Tropical race 4
Tropical race 4 (TR4) is the name given to the strain of the fungus Fusarium oxysporum f. sp. cubense (Foc) that causes Fusarium wilt (aka Panama disease) in Cavendish cultivars. The term TR4 was coined to distinguish this race 4 strain from the ones that affect Cavendish cultivars only in the presence of predisposing factors such as low temperatures. The latter are known as subtropical race 4 (STR4). TR4 is more specifically associated with a particular vegetative compatibility group called VCG 01213, although other VCGs have also been reported to cause Fusarium wilt in Cavendish cultivars in the absence of predisposing factors. It has spread to most banana-producing countries in Asia and in 2013 was reported to be in Mozambique.
Like the other Foc strains, TR4 originated in Asia. It did not evolve from a strain that overcame the resistance of Cavendish cultivars. It should be noted that TR4 has a wider host range than just Cavendish cultivars. In addition to hitherto unaffected cultivars, such as 'Lakatan' and 'Pisang mas', it also causes disease in groups of cultivars susceptible to races 1 and 2, such as Gros Michel, Silk, Pome and Bluggoe.
Like all other soil-dwelling Foc strains, TR4 cannot be controlled using fungicides and cannot be eradicated from soil using fumigants. The capacity of TR4 to survive decades in the soil, along with its lethal impact and wide host range, are among the main reasons it was ranked as the greatest threat to banana production. To avoid further losses to the pathogen, the United Nations' Food and Agriculture Organization (FAO) has called on banana-producing countries to step up monitoring and reporting, and to contain suspected incursions to prevent the fungus from getting established.
- Host range
- Symptoms and diagnosis
- Modes of transmission
- Control options
- Preventing the spread of TR4
- Efforts to address the threat of TR4
- See also on this website
- Further reading
- External links
In Taiwan, symptoms of Fusarium wilt on Cavendish cultivars were first observed in 1967. In 1972, the results of pathogenicity tests suggested that the isolates belonged to race 1, but isolates tested in 1977 in Taiwan were designated as race 4. In the late 1980s, isolates from Taiwan were the first ones to be designated VCG 01213. The vulnerability of Cavendish cultivars to what would become known asTR4 was underscored in the early 1990s, when Fusarium wilt decimated newly established plantations of Cavendish bananas in Indonesia and Malaysia.
By the end of the 2000s, TR4 had been found in Taiwan, Malaysia, Indonesia (Java, Sumatra, Sulawesi, Halmahera, Kalimantan on the island of Borneo, and Papua Province on the island of New Guinea), mainland China (Guangdong, Hainan, Guangxi, Fujian and Yunnan), the Philippines' island of Mindanao and Australia (Northern Territory).
Since then, the number of first records has increased exponentially. Although not formally published, TR4 seems to have been in Oman since at least 2012. At the same time, it was also present in the Katihar and Purnea districts of Bihar in India, though not formally published. TR4 has since spread to the states of Uttar Pradesh, Madhya Pradesh and Gujarat.
In 2013, TR4 was reported to be in Jordan, the first official report of TR4 outside the Southeast Asia-Pacific region. A 2014 survey revealed another infected area north of the original outbreak.
At the end of 2013, TR4 was also reported to be in Africa, where it was confirmed to be in an export banana plantation located in northern Mozambique. It was later found in a second plantation, also in Nampula province. In 2018, a senior government official said that TR4 had been identified further north in the Cabo Delgado province.
In 2015, it was reported to be in Lebanon and Pakistan, as well as Queensland in Australia. An analysis of isolates from Pakistan and the Phillipines showed that they were closely related. The incursion in Lebanon has also been shown to be associated with the one in Jordan.
In 2016, TR4 was found in two farms in Israel. After taking measures to contain the two outbreaks, the National Plant Protection Organization declared in 2018 that the fungal strain had been eradicated from Israel. The declaration drew comments that TR4 had been contained rather than eradicated.
In 2018, it was confirmed to have spread to Myanmar. The analysis of isolates from Laos, Vietnam and Myanmar provided evidence that the particular TR4 strain in these countries was likely introduced from China.
