Can Bananas Fend Off Fatal Fungus?

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The world's most popular bananas — the Cavendish cultivar variety — have come under threat from a fungus that's rapidly spreading around the world. Previously constrained to parts of Asia and Australia, the banana fungus, also known as Panama disease, has also appeared in the Middle East and more of South Asia.

Now the fungus has spread to Latin America — something experts had long feared, which could be disastrous for the worldwide market because that's where the majority of Cavendish bananas are grown. In early August, the Colombian Agricultural Institute announced a national state of emergency, confirming that the fungus had been found in plantations in the north of the country, reports Nature. In an attempt to stop the spread, crops were destroyed and plantations quarantined.

Industry analysts say the Cavendish's days are numbered, but it likely won't happen soon. "These epidemics develop slowly, so the [spread] will take some time,” Randy Ploetz, a plant pathologist at the University of Florida in Homestead, told Nature. "But eventually, it will not be possible to produce Cavendish for international trade."

Once the fungus — Fusarium oxysporum f. sp.cubense, more commonly referred to as Foc — takes hold in the soil, it's almost impossible to eliminate. No one knows exactly how the fungus arrived in these new areas, but some people think it could have arrived with migrant workers who came from Asia to work on local plantations.

The worldwide banana market is difficult to quantify because so many banana producers are small-scale, local farmers, but the Food and Agriculture Organization of the U.N. says global production of bananas was 114 million tonnes in 2017, up from around 67 million tonnes in 2000.

A tangled tale

Bananas have a long history with varieties of the Foc fungus. A different strain all but wiped out the once-popular Gros Michel banana cultivar back in the 1950s. That particular strain is not a threat to Cavendish bananas, which replaced Gros Michel, but they are susceptible to the newest strain, called TR4, which is the one that has spread to Latin America. Cavendish bananas represent about 13% of worldwide banana sales. Other varieties may not be at risk from the fungus, but its spread would hurt farmers across the globe.

The only useful solutions for such farmers is quick action to prevent further plantations from being devastated by the fungus. It's possible to quarantine the affected regions and destroy infected plants, but the fungus will remain in the soil, meaning Cavendish bananas can't be grown there again. The bigger problem is that all Cavendish bananas are all the same — literally. They are all clones of the same banana, which means their reaction to this disease is exactly the same: a complete meltdown best described in this article in Science Alert:

This fungus is incredibly efficient at infecting banana crops, and when it does, it’s devastating. Transmitted through both soil and water, F. oxysporum can lay dormant in the soil for up to 30 years, and it’s virtually impossible for growers to know their crops have it without rigorous testing (which doesn’t exist). Once it latches onto a suitable host, it finds its way to the root system and travels up to the xylem vessels - a plant’s main water transporters.

You can learn more about Panama disease — which can be transmitted to a new location on just a tiny bit of soil — and efforts to curtail it in this video:

The hits keep on coming

The fungus is not the only threat to bananas. In 2013, Costa Rica's $500 million banana industry was in a state of national emergency, according to the Independent, after being hit by mealybugs and scale insects, which affected as much as 20% of the country's crop. The bugs cause blemishes on the fruits, making them unsellable. The increased insect population was blamed on climate change.

In 2016, researchers from the University of California, Davis and the Netherlands sequenced the genomes of three strains of fungus that cause Sigatoka, which hijacks the bananas immune system, according to Science Alert. The update prompted a renewal of dire predictions for bananas as we know them today because this disease has also managed to manipulate the bananas' metabolism.

Oddly enough, there's an upside to the news: The genome sequencing that uncovered how Sigatoka works may also help scientists create disease-resistant varieties of bananas.

"Now, for the first time, we know the genomic basis of virulence in these fungal diseases and the pattern by which these pathogens have evolved," UC Davis plant pathologist Ioannis Stergiopoulos said in an update for the UC Davis website.