Can Science Save the Banana?

(Credit: Shutterstock)

(Credit: Shutterstock)

The banana is the world’s most popular fruit crop, with over 100 million metric tons produced annually in over 130 tropical and subtropical countries. Edible bananas are the result of a genetic accident in nature that created the seedless fruit we enjoy today. Virtually all the bananas sold across the Western world belong to the so-called Cavendish subgroup of the species and are genetically nearly identical. These bananas are sterile and dependent on propagation via cloning, either by using suckers and cuttings taken from the underground stem or through modern tissue culture.

The familiar bright yellow Cavendish banana is ubiquitous in supermarkets and fruit bowls, but it is in imminent danger. The vast worldwide monoculture of genetically identical plants leaves the Cavendish intensely vulnerable to disease outbreaks. Fungal diseases severely devastated the banana industry once in history and it could soon happen again if we do not resolve the cause of these problems. Plant scientists, including us, are working out the genetics of wild banana varieties and banana pathogens as we try to prevent a Cavendish crash.

The Cautionary Tale of ‘Big Mike’

One of the most prominent examples of genetic vulnerability comes from the banana itself. Up until the 1960s, Gros Michel, or “Big Mike,” was the prime variety grown in commercial plantations. Big Mike was so popular with consumers in the West that the banana industry established ever larger monocultures of this variety. Thousands of hectares of tropical forests in Latin America were converted into vast Gros Michel plantations.

But Big Mike’s popularity led to its doom, when a pandemic whipped through these plantations during the 1950s and ‘60’s. A fungal disease called Fusarium wilt or Panama disease nearly wiped out the Gros Michel and brought the global banana export industry to the brink of collapse. A soilborne pathogen was to blame: The fungus Fusarium oxysporum f.sp. cubense (Foc) infected the plants’ root and vascular system. Unable to transport water and nutrients, the plants wilted and died.

A cross-section of a banana plant, infected with the fungus that causes Fusarium wilt. (Credit: Gert Kema, CC BY)

Fusarium wilt is very difficult to control – it spreads easily in soil, water and infected planting material. Fungicide applications in soil or in the plant’s stem are as of yet ineffective. Moreover, the fungus can persist in the soil for several decades, thus prohibiting replanting of susceptible banana plants.

Is History Repeating Itself?

Cavendish bananas are resistant to those devastating Fusarium wilt Race 1 strains, so were able to replace the Gros Michel when it fell to the disease. Despite being less rich in taste and logistical challenges involved with merchandising this fruit to international markets at an acceptable quality, Cavendish eventually replaced Gros Michel in commercial banana plantations. The entire banana industry was restructured, and to date, Cavendish accounts for 47 percent of the bananas grown worldwide and 99 percent of all bananas sold commercially for export to developed countries.

Bananas in Costa Rica affected by Black Sigatoka. (Credit: Gert Kema, CC BY)

But the Cavendish unfortunately has its own weaknesses – most prominently susceptibility to a disease called Black Sigatoka. The fungus Pseudocercospora fijiensis attacks the plants’ leaves, causing cell death that affects photosynthesis and leads to a reduction in fruit production and quality. If Black Sigatoka is left uncontrolled, banana yields can decline by 35 to 50 percent.

Cavendish growers currently manage Black Sigatoka through a combination of pruning infected leaves and applying fungicides. Yearly, it can take 50 or more applications of chemicals to control the disease. Such heavy use of fungicides has negative impacts on the environment and the occupational health of the banana workers, and increases the costs of production. It also helps select for survival the strains of the fungus with higher levels of resistance to these chemicals: As the resistant strains become more prevalent, the disease gets harder to control over time.

Aerial spraying of fungicides on a banana plantation. (Credit: Gert Kema, CC BY)

To further aggravate the situation, Cavendish is also now under attack from a recently emerged strain of Fusarium oxysporum, known as Tropical Race 4 (TR4). First identified in the early 1990s in Taiwan, Malaysia and Indonesia, TR4 has since spread to many Southeast Asian countries and on into the Middle East and Africa. If TR4 makes it to Latin America and the Caribbean region, the export banana industry in that part of the world could be in big trouble.

Cavendish varieties have shown little if any resistance against TR4. Growers are relying on temporary solutions – trying to prevent it from entering new regions, using clean planting materials and limiting the transfer of potentially infected soil between farms.

Cavendish banana trees in China infected with new fungal disease TR4. (Credit: Andre Drenth, UQ, CC BY)

Black Sigatoka and Panama disease both cause serious production losses and are difficult to control. With the right monitoring in place to rapidly intervene and halt their spread, the risks and damage imposed by these diseases can be considerably reduced, as has been recently shown in Australia. But current practices don’t provide the durable solution that’s urgently needed.

