 1512-1521.
During the 1950s, an outbreak of Panama disease almost wiped out 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. Since the 2010s, a new outbreak of Panama disease caused by the strain Tropical Race 4 (TR4) has threatened the production of the Cavendish banana, today's most popular cultivar.
Overview Although fruits of the wild banana plants (Musa spp.) have large, hard seeds, most edible bananas are seedless. Banana plants are therefore propagated asexually from offshoots. Because these rhizomes are usually free of symptoms even when the plant is infected by Fusarium oxysporum f.sp. cubense (Foc), they are a common means by which this pathogen is disseminated. It can also be spread in soil and running water, on farm implements or machinery.
Panama disease is one of the most destructive plant diseases of modern times. It is believed to have originated in Southeast Asia and was first reported in Australia in 1876. The disease did not attract widespread attention until the early 20th century when it began heavily affecting banana cultivations in Panama and Costa Rica. By 1950 it had spread to all the banana-producing regions of the world with the exception of some islands in the South Pacific, the Mediterranean, Melanesia and Somalia.
Panama disease affects a wide range of banana cultivars; however, it is best known for the damage it caused to a single cultivar in the early export plantations. Before 1960, a total reliance was put on the cultivar 'Gros Michel', and it supplied almost all the export trade. It proved susceptible to the disease, and the use of infected rhizomes to establish new plantations caused widespread and severe losses. Some indication of the scale of the losses is demonstrated by the eradication of production on of plantation in the Ulua Valley of Honduras between 1940 and 1960. In Suriname, an operation of was out of business within eight years, and in the Quepos area of Costa Rica, were destroyed in 12 years. Overall fungal diseases—including most prominently Foc—are disproportionately important to small island developing states.
By the middle of the 20th century, resistant cultivars in the 'Cavendish' subgroup were being used as a substitute for 'Gros Michel' in the export trade. For a time these cultivars did prove resilient and grew well, and in some areas remain the clones on which the current export trade is based. However, in several growing areas in the Eastern Hemisphere, these cultivars are falling to TR4. This poses a significant threat to production, because there are currently no acceptable replacement cultivars. It is anticipated by experts that disease surveillance, integrated pest management, breeding of resistant cultivars, and genetic engineering will yield worthwhile results. This conclusion comes from an economic analysis which examined these as investments which governments and international organizations may or may not choose to invest in. Their opinion was that there would be positive yield from these investments, taken as either net present value or the internal rate of return.
A variant cultivar called Formosana (GCTCV-218), a type of Taiwanese Cavendish, has been reported in the popular press as having some resistance to TR4, as well as by Molina et al 2009, although in 2015 Ploetz was of the opinion that it has been evaluated as having no resistance at all. 'GCTCV-119' is another Taiwanese Cavendish used commercially for its TR4 resistance.
The TR4 variant also affects plantains (Musa acuminata × balbisiana) which are an important staple food in tropical regions of the world. Apart from the export trade, 85% of banana production is for local consumption, and many of the cultivars used for this purpose are also susceptible to infection.
