Pests, Clones & The Imperiled Banana

The global banana trade is dominated by a single clone: the Cavendish. However, the ease and affordability of growing a single variety comes at a heavy price, Zaria Gorvett writes.

Anyone who has mistaken a plantain for a banana will agree: not all bananas are created equal. In fact, the fruit are paragons of agricultural achievement, transformed from the bitter, starchy fruit of an obscure Southeast Asian herb, to the eminently edible center of a mega-industry. And it’s only taken 10,000 years.

Cheap, easy to transport, and high in nutrients, the banana is produced to the tune of 145 million metric tons per year–that’s about 170 times the weight of the Shanghai Tower, or 800,000 blue whales. But the beloved banana is under threat: a horde of deadly pathogens are decimating plantations around the globe, putting the livelihoods and staple diet of millions of people at risk.

Termed a “Bananageddon”, it has in fact happened before in living memory. As the export business braces itself for near-extinction, and small-holding farmers sink 50 percent of their capital into pesticides, experts are warning that industry practices are putting crops at risk. Will the developments which have empowered the banana lead to its demise?

The banana’s is an intriguing tale of a ruthless killer, some bizarre genetics, and a sexless clone; and begins with a surprising fact. Despite the existence of between 300 and 1,000 varieties of banana, 97 percent of banana exports belong to a single variety, the Cavendish.

The killer: Panama disease

It has not always been this way. For all its popularity, it turns out the Cavendish–originally from China and named after an English Duke–is actually a stand-in for true banana nobility: the Gros Michel. Reportedly superior in flavour and sweetness, “Big Mike” was the prevailing banana for export until the plants were beset with sickness in the 50’s. Their assailant was a resilient fungi found in the soil. There was no escape; Tropical Race 1 Fusarium oxysporum (TR1) infected the plants from the bottom-up, slowly rotting its victims’ internal plumbing until they died of thirst. The so-called Panama disease destroyed hundreds of thousands of hectares of the crop, bankrupting landowners as it spread. Pristine rainforest was felled in Latin America to clear uninfected land for planting, but inevitably the banana business succumbed. The Gros Michel was no more, and the Cavendish–thought to be resistant– was briskly adopted.

The modern crisis is eerily familiar. By 1990, the Cavendish was cultivated across the globe, where the assurance of this new variety did not last long. A new pathogen was on the horizon, and this time it affected plantations in South East Asia. A fresh, virulent strain of F. oxysporum called Tropical race 4 (TR4) had emerged, a revelation which plunged the industry into a fresh wave of panic. While for the moment the disease is confined to the Eastern hemisphere, it is highly transmissible and expected to spread.

What makes bananas vulnerable to disease?

As one of the first domesticated crops, bananas have been at the vanguard of human innovation for some time. In fact, long before Dolly the sheep or Brave New World, ancient banana farmers were honing their cloning techniques. Banana plants are infertile; they haven’t had sex for thousands of years. Instead, they have been tenderly dispersed by farmers, who remove and re-plant underground shoots which grow out of their root systems, called suckers. This method of fruit cultivation yields genetically identical banana plants each time, and is popular because sterile plants invest less in producing seeds. Yet bananas are missing out on an extraordinarily useful process.

Sexual reproduction, in which gene variants are shuffled and new combinations concocted, increases the genetic diversity of a population and helps organisms to adapt. All this means that eating a banana is an astonishingly consistent experience, down to the very last chromosome, and that they are more than a little evolutionarily decrepit. In the arms’ race between bananas and their enemies, the plant is thousands of years behind.

Growing single clones makes plantations especially vulnerable, because genetically identical plants tend to be susceptible to the same pathogens. Once they have arrived, they spread rapidly and can easily wipe out an entire crop.

Dr. Elizabeth Aitken, associate professor in plant pathology at the University of Queensland told Asian Scientist Magazine “this [vulnerability] was demonstrated in 1940’s Europe with devastating effects, when the potato crop failed due to Phytophthora infestans–which caused what we now know as the Irish Potato Famine. At that time potato production in Europe was dependent on only a very few introduced potato lines from South America”.

Modern farming practices have compounded the problem. Bananas, like many crops, are grown in monoculture, where a single plant is grown over a wide area. This highly efficient method of farming has expanded rapidly, turning an industry dominated by smallholdings into a commercial business of sprawling fields and high-yields. Plantations can encompass hundreds of hectares and consume vast quantities of agrochemicals.

Professor Randy Ploetz at the University of Florida told Asian Scientist Magazine, “Monocultures of susceptible clones like Cavendish hasten the spread and increase the ultimate impact of a problem like Panama disease. Nonetheless, monocultures are the least expensive way of producing high volumes of fruit. Although producing bananas in heterogeneous plantings of different cultivars or mixtures of other crops would slow the spread of the disease and reduce its impact, it would also increase costs where the disease is not found.”

Why can’t farmers just breed new banana varieties?

To understand why banana breeding is difficult, it helps to compare genomes. The human genome is bundled up into chromosomes, and all our cells have two identical copies of each (46 in total). There is just one process which requires fewer: sex. Sperm and egg cells only get one copy of each chromosome (23), so that when they combine, voila: our offspring have the same number as their parents. Most mammals and some plants, including wild bananas, follow suit, we are “diploid”.

However, occasionally something goes wrong. Some parental banana plants fail to divide their chromosomes in two, resulting in pollen or egg–the banana equivalents of sperm and egg–with two sets of chromosomes instead of one. When these fuse with normal sperm/egg, the offspring have three sets of chromosomes, they are “triploid”. And this is exactly what happened to the domestic banana, a sterile mutant with a third too many gene copies. It’s what makes the fruit larger than wild bananas, and seedless, but it also means that breeding existing varieties to create new ones is extremely difficult.

Or is there another way? Professor Ploetz is optimistic.

“Biotechnology has provided solutions to other disease problems. The best current example on a fruit crop is probably GM (genetically modified)-papaya that resists ringspot disease.”

The plight of the banana has the potential to wreak havoc. As food security in the developing world becomes increasingly precarious, and the banana export business faces crisis, new measures to protect the crop from pests are urgently needed. Monocultures of crops and livestock have left agriculture increasingly dependent on pesticides and vulnerable to mass annihilation. Farmers have a stark choice: diversify, or risk becoming the next example of agricultural collapse. But will farmers heed the warning?


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Zaria Gorvett is a freelance science writer based in the UK. She graduated with a bachelors degree in biological science from the University of Exeter, UK and a masters degree in medical microbiology from the London School of Hygiene and Tropical Medicine, UK.

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