Cover crops are hosts to the fungus that causes banana fusariosis

Research carried out in the Vale do Ribeira region, SP, identified three species of cover plants that act as potential hosts of the fungus Fusarium oxysporum f. sp. Cubense (Foc), which causes banana fusariosis, also known as Panama disease, the most destructive disease of this crop in the world. Of the six species evaluated, sunn hemp, forage radish and jack beans were identified as potential hosts of the pathogen. The discovery could contribute to optimizing management strategies for banana plantations affected by fusariosis, in addition to helping to contain the spread of the pathogen in plantations. 

The study is part of a project funded by the São Paulo Research Foundation (Fapesp), carried out by researchers from Embrapa Meio Ambiente (SP), Embrapa Digital Agriculture (SP), Instituto Agronomic de Campinas (IAC), Agência Paulista de Tecnologia dos Agronegocios (APTA), from Vale do Ribeira, Coordination of Comprehensive Technical Assistance (CATI) and  Alliance of Bioversity International and the International Center for Tropical Agriculture (CIAT), and was the subject of the Masters dissertation by Maria Laura Nascimento, defended at the Federal University of Espírito Santo (Ufes) and developed in the laboratories and experimental areas of Embrapa Meio Ambiente. The results bring a new scientific approach to controlling the progression of the disease. “Current management does not consider the existence of alternative hosts. It is more aimed at the crop itself and the soil, or the area itself, such as the destruction of diseased plants, disinfestation of tools, control of the traffic of machines, people and animals in the area, and soil management”, explains Nascimento.

The use of cover crops is a common practice in controlling weeds, pests and diseases. Species that act as hosts, when infected by Foc, can serve as a source of fungus growth and favor the occurrence of the disease in the field.

In the study, Fusarium colonies were isolated in roots and aerial parts of weeds, collected close to banana trees affected by fusariosis, to verify their potential as alternative hosts. Today, little is known about the role of these plants as a possible source of dispersion and persistence of the disease in the field.

The research points out that the appropriate choice of cover crops in infested areas is relevant and must take into account their capacity as Foc hosts, so as not to contribute to the increase in reproductive structures of this fungus in the field.

The work also evaluated the potential of the banana borer (Cosmopolites sordidus) as a vector of the disease. When analyzing the dynamics of the interaction between the banana borer and the fungus that causes fusariosis in a greenhouse, a greater intensity of the disease was observed in plants infested with the fungus and in the presence of insects. It was also possible to isolate colonies of the fungus associated with the exoskeleton of adult insects collected in banana plantations. However, when testing pathogenicity for banana plants, none of the isolates reproduced characteristic symptoms of the disease in plants. “The results advance the understanding of the interaction between the fungus and the insect and reinforce the need to manage the banana borer in the field”, highlights Jeanne Prado, researcher at Embrapa Meio Ambiente.  

Plant species evaluated

According to Nascimento, the main species of plants collected in the region were brachiaria, bitter grass, silk grass, guanxuma, maria pretinha, serralha, trapoeraba and clover.

She explains that two studies were carried out to check possible alternative hosts, one with weeds and the other with vegetation cover. For weeds, isolation was made from material collected in the field in areas adjacent to banana trees infected with fusariosis. “As they were close and the fungus is dispersed in the soil, if they were hosts, there would be a possibility that they would also have been colonized by Foc”, she notes.

Nascimento explains that for cover crops, the methodology adopted was different. Instead of collecting naturally infected plants, the seeds were planted in artificially infested soil in a controlled environment. Around 80 days later, fragments of roots and aerial parts of each plant were analyzed to check whether or not they had been colonized by the fungus.

"Of the six species evaluated, it was possible to isolate at least one colony of the fungus in each of the species of Crotalaria ochroleuca, forage turnip, jack bean and Crotalaria spectabilis. Black oat and millet, however, did not show potential as alternative hosts", he adds.

Researcher Miguel Dita, from the Alliance of Bioversity International and the International Center for Tropical Agriculture (CIAT), explains that Foc's ability to survive inside other plant species without causing the disease has already been confirmed, but this information needs to be included in the context of field management, such as, for example, the use of cover crops. The researcher emphasizes that this research is important because, in addition to helping to manage banana fusariosis, which already occurs in the country, it prepares Brazil for the eventual arrival of tropical race 4, an even more destructive variant of the fungus, which does not yet occur. in the national territory. 

Fusariosis: the “banana pandemic”

Banana fusariosis or Panama disease, caused by the soil fungus Fusarium oxysporum f. sp. cubense (Foc), is the most destructive disease of banana plants. Foc race-1 was responsible for the epidemic that had a major impact on the banana export industry in the Americas in the middle of the last century. The disease was detected in Brazil, in 1930, in the municipality of Piracicaba, state of São Paulo, where it destroyed around one million banana plants in around four years. 

The pathogen is widespread in all regions of Brazil and since the fungus entered the country, the highly susceptible Maçã banana has been replaced by resistant varieties from the Cavendish subgroup. The tropical race 4 of this pathogen does not yet exist in Brazil, but the vulnerability of the main varieties present in the country has already been confirmed. Prata, Maçã and Nanica.

 Typical symptoms of the disease are: yellowing, wilting, cracking of the sheath bundle and breakage of the petiole. Older leaves show yellowing, which is later also observed on young leaves. This symptom begins at the edges of the leaf blade, which progresses until it reaches the main vein. 

In addition to yellowing, wilting also occurs, leading to the breakage of the petiole next to the pseudostem, giving the plant the appearance of a closed umbrella. Internally to the pseudostem or rhizome, there are reddish-brown spots and, externally to the pseudostem, there is vascular discoloration, with a light center. (source: Phytopathology Manual, volume 2, from the “Luiz de Queiroz” School of Agriculture – Esalq/USP)

In the last decade, the tropical race 4 epidemic has suddenly accelerated, spreading from Asia to Australia, the Middle East, Africa and, more recently, Latin America, where the majority of bananas sent to supermarkets in the Northern Hemisphere come from.

Currently, the tropical race 4 is present in more than 20 countries, sparking fears of a "banana pandemic" and a shortage of the world's most consumed fruit. (source: BBC News Brasil).

Banana and its importance for food security

Banana is one of the most produced and consumed fruits in the world, with great social and economic relevance. It is a basic and important food for food security. In 2019, global banana production reached around 115 million tons.

In Brazil, it is the most consumed fruit. According to the Family Budget Survey (POF) from the Brazilian Institute of Geography and Statistics (IBGE), from 2017/2018, Brazilians consume an average of 25 kilos of bananas per capita/year.

Regarding production, the country has favorable climatic characteristics for banana cultivation, meaning that it is produced in all states throughout the year. According to IBGE estimates, national production in 2021 was 7 million tons in an area of ​​465,9 thousand hectares. Brazil is the fourth largest banana producer in the world, behind only India, China and Indonesia.

However, despite being extremely attractive, as it allows a quick return on capital and a continuous flow of production, banana farming faces strong limitations due to the occurrence of diseases, among which fusariosis stands out, with losses that can reach 100%.