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The Cavendish banana variety accounts for 95% of all bananas sold commercially, and is seedless, making it extremely convenient to eat. But seedlessness also means sterility – it cannot reproduce through the normal process of sowing seeds. Today’s commercial banana industry relies almost entirely on the Cavendish banana, as selling only one variety makes harvesting, packaging and shipping more cost-effective, and provides a uniform product. However, it also means that the vast majority of the world’s bananas are clones, and if something affects one plant, it affects them all. This is exactly what is happening. A banana fungus that causes black leaf disease – Mycosphaerella fijiensis (Morelet) – has emerged, threatening the world’s entire banana crop. The industry’s only defence is to spray plantations with large amounts of fungicides, which has both human health and financial implications. The Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, which has pioneered the use of tissue culture for mutagenesis breeding, is now developing banana mutants that are resistant to the fungi that threaten the plants.
There are more than 1,000 varieties of bananas in the world, according to the Yearbook of Plant Genetic Diversity. They range in color from red to black and green to maroon, from sweet bananas that can be eaten straight off the tree to starchy bananas that require cooking. They also vary in nutritional value, with one Nigerian variety being used to treat infertility. Yet shoppers may only find one type of banana in their local supermarket.
The same variety of banana will be sold at the market across town, in the next region or country – and yes, in nearly all of the world’s supermarkets. The entire global commercial banana industry depends on one sweet, seedless banana variety – the Cavendish.
The variety was adopted by the commercial industry because of its resistance to a disease that threatened the banana world in the 1960s. Today, history is repeating itself. Another banana disease, black sigatoka, is spreading across the globe, and the Cavendish banana, which has no resistance to it, is being attacked. The threat is particularly severe because of the way these bananas are propagated: they are essentially clones, which means that if one plant is at risk, all are at risk.
Fighting banana fungus is a race against time
Planting a new variety that is less susceptible to black spot would require the industry to retool its entire processing infrastructure — a drastic and expensive measure. So producers instead rely on fungicides that are sprayed from the air every six days — fungicides that have serious side effects on human health, including stunted growth in children and miscarriage. The fungicides are also expensive to use, making them unaffordable for many of the estimated 400 million locals who rely on bananas to feed their families or earn extra income.
Unless current global varieties are resistant to Black Sigatoka, spraying with fungicides will continue, which is why the Joint FAO/IAEA Division, a global pioneer and leader in plant genetic mutation, is racing against time to urgently work with countries to develop new varieties with resistance traits.
The quest for resistance to fungi is a numbers game
In the case of bananas, the mutation process requires irradiating thousands of seedlings with gamma or X-rays to induce random mutations. Screening is then required to see if the mutations affect genes in a way that results in the desired trait—in this case, resistance to the banana leaf spot disease. It’s basically a numbers game: the better the screening technology, the faster the chances of detecting a specific, unique, improved banana.
So far, the Joint FAO/IAEA Plant Breeding and Genetics Division laboratories have produced three banana plant mutants that are resistant to the black spot virus toxin under laboratory conditions. The next step is to take these seedlings to the field to determine if the bananas they produce outside the laboratory are still resistant.
The Joint FAO/IAEA Division’s plant mutation research aims to help smallholders and medium-sized producers. Its commercial bananas have increased Sudanese farmers’ yields by 30 percent, and it has introduced micropropagation techniques to 600 Sri Lankan families, increasing their incomes 25-fold – a technique so successful that the Sri Lankan government has recommended that local farmers consider switching from subsistence rice to value-added bananas.
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