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Column: Popular banana variety faces fatal fungal threat

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Lowly bananas are the world’s most popular fruit. Sadly, the yellow fruit is in danger.

A fungus, fusarium, wiped out tens of thousands of acres of Cavendish bananas in Australia and Southeast Asia in the 2010s. The resilient fungus has gained ground in Africa and the Middle East, having hitched a ride on the boots of workers helping to establish new plantations.

Latin America, the source of virtually all bananas eaten in the Unites States, is next. In August authorities in Colombia declared a national emergency after confirming that the Panama disease had reached Latin America. “Once you see it, it is too late, and it has likely already spread outside that zone without recognition,” said one expert quoted by National Geographic.

The Cavendish banana is the ultimate fruit package. No other variety has both the sweetness and texture for packing. In Australia researchers are planting both genetically modified strains and conventionally bred hybrids in the hopes of preventing a banana apocalypse, a world with no bananas on store shelves.

Since its discovery and identification in 1989 in samples from Taiwan, banana farmers have been fighting the fusarium wilt, also known as Panama disease, Tropical Race 4 or TR4.

Fusarium is a tricky adversary. Fungicides and fumigants are ineffective, it is extremely contagious and it can lie dormant for decades. It tricks farmers into thinking they have eradicated it only to find plants rotting from the inside.

The only recourse once discovered is to kill all the plants and start over.

Some researchers fear that in a few years affected plantations will not be able to grow because no replacement is available.

Both biotech researchers and breeders tried unsuccessfully for decades to either fortify the Cavendish to resist disease or hybridize a replacement for the Cavendish. The thick-skinned, slow-ripening Cavendish has dominated banana exports to the tune of a $12 billion global business.

Banana farmers had reported soon after TR4 that a subspecies of the Musa acuminata variety, which grows wild across Malaysia and Indonesia, was growing happily in plantations devastated by TR4.

After years of painstaking research, James Dales’ lab at Queensland University of Technology in Australia identified several candidate genes that seemed worth testing. After three more years Dale inserted genes from M. acuminata, growing them first in test tubes, then in whole plants, which takes about a year to grow into plants that can grow in soil.

End of story? Not quite. No one wanted to pay for field trials because growers believed they could manage the disease. Another three years passed before Dale found a facility to produce transgenic bananas. A small field trial was “extremely positive,” Dale said. Four of six plants showed resistance to TR4, an impressive and unusual response.

This may be the best hope for making the Cavendish resistant to TR4 while keeping the taste and texture that have made it so successful.

Botanists and horticulturalists in locations around the world are collecting wild bananas to see which, if any, have resistance to TR4. Those that do are bred with Cavendish with the hope that the Cavendish hybrid will retain its commercial and consumer appeal.

This is not the first rodeo for the banana. In the early 20th century, the world’s banana was the Gros Michel, a short, straight and stubby cultivar. A fungus, Tropical Race 1 (TR1), nearly drove it to extinction.

The first strain of the Cavendish was a variety from a Chinese greenhouse that showed resistance to the TR1 fungus while retaining the same shipping durability. The bananas in stores today are all clones from an English hothouse.

There may be a new banana hiding somewhere in the genome. It could be that TR4’s march to Latin America may be inevitable, and breakfast may never be the same.

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