Tropical race 4 - threat to global banana farming

Could it be that the time has come for banana trees to become extinct? More than twenty years ago, researcher Emile Frison, then Director of the International Network for the Improvement of Banana and Plantain, stated that bananas were at risk of extinction in an article published in New Scientist (newscientist.com/article/mg17723784-800-going- bananas/). At the time, the advance of Black Sigatoka was the biggest threat.

But what has changed in banana farming in these two decades? In the export production segment that predominantly cultivates banana trees from the Cavendish Subgroup, such as Nanica, Grande Naine, Water Banana, there was a significant increase in production costs due to Black Sigatoka. In Central America, up to 60 to 70 fungicide applications are made per year, with planes flying over banana groves once a week. In areas of production for local consumption, producers who are unable to carry out adequate chemical control of the disease opt to grow more resistant varieties or simply stop planting bananas.

Despite all the damage caused by Black Sigatoka, which did not extinguish banana trees, a new threat has emerged in recent years and has been worrying producers and scientists. This is the tropical race 4 of the fungus Fusarium oxysporum f.sp. cubense (FOC), known as TR4.

Recently, a study published in the scientific journal Frontiers of Plant Science brought together researchers from Brazil, Colombia and Mexico to discuss the spread and impact of TR4 in Latin America and the Caribbean, focusing on epidemiological aspects and the most recent evidence-based management options. scientific. The study authors consider that this new breed of Fusarium represents an undeniable threat to global banana production and its spread to the main banana producing countries in the Americas is only a matter of time.

Fusarium wilt of banana plants has intensified in recent years, with the rapid spread of TR4. Since 2018, the number of affected countries has increased from 16 to 23, which represents a significant challenge for researchers, producers and National Plant Protection Organizations around the world (Figure 1).

The potential impact of TR4 in Latin America and the Caribbean is particularly worrying. This region has seven of the ten main banana exporting countries, with this fruit being essential for food security and income generation. In Colombia, where TR4 was detected in 2019, the disease has already spread from La Guajira to Magdalena and currently affects 20 large commercial export farms. In Peru, the disease was detected in 2021 and, although still restricted to the northern region, flood irrigation and heavy rains associated with Cyclone Yaku drove the spread of the pathogen. By 2024, more than 400 small organic banana producers have already been affected. In Venezuela, TR4 was detected in 2023, with affected plantations spread across three states. Worryingly, TR4 has also been confirmed in plantains, a staple food in the region. Currently, national responses in Latin America and the Caribbean are mainly based on preventive exclusion and reactive measures related to banana plantation management. However, the continuous and silent progression of the disease suggests that the disease may already be present in other producing regions.

The threat derives from the persistence of the fungus in the soil for up to 30 years, even in the absence of banana trees. Spore survival is considered the main mechanism, but multiplication on other host plants also contributes to the persistence of spores. When soils are contaminated with TR4, susceptible varieties cannot be replanted.

O Fusarium is a soil fungus that penetrates banana trees through secondary roots. Once inside the plant, the pathogen colonizes and destroys the vascular system, causing external symptoms that begin as yellowing and wilting from older to younger leaves (Figure 2A) and internal symptoms characterized by necrosis in the vascular system (Figure 2B and C ) and rhizome discoloration (Figure 2D). Infected plants may also experience delayed development and abnormal leaf growth. A comparison between affected and healthy plants (Figures 2E, F and G) illustrates that FOC deprives affected plants of water and nutrient absorption, preventing normal fruit production and causing plant death. Once an epidemic begins, if management measures are not implemented in a timely manner, widespread destruction can quickly occur (Figure 2H).

The key components of an integrated approach to TR4-compliant banana farming systems are plant exclusion, early detection and destruction; biosecurity; resistant varieties; biological control and integrated practices aimed at soil health and cropping systems.

