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Hope for Bananas: A Joint Effort Against Fusarium oxysporum f. sp. cubense TR4

Posted: April 23, 2016

Ph.D. candidate Freddy Magdama reports on the emergence of FocTR4 and int'l efforts to address this banana industry threat.

Banana is one of the most important fruits in the world, representing a main export product for many agriculture-based countries and a staple food for millions of people. Currently, almost all the banana produced for export, accounting for 15 percent of the global production, relies on cultivars from a narrow genetic base, the subgroup Cavendish. The remaining 85 percent, approximately 123 million metric tons, is derived from other locally-grown cultivars that are used for local trade and family consumption.

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Figure 1. Trade of native banana cultivars as a source of income for local Ecuadorean farmers and families. Picture taken at Bucay, Guayas province (Ecuador). Image: Freddy Magdama

The fate of bananas recently has been portrayed in the media as having an inevitable ugly end due to the emergence of a new variant of Fusarium oxysporum f. sp. cubense (Foc), the causal agent of Panama disease or Fusarium wilt of banana.

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Figure 2. Symptoms and signs of Fusarium wilt of banana: (a) yellowing and wilting of leaves; (b) cracking of the pseudo-stem; (c) colonization of the leaf petiole by the pathogen; (d) infected corm; (e) initial symptoms of the disease; (f) morphological and reproductive structures of F. oxysporum; (g, h) reddish-brown discoloration caused by Foc. Image: Freddy Magdama

This new race, known as FocTR4 (tropical race4), is capable of affecting Cavendish as well as several other cultivars. It is estimated that approximately 80 percent of all cultivated banana varieties are susceptible to the disease caused by this race, and this is the reason why it constitutes a threat to the banana industry. More importantly, it is a threat to the livelihood of millions of people in developing nations whose subsistence depends on these cultivars. The pathogen reportedly is being dispersed to new areas outside of its native range, southeast Asia: South Africa and the Middle East.

Quarantine procedures and the development of new management strategies are the main focus for researchers until new resistant lines that are suitable for the market are developed. The scientific community fears that this new race will reach South America, where the majority of banana production for export is found. Having said that, I want to share some of the insights from a conference I recently attended.

A mix of fear, anxiety, frustration, anger, and overall, curiosity, were some of the feelings expressed by participants attending the recent XXI International Meeting of ACORBAT-Association for Cooperation in Banana Research in the Caribbean and Tropical America and VI International Banana Congress sponsored by Costa Rica's national banana corporation Corbana.  This event, held April 19-22 in Miami, Florida, is considered one of the most important in the banana industry due to its multidisciplinary focus.

During the course of this three-day meeting delegates from forty-three countries listened to a series of talks on plant nutrition, pest management, quarantine, trade, and new technologies for improving banana production. However, because ACORBAT and Corbana put the major emphasis on TR4  by bringing together a great panel of experts dealing with this problem, the most-attended sessions were those addressing advances made on fighting TR4.

Dr. Randy Ploetz from the University of Florida opened his session by describing the current situation of the disease and warned growers against the poor quarantine measures evidenced in the countries where the pathogen has recently been reported. Dr. Ploetz mentioned the need to validate experiments in field trials since the majority of research concerning TR4 has been carried out under greenhouse conditions. The message was clear: a multidisciplinary effort is required to contain the pathogen until breeding programs can provide a new cultivar with the appropriate level of resistance, both at regional and intercontinental levels.

Dr. Gus Molina from Bioversity International, a scientist who has been dealing with the disease for more than forty years, emphasized that the problem of bananas is due to the industry’s resilience on monoculture. In the Philippines where the disease has been present for several years and mainly affecting small farmers, adopting a new set of somaclones named Giant Cavendish Tissue Cultured Variants (GCTCVs) has been identified as a viable alternative to surviving the presence of TR4. These new grown somaclones, chosen among thousands of generated lines with induced mutations, have presented resistance in soils highly infested with the pathogen for more than three years now.

Similarly, Professor Yi Ganjun, from the Fruit Tree Research Institute, Guangdong Academy of Agricultural Sciences, has also developed two other Cavendish-like somaclones with some level of resistance to FocTR4, named ZJ4 and ZJ6. Both lines have been tested in field conditions, where low levels of disease incidence have been observed. He also showed promising results using gene silencing by transforming bananas with two genes involved in ergosterol biosynthesis using embriogenic cell suspensions and shared some results about the screening for resistance in local and foreign banana germplasm. It was gratifying to know that within this collection there is a set of accessions, plantains and East Africa highland bananas (EAHBs), which has shown a high level of resistance against TR4 and can be employed in breeding programs.

A parallel effort is being carried out in the Netherlands at Wageningen University and Research Centre under the direction of Dr. Gert Kema. Professor Kema, however, expressed that the level of resistance required for fighting TR4 is not that which is observed in somaclones, which he described as being at most less susceptible to the disease but not completely immune. According to Prof. Kema, using somaclones would only delay the problem. He maintains his position on the implementation of genetically modified organisms (GMOs) and urges the banana industry to work on the perception of consumers on the adoption of this technology.

In this area of research Dr. James Dale from Queensland University of Technology (QUT) is the one leading the campaign. He also announced that FocTR4-resistant GMO Cavendish bananas are currently being field-tested in Australia with promising results. Dr. Dale mentioned it might take five to six years until this new cultivar reaches the market and undergoes regulations.

Regardless of these great advances in developing new cultivars, representatives from the Food and Agriculture Organization of the United Nations (FAO) and the International Regional Organization for Plant and Animal Health (OIRSA) called for improvement of quarantine procedures and reminded the audience to keep vigilance and strengthen all efforts to avoid the incursion of this pathogen to new areas.

Professor André Drenth from the University of Queensland-Australia revealed the case of FocTR4 false positives. The current detection methods for FocTR4 are not reliable and have led to the destruction of several hectares of bananas in Australia. Dr. Drenth advised being cautious when reporting new outbreaks and pointed out the lack of understanding of many basic pathological and epidemiological facts of this important pathogen.

The highlight of this conference was the decision taken by the scientific committee of ACORBAT to keep this international congress out of the banana-producing countries for at least six years due to the high risk it represents to the banana industry in bringing people from all over the world, including those from countries where TR4 is present.

There are still a lot of questions to be answered and the need for research is unquestionable. Thus, the knowledge acquired in this conference in conjunction with the results of my research will bring new perspectives to the study of this disease.