Bananas under threat in Africa from soil fungus
Saturday May 20 2023
Bananas in Africa are threatened by a soil-borne fungus, Fusarium oxysporum (Foc TR4), which has caused significant losses in other countries where banana is produced.
The highly virulent Fusarium wilt of banana (FWB) disease popularly known as Panama disease, has in the past 10 years spread from Southeast Asia, where it was restricted for almost 20 years, to other parts of the world including Africa. It is the first disease of bananas to have spread globally in the first half of the 20th century.
Foc TR4 affects many banana varieties, including the widely produced and exported Cavendish variety. Next to Cavendish cultivars, TR4 affects a wide range of banana germplasm, including locally important varieties, and East African Highland bananas — crucial cash crops and staple food for millions of people in the region. Locally, different clonal banana varieties are sold, in contrast to the global banana trade dominated by clonal Cavendish varieties. These large banana monocultures are extremely vulnerable to numerous diseases.
A previous related survey was conducted over two seasons in banana-based subsistence farming systems in Rwanda, Burundi, north-western Tanzania (Kagera and Kigoma regions) and eastern DR Congo (South Kivu province), to investigate the distribution and incidence of banana FWB as related to cropping systems, edapho-climatic and socio-economic factors. FWB incidence was found generally high in the region, as 54.1 percent of all farms had disease incidence higher than 40 percent, with Tanzania having the highest (63.6 percent).
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Farm age correlation
For the first time the occurrence of FWB in Rwanda and Burundi, suggests that strategies for its management in east and central Africa should include raising farmers’ awareness on pathogen spread mechanisms and enhancing their access to disease-free planting materials.
This study showed that disease incidence was lower in farms growing cultivar mixtures and at higher altitudes (above 1,600m Above Sea Level). Also, a significant association of FWB and farm age was observed whereby disease incidence was highest in farms aged between 10 and 30 years.
The local varieties are essential for food security in the Great Lakes Region where banana is a major staple crop that already suffers from manifold other pests and diseases, such as nematodes, weevils, Xanthomonas bacterial wilt, and black leaf streak disease, also known as Black Sigatoka.
The region is said to have one of the world’s highest per capita banana consumptions of 400 to 600 kg.
In most major banana-producing regions, TR4 incursions have been reported, and TR4 is spreading globally from its Asian center and in 1876 in Australia, where it was first identified, to other banana-growing regions.
In West Africa, the Caribbean and Tropical America, the disease has spread widely.
Symptoms include yellowing, stunting, and death of seedlings and yellowing and stunting of older plants. Infected plants wilt readily, lower leaves yellow and dry, the xylem tissues turn brown, and the plant may die. In the early stages of disease, the roots are not rotted.
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International bananas at risk
The Food and Agriculture Organization of the United Nations (FAO) has warned that the disease puts the international production of bananas at risk.
Cavendish banana plantations in Mozambique, where the disease was first reported on two commercial banana farms in 2013, have shown the most severe external symptoms, caused by TR4. “Formal confirmation was only published in 2020,” say the scientists in their paper published in the journal Plant Disease, on May 8.
The team of researchers led by Anouk van Westerhoven of Wageningen University & Research and Utrecht University confirmed the presence of TR4 beyond the farm boundaries with the initial infestations, indicating its uncontrolled spread in Mozambique.
This alarming result demonstrates the failure of previous management methods. The uncontained disease spread therefore invokes immediate action to protect banana production and, subsequently, the livelihood of millions in Africa.
“The spread of the disease to other farms in the country strongly suggests that TR4 was not successfully contained. This underscores the failure of the implemented management strategies, which threatens food security in East Africa,” they add.
It is suspected that human factors such as increased international travel or environmental and climate changes likely drove the pathogens’ emergence, evolution, and dissemination to novel geographic regions or ecological niches.
“Often new incursions remain unnoticed and once fungal pathogens are endemic, successful disease management is basically unfeasible, as exemplified by the very few examples of successful eradication. Such cases often rely on fungicides and thorough eradication of host plants, illustrating the importance of an accurate understanding of the host range of a pathogen,” said the scientists.
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They noted that effective and open science at local and global scales are crucial to enable a rapid and coordinated response to emerging and invasive fungal diseases like these.
“TR4 continues to disseminate, irrespective of implemented strategies, and we observe that new incursions often do not lead to effective and transparent responses and data sharing, which are required to improve disease control. The recently reported uncontrolled dissemination of FWB in Mozambique is a serious threat to African food security and global banana production.
Now, nearly 10 years after its introduction to Africa, we call for radical eradication strategies of TR4, along with proactive screening for resistance of African banana germplasm and intensified breeding programs for this important staple crop,” they said.
No banana resistant
The FAO has warned that no commercially available banana variety is resistant to TR4, and consequently, surveillance and disease management are currently the only strategies to control its further dissemination.
To prepare member countries in the Southern African Development Community (SADC) against the possible incursion of Foc TR4, the FAO has organized training to help increase awareness on Foc TR4 in Africa and identify Foc strains collected from the region. There have also been efforts to develop guidelines and provision of training on prevention and management of the disease.
World banana trade has skyrocketed in recent years, with an estimated export volume of 21 million metric tons in 2019, according to the FAO.
The disease outbreaks were vital for playing an important part in the desertion and transition of 'Gros Michel' to the Cavendish subgroup in trade. Presently, both export and smallholder production, the above banana cultivars are the most widely cultivated worldwide.
Van Westerhoven and colleagues collected fungal samples from 13 symptomatic banana plants found throughout northern Mozambique, then tested the samples using molecular diagnostics and greenhouse pathogenicity assays. The samples tested positive for TR4, which prompted the researchers to investigate the genetic variation and potential origin of TR4 in Mozambique.
Spreading at reproduction
Based on the little amount of genetic variation revealed in this investigation, the researchers speculate that TR4 is spreading at reproduction.
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Corresponding authors Gert Kema and Michael Seidl explained, “Likely the cultivation of Cavendish bananas—a banana variety that stopped the previous epidemic affecting Gros Michel bananas in the 1950s — is now a vehicle for worldwide dissemination, as global banana cultivation is dominated by the highly susceptible Cavendish clones. Moreover, there is a lot of trafficking in the banana world. Mobile working crews, international labour hires, and many of these workers and their managers are unaware of the danger of fungal diseases.”
Despite this information, there are still knowledge gaps that hinder successful containment of the unruly fungus. “Unfortunately, we do not have access to on-farm data, which is essential to monitor the disease,” Kema and Seidl say.
he dire situation requires further action, research, and transparent data-sharing to implement new management strategies — such as generating and releasing genetically diverse and resistant germplasm for growers in Africa — which will hopefully peel back the ramifications of this complex fungal disease.