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Dangerous plant disease: banana is threatened with extinction

2019-09-03T13:25:24.001Z


The plant pest TR4 has reached Colombia. Because there is no antidote, the banana could soon disappear completely from the fruit shelf. An alternative variety is not in sight.



Those who want to illustrate the economy of shortages in the former GDR with a single product are welcome to choose the banana, which is notoriously rare in eastern Germany. Meanwhile, it is the second most popular fruit of the Germans after the apple - and supply bottlenecks with the fruit in local supermarkets, no one has to waste a thought. That could change soon.

Since the 1990s, a deadly fungal disease has spread worldwide and has now reached South America with its first detection in Colombia - the region from which almost all bananas in German supermarkets originate. Central and South America supply 99 percent of the bananas that can be found in Germany, just under a million tons a year, reports the German Fruit Trade Association (DFHV).

"It is to be feared that no bananas of the Cavendish variety will be available for the German market in the foreseeable future," said the DFHV. 90 percent of bananas imported to Germany belong to this variety. Responsible is the so-called Panama disease. It would not be the first time that a banana variety is running short because of her.

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Under the term panama disease are variants of the fungus Fusarium oxysporum f. sp. cubense, which specializes in different types of bananas. The first known form of the pathogen made sure that in the sixties the then most widespread banana variety Gros Michel disappeared from the offer - and the Cavendish in the first place became the standard banana.

If the Cavendish dies, there are hardly any alternatives left

Gros Michel, also known in the US as "Big Mike", tasted more intense than the Cavendish and was better transported because of their thicker shell. That did not help, however, when about sixty years ago, the deadly pathogen Tropical Race 1 (TR1) first appeared in Panama. A means that can protect the plants from the fungus, there is not today.

Thus, TR1 spread from Central America to South America, Africa, Asia and Australia. Afflicted plantations had to be cleared and were then no longer useful for banana cultivation. Farmers had no choice but to replace Gros Michel with the Cavendish that we eat today.

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She had proven to be TR1-resistant. But the harmful fungus that triggers the panama disease can adapt to different varieties of banana and now also brings the Cavendish in danger. This time, however, there is no resistant variety that could be grown instead.

World's most important banana export region affected

"Tropical Race 4 (TR4)", as the newer pathogen is called, was initially sporadically found in Southeast Asia. Over time he spread to the Middle East, Australia and Africa. India and China are already affecting the world's largest banana producers.

In South America, TR4 could soon reach Brazil and Ecuador, the third and fourth largest banana producers in the world (see chart). South America is the most important export region with an annual sales value of six billion dollars.

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Although the affected states have been trying for years to contain the pest by means of protective measures, no country has yet succeeded in the long term. Colombia also declared a national emergency after the discovery and announced that it would burn down the 170 hectares of farmland affected. At ports, airports and borders there should be more controls.

But once the pathogen reaches a region, it can hardly be stopped. TR4 lives in the ground. An earth-polluted shoe sole or a plant part is enough to carry the fungus from one plantation to the next. The pathogen also survives in the water. Once in the ground, he can infect banana trees years later.

The fungus penetrates through the roots into the vascular system of the bananas and spreads there until the plant starves and dies of thirst - on the outside this is recognizable by yellow, drooping leaves. Panama disease is not a danger to humans.

"The worst, craziest monoculture in the world"

That it is so difficult to contain is also due to the way bananas are grown. Unlike wild bananas, modern culture concerns can not multiply on their own because they have no seeds. These were bred out for taste reasons. The plants are now increasing, as farmers put cut shoots in the ground.

All Cavendish bananas are therefore genetically identical clones. "Bananas are the worst, craziest monoculture in the world," said banana specialist Gert Kema from the Dutch University of Wageningen to SPIEGEL in 2016. The clones have virtually nothing against pathogens. If a fungus succeeds in infesting a single such plant, the entire population is at risk.

How fast the disease will now spread in South America, can be difficult to estimate. It usually takes a while. For example, TR4 took just under 20 years to reach the north of the neighboring state of Queensland from the Northern Territory in Australia, where it was first discovered in 1997.

"These epidemics are developing slowly," said Randy Ploetz, a plant pathologist at the University of Florida of the news section of the journal Nature. "But eventually it will not be possible to produce Cavendish for international trade." In that case, banana shelves could actually be empty even in German supermarkets.

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"The banana we know is extremely threatened - and a successor is not in sight," warned Kema three years ago. Although it is conceivable that a TR4-resistant banana variety can be grown again by crossing different varieties. So far, however, this was not very promising.

Experts are therefore increasingly placing their hopes on trials with the collateral scissors Crispr. Queensland University scientists have found that the Cavendish carries a gene that protects a variety of wild bananas from the dangerous TR4 fungus. This is barely read in the crop. You now want to increase the gene's activity with the help of Crispr.

Whether this succeeds is still unclear. But even if the project succeeds, Germany will not secure the supply of bananas. In July 2018, the European Court of Justice found that Crispr genetically modified plants are subject to strict European genetic engineering law. The import of a TR4-resistant banana produced in Europe would therefore be prohibited.

Source: spiegel

All tech articles on 2019-09-03

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