In addition to Cavendish cultivars, TR4 affects cultivars susceptible to races 1 and 2 as well as hitherto unaffected cultivars such as, 'Barangan' (Lakatan subgroup, AAA genome group) and 'Pisang Mas'. The often cited figure that TR4 affects cultivars that account for more than 80% of the world's banana production assumed that Plantains were also susceptible. At the time, however, the only Plantain-like material that had been evaluated against TR4 were hybrids produced by breeders. The reaction of the Plantains domesticated in Africa, along with another group of locally domesticated bananas, the East African highland bananas (EAHB), was not known. The first field screening of these two subgroups was conducted in 2011-2012 in the Philippines using accessions from the ITC genebank. Most of the accessions tested were slightly to moderately susceptible. Except for the 'Obubit Ntanga' Plantain accession that was still symptom-free after 10 months (a relatively short time given the perennial nature of most banana production), the disease incidence was below 5%. with the exception of 'Ibwi', for which the disease incidence was 29%. However, the ploidy of the ITC accession called Ibwi (2x/3x) suggests that the material tested might not be the EAHB cultivar 'Ibwi'.
The FHIA improvement programme has produced hybrids that are resistant to races 1 and 4, while the Taiwan Banana Research Institute (TBRI) has released Giant Cavendish tissue-culture variants (GCTCV) that are partially resistant to TR4. In field trials conducted in China, FHIA-01, FHIA-02, FHIA-18, FHIA-25, Pisang Jari Buaya, Rose (AA), and to a lesser extent GCTCV-119 and FHIA-03, have shown resistance to TR4. In a field trial conducted in the Philippines, only 1% of the GCTCV-219 plants exhibited symptoms of Fusarium wilt in the second crop cycle, whereas none of plants of the Cardava cultivar (Saba subgroup) did.
Symptoms and diagnosis
The symptoms of a TR4 infection are the same as those caused by any other Foc strains (see symptoms of Fusarium wilt).
However, the wide host range of TR4 makes it difficult to diagnose TR4 on non-Cavendish bananas that are susceptible to other strains. For example, a Gros Michel infected with TR4 would not raise alarm because the assumption would be that it is infected with a race 1 strain.
The quickest way to confirm a TR4 infection is by analysing tissue samples using a TR4-specific PCR test. Fungal isolates can also be analysed to determine their vegetative compatibility group (VCG). The VCG associated with TR4 is 01213.
Modes of transmission
TR4 can be spread through infected planting material, infested soil and water. For more information, see the section on the modes of transmission of Fusarium wilt.
Like all the other Foc strains, TR4 cannot be controlled using fungicides and cannot be eradicated from soil using fumigants. As a result, the spread of TR4 has led to an increase in research on biological control and the role of the soil microbial community in suppressing the pathogen.
Crop rotation with a non-banana crop that has anti-fungal activity has been used to reduce losses. In China, farmers have been growning bananas in the presence of TR4 by rotating them with Chinese leek (Allium tuberosum). Chinese leeks has also been used as an intercrop.
The solution best adapted to the continued production of bananas in infested soils is replacing susceptible cultivars with resistant ones. However, given TR4's wide host range, virulence and persistence in the soil, experts stress the importance of preventing the spread of the fungus (see Preventing the spread of TR4).
The FHIA improvement programme has produced hybrids that are resistant to races 1 and 4, while the Taiwan Banana Research Institute (TBRI) has released Giant Cavendish tissue-culture variants (GCTCV) that have been selected for their increased resistance to TR4.
In field trials conducted in China, FHIA-01, FHIA-02, FHIA-18, FHIA-25, Pisang Jari Buaya, Rose (AA), and to a lesser extent GCTCV-119 and FHIA-03, have shown resistance to TR4. Preliminary results from a field trial conducted in the Philippines in 2011-2012 suggest that EAHB and Plantain might be relatively resistant to TR4. Most of the ITC accessions screened displayed little or no sign of Fusarium wilt. The one exception was Ibwi (ITC1465), whose ploidy (2x/3x) suggests that the accession might not be representative of the Ibwi cultivar. It is possible that the wrong accession was introduced to the ITC. In a separate field trial conducted in the Philippines, only 1% of the GCTCV-219 plants exhibited symptoms of Fusarium wilt in the second crop cycle, whereas none of plants of the Cardava cultivar (Saba subgroup) did.
Two genetic engineering strategies, one involving the introduction of a resistance gene isolated from a wild relative of the banana and the other of an anti-apoptosis gene derived from a nematode , are being tested in Australia. Two of the evaluated lines were still free of the disease after three years of a field trial conducted in the Northern Territory.
Preventing the spread of TR4
Even though the threat posed by TR4 has been widely recognized, few TR4-free countries have taken the steps to prevent the entry of the fungal strain or to contain it when it was first detected. By 2018, only two of the 16 countries that are known to have TR4 had taken immediate action to contain the fungal strain when it first showed up in an area: Australia in 2015 and Israel in 2016.