Distribution Not all banana-producing countries have been affected by the outbreak of Panama disease. TR4 was first identified in Taiwan,Ploetz, R. C. (2006). "Panama disease, an old nemesis rears its ugly head: part 2, the cavendish era and beyond." Plant Health Progress: 1-17. and from there rapidly spread to Indonesia, China, Malaysia, Australia and the Philippines.Ordonez, N., M. F. Seidl, C. Waalwijk, A. Drenth, A. Kilian, B. P. Thomma, R. C. Ploetz and G. H. Kema (2015). "Worse comes to worst: bananas and Panama disease—when plant and pathogen clones meet." PLoS pathogens 11(11): e1005197. The disease was then identified in Jordan in 2013.Garcia-Bastidas, F., N. Ordonez, J. Konkol, M. Al-Qasim, Z. Naser, M. Abdelwali, N. Salem, C. Waalwijk, R. C. Ploetz and G. H. J. Kema (2014). "First Report of Fusarium oxysporum f. sp cubense Tropical Race 4 Associated with Panama Disease of Banana outside Southeast Asia." Plant Disease 98(5): 694-694. TR4 later spread to VietnamHung, T. N., N. Q. Hung, D. Mostert, A. Viljoen, C.-P. Chao and A. Molina (2017). "First report of Fusarium wilt on Cavendish bananas, caused by Fusarium oxysporum f. sp. cubense tropical race 4 (VCG 01213/16), in Vietnam." Plant Disease: PDIS-08-17-1140-PDN. and Laos,Chittarath, K., D. Mostert, K. S. Crew, A. Viljoen, G. Kong, A. Molina and J. E. Thomas (2017). "First report of Fusarium oxysporum f. sp. cubense tropical race 4 (VCG 01213/16) associated with Cavendish bananas in Laos." Plant Disease: PDIS-08-17-1197-PDN. as well as to the Middle East being reported in Pakistan and Lebanon.Ordonez, L. N., F. Garcia-Bastidas, H. B. Laghari, M. Y. Akkary, E. N. Harfouche, B. N. al Awar and G. H. J. Kema (2016). "First Report of Fusarium oxysporum f. sp cubense Tropical Race 4 Causing Panama Disease in Cavendish Bananas in Pakistan and Lebanon." Plant Disease 100(1): 209-210. In 2015, the disease then spread to Africa, being informally announced in Mozambique and Oman. In August 2019, TR4 arrived in Colombia.
Symptoms
Infection by Foc triggers the self-defense mechanisms of the host plant causing the secretion of a gel. This is followed by the formation of tylose in the vascular vessels which blocks the movement of water and nutrients to the upper parts of the plant. The infection begins at the tips of the feeder roots and then moves on to the rhizome. Sign of the disease is most noticeable as a dark stain where the stele joins the cortex. As the disease develops, large portions of the xylem turn a reddish-brown colour. Externally, the oldest leaves start turning yellow and there is often a longitudinal splitting of the lower part of the outer leaf sheaths on the pseudostem. The leaves begin to wilt and may buckle at the base of the petiole. As the disease progresses, younger leaves are affected, turn yellow and crumple, and the whole canopy begins to consist of dead or dying leaves. The leaf symptoms of Fusarium wilt can be confused with those of Xanthomonas wilt. In plants affected by Fusarium, yellowing and wilting of the leaves typically progresses from the older to the younger leaves. The wilted leaves may also snap at the petiole and hang down the pseudostem. In plants affected by Xanthomonas, the wilting can begin with any leaf and the infected leaves tend to snap along the leaf blade.
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
Foc is a member of the Fusarium oxysporum species complex, a group of ascomycete fungi with morphological similarities.Ploetz, R. C. (2006). "Fusarium wilt of banana is caused by several pathogens referred to as Fusarium oxysporum f. sp. cubense." Phytopathology 96(6): 653-656.Snyder, W. C. and H. Hansen (1940). "The species concept in Fusarium." American Journal of Botany: 64-
67. 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.).Baayen, R. P., K. O'Donnell, P. J. M. Bonants, E. Cigelnik, L. Kroon, E. J. A. Roebroeck and C. Waalwijk (2000). "Gene genealogies and AFLP analyses in the Fusarium oxysporum complex identify monophyletic and nonmonophyletic formae speciales causing wilt and rot disease." Phytopathology 90(8): 891-900. Foc mainly infects banana (Musa) species. The special form cubense has been subdivided into four different races, which 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 and has been renamed 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.Ploetz, R. C. (2005). "Panama disease, an old nemesis rears its ugly head: part 1, the beginnings of the banana export trades." Plant Health Progress(December): 1-10.
Tropical Race 1/TR1
/ is also found in Paspalum fasciculatum, Panicum purpurescens, Ixophorus unisetus, and Commelina diffusa in Central America. These weeds may be acting as an inoculum source. , TR1 is found in Queensland.
Tropical Race 2/TR2
, / is found in Queensland.
Tropical Race 3/TR3
/ is a pest of Heliconia ornamental flowers. Formerly reported to be a lesser pest of Musa balbisiana seedlings and of Gros Michel, but that is no longer thought to be true. Now renamed Fusarium oxysporum f. sp. heliconiae.
Tropical Race 4/TR4
/ belongs to vegetative compatibility group 01213/16. All cultivars which are susceptible to Race 1 and Race2 are susceptible to TR4 (see and ).
Subtropical Race 4/STR4