Getting Started on Banana Genetic Research

If there’s a lesson to be learned from the sad history of Gros Michel, it’s that reliance on a large and genetically uniform monoculture is a risky strategy that is prone to failure. To reduce the vulnerability to diseases, we need more genetic diversity in our cultivated bananas.

Local banana varieties in southern China. (Credit: Andre Drenth, UQ, CC BY)

Over a thousand species of banana have been recorded in the wild. Although most do not have the desired agronomic characteristics – such as high yields of seedless, nonacidic fruits with long shelf life – that would make them a direct substitute for the Cavendish, they are an untapped genetic resource. Scientists could search within them for resistance genes and other desirable traits to use in engineering and breeding programs.

To date, though, there’s been little effort and insufficient funding for collecting, protecting, characterizing and utilizing wild banana genetic material. Consequently, while almost every other crop used for food production has been significantly improved through plant breeding over the last century, the banana industry has yet to benefit from genetics and plant breeding.

But we have started taking the first steps. We now know the genome sequences of the banana and the fungi that cause Fusarium wilt and Sigatoka. These studies helped illuminate some of the molecular mechanisms by which these fungal pathogens cause disease in the banana. That knowledge provides a basis for identifying disease-resistant genes in wild and cultivated bananas.

Researchers now have the tools to identify resistance genes in wild bananas or other plant species. Then they can use classical plant breeding or genetic engineering to transfer those genes into desired cultivars. Scientists can also use these tools to further study the dynamics and evolution of banana pathogens in the field, and monitor changes in their resistance to fungicides.

Availability of the latest tools and detailed genome sequences, coupled with long-term visionary research in genetics, engineering and plant breeding, can help us keep abreast of the pathogens that are currently menacing the Cavendish banana. Ultimately we need to increase the pool of genetic diversity in cultivated bananas so we’re not dependent on single clones such as the Cavendish or the Gros Michel before it. Otherwise we remain at risk of history repeating itself.

 

This article was originally published on The Conversation. Read the original article.

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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    What is the point of enslaving Central America if they cannot deliver to spec Big Mikes to be sliced into our Wheaties? Let’s get Franken-bananas online – yellow, beautiful, disease-proof, delicious bananas so large that Donald Trump is accused of developing them. Mellow Yellow for the masses!

    has negative impacts on…occupational health of the banana workers” Hire History, English, Ethnic Studies, and Psychology majors. Stop whining, start doing. If you cannot gene-gineer, swallow your pride and go to the street for a scientist.

  • Robert Bilicki

    I remember eating “Gros Michel” when I was a child. The taste and smell was wonderful. Nothing like today’s bananna’s

    • wgone

      It is still grown and sold in the Tropics. Its not as large and puffed up as the cavendish, but its much sweeter and flavourful.

  • Jim Sluyter Jo Meller

    Given these chemical inputs and their apparent necessity, how are organically certified bananas being produced? I have heard, unconfirmed, that the organic producer simply moves on to new ground when problems arise. Certainly problematic if true. Any insights?

  • John Pollard

    I remember bananas being different when I was young but I never new the story of why this was so – I grew up in the 50’s – now I know.

  • wgone

    I think all this BS about the banana is being over blown. There are literally
    hundreds of varieties of bananas. WHere i live Gros Michel is still
    grown and sold in the markets. What is happening is that commercial
    growers will have to adopt a different strategy to grow on such a large
    scale and deal with the parasites etc. There must be other crops grown together with the bananas or something like that. But come on its not the end of the banana.

    • Small_Businessman

      I’m curious where you are. I’d love to get my hands on some Gros Michels again. It’s been way too many years since I’ve been able to enjoy one.

      • wgone

        I live in the Caribbean. It would grow anywhere that it previously grew. But dont expect fruit thats over-pumped up with chemicals and without blemish. YOu have to buy from smaller vendors who are planting for themselves. It never went extinct, it just became nonviable as a commercial crop. There are hundreds of different varieties of banana and plantains, many of them taste better than gros michel, but they are smaller and we call them figs.

  • The History Man

    The British have just voted for Brexit on the basis of arguments such as that the EU was trying to make them buy straight rather than curved bananas. I wonder how they will react to a world where there are NO bananas!

  • Don Huntington

    We lived for four on Mindanao in the Republic of the Philippines. Huge tracts of Doyle bananas were growing nearby — each towering plant having so much fruit that poles were necessary to keep them from falling over. I never knew anyone to ever eat a banana off one of those plants. We ate what they called Native Bananas that were smaller and grew on much smaller trees. However, the taste was wonderful — so good that after returning to the States it was a couple decades before I could bring myself to eat our commercial bananas. Perhaps the Native Bananas were of the Gros Michel variety.

  • http://www.charlesriverbiotech.org Dr. Jeeri R Reddy Sr

    Plant Pathologists must be on site with a lab to monitor diseases of Banana plantation to save the crop. Millions worldwide depended on the fruit for rich potassium source and also help the world hunger

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