- Designate TR4 as a quarantine pest;
- Set up a monitoring system to promptly detect incursions;
- Enact regulations that allow the national plant protection organisation to intervene on farms, including:
- Conduct inspections;
- Collect samples;
Enforce the destruction of plants.
Contingency plans are typically developed by the authorities responsible for planning and responding to incursions of pests and diseases. They cover the technical and regulatory aspects of confirming suspected cases of and stopping the pest or pathogen from getting established.
Generic contingency plans can also be developed. OIRSA, a regional plant protection organization, whose membership includes Belize, Costa Rica, the Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nicaragua and Panama, developed one against TR4.
At the farm level, containment (keeping the pathogen in) and exclusion (keeping the pathogen out) are two sides of the same coin. Except for the actions specific to isolating an infested area, the biosecurity measures aimed at preventing the fungus from escaping an infested farm are essentially the same as the ones growers can take to protect their farm. But whereas exclusion is the responsibility of the producer, containment can be legislated.
TR4 is mostly spread by infected planting material and contaminated soil and water. In Australia, the biosecurity measures banana growers have been encouraged to implement were designed to halt the movement of the pathogen along these pathways.
The primary line of defence is the exclusion of all non-essential visitors, vehicles and plant material from outside. This is part of a strategy to manage people and vehicle access that is called differential access zoning. For banana farms, three key zones are proposed:
1. The exclusion zone for vehicles that don’t need to enter the farm;
2. The separation zone for essential vehicles that are low risk (i.e. not associated with field production) and which are usually subjected to cleaning/disinfection procedures;
3. The farming zone, where farming activities take place, is physically separated from the other zones to manage the risk of cross-contamination.
Some of the actual practices put in place include physical barriers, facilities for footwear change at zone boundaries, and vehicle wash-down facilities that use disinfectants such as Farmcleanse®, Sporekill® or Domestos®.
These practices also provide a barrier to the entry of other pests, diseases and weeds. Managing the movement of water and soil can also have a beneficial impact on the environment.
The severity of the damage depends on interactions between the strain, its host and environmental conditions.
TR4 has devastated commercial plantations of Cavendish bananas in Taiwan, Indonesia, Malaysia and Australia’s Northern Territory.
In mainland China, the strategy of establishing Cavendish plantations in TR4-free areas to stay ahead of the disease has led to the spread of the fungus to all the main banana-growing provinces and at least 3 countries in the Greater Mekong area.
In the Philippines, the extent of the damage in Cavendish plantations has not been documented. The Mindanao Banana Farmers and Exporters Association, which represents small-scale farmers growing Cavendish cultivars for the export market, has reported that about 5,900 hectares of their members’ aggregate plantation area had been infected, including 3,000 hectares that have been abandoned. Some growers say their farm was infected by run-off from a nearby large commercial farm.
In the few instances in which losses to TR4 have been estimated, they amounted to 121 million USD in Indonesia, 253.3 million USD in Taiwan and 14.1 million USD Malaysia.
In Africa, where TR4 was reported for the first time in 2013 in an export plantation of northern Mozambique, the number of symptomatic plants had risen to more than 570,000 (out of a total of more than 2.5 m plants) by September 2015. The plantation has since scaled down its operations. TR4 has also been detected in another plantation.
For more information, see the section on the impact of Fusarium wilt.
Efforts to address the threat of TR4
Following the first confirmed case of TR4 in Queensland, Biosecurity Queensland, in partnership with the Australian Banana Growers' Council, set up a programme of surveillance and containment. The farm was bought by the Australian Banana Growers Association in late 2016 with the objective of shutting down the farm and destroying all the banana plants.
Latin America and the Carribean
OIRSA, a regional organzation for plant and animal health, has 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 available in Spanish only.
In 2014, the Caribbean Agricultural Research and Development Institute (CARDI) organized a seminar and a training workshop to raise awareness of the potential threat of TR4 as a key step to prevent its introduction to the Caribbean.
In December 2013, a task force on TR4 was set up within the framework of the World Banana Forum. In December 2014, the FAO held a consultation with a group of international experts to agree on the framework for a global programme. The plan would work on three main fronts: preventing future outbreaks, managing existing cases, and strengthening international collaboration and coordination among institutions, researchers, governments and producers.
See also on this website
- NPPO declares TR4 eradicated from Israel
- TR4 spreading in India
- First report of TR4 in Myanmar
- Third case of TR4 in Queensland, Australia
- Infected nursery plants suspected in TR4 outbreak