Tropical race 4
Tropical race 4 (TR4) is the name given to the strains of the fungus Fusarium oxysporum f. sp. cubense (Foc) that cause Fusarium wilt (aka Panama disease) in Cavendish cultivars. The term TR4 was coined to distinguish these strains from the ones that also affect Cavendish cultivars, but only in the presence of predisposing factors such as low temperatures. The latter are known as subtropical race 4 (STR4).
The term TR4 usually refers to isolates belonging to a particular vegetative compatibility group called VCG 01213/16, although other VCGs have also been shown to cause Fusarium wilt in Cavendish cultivars. It should be noted that TR4 has a wider host range than just Cavendish bananas. 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.
The strain associated with TR4 was identified in 1990 in samples from Taiwan. For the next 20 years or so, the distribution of TR4 was limited to Asia (where TR4 and the other strains of Foc originated) and Australia's Northern Territory. In 2013, TR4 was reported to be Jordan, the first record of TR4 outside the Asia-Pacific region. Later that year it was also reported to be in Africa.
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. The severity of the damage depends on interactions between the strain, its host and environmental conditions. 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.
By the turn of the 20th century TR4 had been observed 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).
It has been in Oman since at least 2012. In 2013, TR4 was reported to be in Jordan (a 2014 survey revealed another infected area north of the original outbreak) and Mozambique (where it has since been observed in a second plantation ). In 2015, it was reported to be in Lebanon and Pakistan, as well as Queensland in Australia. Its presence has been reported in India, though not formally published . In 2017, it was reported in Laos and Vietnam.
See Race 4 for more details.
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 majority of TR4 isolates belong to the VCG 01213/16 complex.
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.
The use of tissue-culture plantlets prevents the spread of the disease through planting material, but once the fungus is established, the solution best adapted to the continued production of bananas in infested soils is replacing susceptible cultivars by resistant ones (see Host range above). However, given TR4's wide host range, virulence and persistence in the soil, experts stress the importance of preventing the spread of the fungus.
To prevent the entry and/or exit of TR4 to and from banana plantations, growers have been setting up strategically located foot and vehicle baths filled with surface sterilants (e.g Farmcleanse®, Sporekill® and Domestos®). Following the detection of TR4 in Queensland, Australia, Biosecurity Queensland published a document on best practices to minimize the risk of spreading TR4.
The biosecurity measures Australian banana growers have been encouraged to implement are designed to minimize the movement of the pathogen in planting material and through contaminated soil and water.
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.
Once the fungus has become established, the solution best adapted to the continued production of bananas in infested soils is replacing susceptible cultivars by resistant ones.
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 less susceptible to TR4 than the original material.
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 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.
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. 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. 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
- Infected nursery plants suspected in TR4 outbreak
- Evaluation of TR4-resistant genetically modified Cavendish banana plants
- First reports of TR4 in Laos and Vietnam
- Second TR4 incursion in Australia's Tully Valley
- TR4-resistant GM bananas to be trialled in Australia