Panama disease

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. 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.

thumb|200px|left|[[Gros Michel|alt=Bunches of Gros Michel bananas on a table]]

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.

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. 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. as well as by Molina et al 2009, 'GCTCV-119' is another Taiwanese Cavendish used commercially for its TR4 resistance. 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.

Symptoms

thumb|right|The fungus migrating up through the plant stem|alt=A banana tree cut horizontally to show the fungus development in the interior of the tree

thumb|right|Banana trees wilting|alt=Wilted banana trees due to the fungus

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 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.
With proceeding infection, the banana pseudostem can split, and eventually the whole plant collapses. 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.). 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.

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.

Tropical Race 2/TR2

, / is found in Queensland. Formerly reported to be a lesser pest of Musa balbisiana seedlings and of Gros Michel, but that is no longer thought to be true. / 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

/ is a subtropical race and does not become symptomatic on Cavendish until the trees are stressed by cold. 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 to 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 strain that devastated the Gros Michel banana became widespread in the Western Hemisphere during the 20th century, while genetically distinct strains of the pathogen were present in Southeast Asia, where the disease is believed to have originated; the Cavendish banana is susceptible to some of these strains. 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).

"Banana Sol" incident in Jamaica

In April 1932, amidst the lack of effective treatments, The Daily Gleaner published reports of a new chemical cure called "Banana Sol." The product was marketed by Zenon Ioannon Solomides, a trained entomologist and member of the Linnean Society of London. Prominent planters and merchants lobbied the Jamaican government to invest in the product, despite it violating existing agricultural quarantine policies.

Subsequent investigations by Jamaican agricultural officials revealed that Solomides had a history of marketing ineffective agricultural solutions, including a failed cure for silver leaf disease in England. By May 1932, banana plants in Jamaica that had been treated with "Banana Sol" succumbed to the fungus, proving the cure ineffective. Solomides was declared bankrupt in London in 1936. 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.

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. Ecuadorian 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.

Australian quarantine

A farm in In Tully, Queensland, was quarantined and some plants were destroyed after TR4 was detected in March 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 Gert Kema, based at Wageningen University, Netherlands, 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 (BQ) to impose quarantine conditions.

Outside experts were brought in to review BQ's performance in 2021. Their assessment credits BQ with quick and effective response which is being emulated by other countries. Thus far TR4 continues to be contained to the Tully valley only, and containment is thought to be possible as long as accidental human movement and transport in flowing water can be halted.

Spread to Colombia

In August 2019, authorities in Colombia declared a national emergency after confirming that 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.

The disease is dispersed by spores or infected material that travel in surface water or farming activities. Spread of the disease is exacerbated the method of propagation. Suckers are taken from one plant and clonally propagated to grow new plants. About 30 to 40% of suckers from a diseased plant are infected, but not all show symptoms, so the chance of growing a new, already infected plant is fairly high. The disease is also 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. 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. Injecting the host plants with carbendazim and potassium phosphonate appears to provide some control but results have been inconclusive. Heat treatment of soil has also been tried in the Philippines but the pathogen is likely to reinvade the treated area. Early research into Foc often was conducted by large companies with a financial interest in banana productivity, especially the United Fruit Company and pathologist Frederick Wellman in the 1920s.

Modified bananas developed in collaboration by Ugandan and Belgian scientists were reported in 2008 to be grown experimentally in Uganda. In Australia the movement, sharing, and sale of propagation material is heavily restricted, especially between states, to slow down the combined threat of TR4, Bunchy Top, and leaf spot. The gene is () totally effective against TR4. It is unexpressed in Cavendish but has been found expressed in the diploid M. a. ssp. malaccensis. It may be possible to produce an expressing Cavendish with CRISPR.

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 Foc. Taiwanese researchers believe that the onset of TR4 was linked to soil degradation caused by the use of chemical fertilizers. Hong et al 2020 achieved significant suppression with a chilli pepper/banana rotation. This success is not merely applicable to Musa or even crops in general - it suggests that a similar "rotation" concept in livestock is advisable. The resistance of different banana cultivars to the pathogen is under scrutiny. The FAO provides a sanitation and diagnostic manual.

Timeline of geographic spread

1997

The first detection in Australia occurs near Darwin, Northern Territory.

2005

Chloris inflata, Euphorbia heterophylla, Cyanthillium cinereum, and Tridax procumbens, in banana farms in Australia known to be infected with TR4. Sample taken from a suspect tree on a farm in Mareeba, was questioned after this detection was reported, and is no longer used by BQ.

2018

TR4 is found widely throughout Southeast Asia.
TR4 is found in Laos, Vietnam, Taiwan, Malaysia, Borneo, Indonesia, mainland China, Philippines, Jordan, Mozambique, Pakistan, Lebanon, Oman, India, and North Queensland in Australia. The appearance of TR4 in Peru threatens its of plantations.