Exclusion is based on phytosanitary surveillance and the implementation of biosecurity measures at the country, region, farm or field level. Biosecurity encompasses all measures to protect society and its members from the negative effects of the spread of pests and diseases. Biosafety, as well as any other TR4 management practice, must be based on the epidemiology of the disease. Pathogen dispersal factors and pathways need to be understood through site-specific approaches. Regardless of the size of the farm or the economic condition of the producer, biosecurity principles must be part of daily work in banana plantations.

Without a doubt, the use of resistant varieties is the best strategy for dealing with Fusarium wilt. However, genotypes fully resistant to TR4 are not yet available to replace susceptible varieties of Cavendish banana or other commercial varieties.

The development of transgenic plants and gene editing offers a new path to producing disease-resistant varieties. Recently, the Australian government approved the commercialization of the genetically modified QCav4 variety, resistant to TR4 and belonging to the Cavendish Subgroup. Although this represents progress in regulatory terms, the logic of widespread conversion to this new variety still needs to be tested. The production and export of these varieties will also face important challenges: 1) varying authorization processes for field validation trials in different countries; 2) time lag between field validation and commercial production licenses; and 3) acceptance in the markets of importing countries, such as those in the European Union. In more than 100 years of banana exports, variety switching in export banana farming has occurred only once, when R1 became widespread in the 1960s. Small-scale producers for domestic markets, although operating with more variety flexibility, They are highly dependent on low-quality planting material that ends up spreading pests and diseases. A switch to tissue culture planting material would represent a major logistical and cost transition.

Replacing susceptible varieties that are accepted by the market may still take time. It is most likely that cultivation systems will be adopted that contribute to the suppression of the disease and greater plant vigor. Soil is a dynamic ecosystem filled with complex interactions that can significantly affect the prevalence and severity of Fusarium wilt and banana production. In Taiwan and the Philippines, producers adopt fallow periods, tolerant varieties and microbial products to recover production and try to live with the pathogen. The expectation is that the challenge of producing bananas on land affected by TR4 will only increase as the disease spreads.

As an example of management practice focused on improving soil health, there is control of soil acidity. Soil pH is the indicator most related to Fusarium wilt. Acidic soils have often been associated with severe expression of the disease. It is known that increasing pH to levels still favorable for bananas, but not FOC, can be effective in reducing disease. However, soils with low acidity (pH above 7) do not guarantee the suppression or reduction of the disease. Banana plantations heavily affected by TR4 in northern Peru and Venezuela have soil pH greater than 7,5. In northeastern Brazil, we also have areas affected by Race 1 that have high pH.

Nitrogen fertilization is a factor that also affects the severity of the disease. Both the dose and the source of N are related to wilting due to Fusarium. Higher doses of N are often associated with increased disease intensity. When stimulating growth, an imbalance may occur in the balance between the physiological mechanisms that govern the relationship between increased plant defenses and growth. It is as if the plant was channeling its energy into growing instead of arming itself to fight the pathogen. Ammoniacal sources such as ammonium sulfate, DAP or urea, which increase the availability of NH4 and cause acidity, can also increase the disease. Using nitric N sources and applying doses that meet the plants' demands without compromising their defense are recommended measures to increase plant resistance to wilt due to Fusarium.

Finally, it is known that organic matter greatly influences the structure, nutrient availability, microbial activity and general health of the soil. Due to these multiple functions, the addition of organic material can have positive implications on the incidence and severity of plant wilt. Fusarium. Cultivation systems and management aimed at increasing and diversifying the supply of organic matter are essential aspects to increase soil suppressiveness and enable coexistence with wilt due to Fusarium of the banana tree.

Although we do not believe in the extinction of banana trees, we are convinced that banana farming will need to undergo major changes in the medium term to guarantee its sustainability. The threat of TR4, combined with the consequences of climate change, makes the current situation much more challenging than it was twenty years ago, when there was already talk about the possible extinction of bananas. Adapting to new realities and seeking innovative solutions is critical to ensuring bananas remain a vital part of food security and the global economy.

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