Tropical race 4
Tropical race 4 (TR4) is the name given to the fungal strains of Fusarium oxysporum f. sp. cubense (Foc), the fungus that causes 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, and have since become 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 Taiwan1. For the next 20 years or so, the distribution of TR4 was limited to parts of Asia and Australia's Northern Territory. In 2013, TR4 was reported to be Jordan2, the first record of TR4 outside the Asia-Pacific region. Later that year it was also reported to be in Africa3. 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 production4. 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 established5.
By the turn of the 20th century TR4 had been observed in Taiwan, Malaysia6 (including Sarawak on the island of Borneo), Indonesia (Java, Sumatra, Sulawesi, Halmahera, Kalimantan7 on the island of Borneo, and Papua Province8 on the island of New Guinea), mainland China (Guangdong9, Hainan10, Guangxi, Fujian and Yunnan), the Philippines' island of Mindanao11 and Australia (Northern Territory12).
In 2013, TR4 was reported in Jordan13 (a 2014 survey revealed another infected area north of the original outbreak14) and Mozambique15 (where it has since been observed in a second plantation 16). In 2015, it was reported in Lebanon and Pakistan1718, as well as Queensland in Australia19. It has also been in Oman since at least 2012202122.
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)7 and 'Pisang Mas'. The often cited figure that TR4 affects cultivars that account for more than 80% of the world's banana production23 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 breeders24. 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 susceptible25. 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/3x26) 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 TR427. 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 TR428. 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) did29.
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 test30. 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. 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 soil31, experts stress the importance of preventing the spread of the fungus232. Following the detection of TR4 in Queensland, Australia, Biosecurity Queensland published a document on best practices to minimize the risk of spreading TR433.
For more information, see the section on control options of Fusarium wilt.
TR4 has devastated commercial plantations of Cavendish bananas in Taiwan, Indonesia, Malaysia and Australia’s Northern Territory34. 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 provinces35. 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 abandoned36. Some growers say their farm was infected by run-off from a nearby large commercial farm37. 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 Malaysia38.
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 201539. TR4 has also been detected in another plantation40.
For more information, see the section on the impact of Fusarium wilt.
Efforts to address the threat of TR4
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 only46.
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 Caribbean48.
In December 2013, a task force on TR4 was set up within the framework of the World Banana Forum49. In December 2014, the FAO held a consultation with a group of international experts to agree on the framework for a global programme32. 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
- Australia's TR4 incursion one year on
- The trap of extinction stories on bananas
- Social and psychological impacts of the TR4 incursion in Queensland, Australia
- Why screening protocols matter
- TR4 quarantine lifted on second Queensland farm