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. 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 30,000 hectares (74,000 acres) of plantation in the Ulua Valley of Honduras between 1940 and 1960. In Suriname, an operation of 4,000 hectares (9,900 acres) was out of business within eight years, and in the Quepos area of Costa Rica, 6,000 hectares (15,000 acres) 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, and from there rapidly spread to Indonesia, China, Malaysia, Australia and the Philippines. The disease was then identified in Jordan in 2013. TR4 later spread 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.
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:
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