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In this post, I originally intended to focus on the evils of monoculture, seen through the lens of the Cavendish banana. Over the past few years, articles about the banana’s impending demise from disease turn up sporadically, so I thought I could use this as another way to preach from my soapbox about sustainable agriculture. However, after digging deeper into both monocultures and bananas, I unearthed information that turned this post from mostly a soapbox to a report on advances in plant and fungal genetics, with a short detour through the definition of monoculture and a few words about the dangers thereof.
Are bananas grown as a monoculture?
The simplest definition of monoculture is the practice of growing a single crop in a field during one growing season. Polyculture is growing more than one crop in the  same field at the same time, such as interplanting corn and beans. That’s it. Many farmers rotate crops; they plant a different crop in a given field each year in a regular sequence. Many people, myself included, think of a monoculture as the same crop—often the same species or variety—in the same field year after year.
Commercially-grown bananas, however, meet my conception of a monoculture in spades. Not only is the same crop grown in the same fields every year, but growers also produce new Dwarf Cavendish banana plants, the most widely-grown of several Cavendish clones, vegetatively. These bananas are parthenocarpic (seedless). Vegetative reproduction ensures that each plant is a clone of every other plant; they have identical genotypes. So yes, bananas are a monoculture.
And this is bad because…
Remember the Irish potato famine? Most of the potatoes planted in Ireland were of one variety. This lack of genetic variation exacerbated the blight’s severity. When a pathogen attacks a vulnerable plant variety, all plants of that variety are at risk. The Understanding Evolution website points out that “…evolutionary theory tells us that relying on crops with low genetic variation can lead to disaster.”
Now commercial growers overwhelmingly rely on one cloned variety of banana, grown as a monoculture, for export. Ironically, Central and South American banana producers switched to Cavendish from the Gros Michel, or Big Mike, banana in the 1950s-’60s when Big Mike succumbed to the first wave of Panama disease, a form of Fusarium wilt. Cavendish plants aren’t susceptible to this strain of Fusarium, and their fruits are well-suited to large-scale commercial enterprises. Picked green, these bananas take several weeks to ripen, allowing for longer shipping and storage times. Unfortunately, Cavendish isn’t as tasty as Gros Michel, but consumers don’t appear to mind.
Given what we know about the Irish potato famine and the decline of Gros Michel, you’d think that banana growers would try to tap into some of the genetic diversity available in wild bananas. With 47 percent of the world banana crop devoted to Cavendish monoculture, banana producers and aficionados are poised on the precipice should another blight arise.
…here comes Tropical Race 4
And that’s exactly what’s happening. A new form of Fusarium fungus, called Tropical Race 4, has emerged in Asia and is decimating banana plantations. The disease is easily transmitted by soil and water and perhaps by air. The disease spores migrate so easily that this year’s International Banana Congress moved to Costa Rica from Miami out of concern that attendees might spread the disease. Plantations infected with TR4 will take years to recover, if indeed they can recover. Fortunately, TR4 hasn’t yet hit Central and South America, so there’s time to work on a solution.
Unfortunately, breeding new disease-resistant banana varieties has traditionally taken decades. The British started in the early 1920s attempting to breed a Panama disease-resistant Gros Michel banana. The challenge of breeding tasty, seedless
bananas, though, is complicated by banana triploidy and tetraploidy. Natural mutations have created sterile triploid and tetraploid varieties. Hybridizers typically cross diploid plants (which have seeds) with triploid plants, sort through the progeny, and select likely candidates for more study. This process is long, painstaking, and complicated. No useful new hybrids or cultivars have emerged in 60 years. Over the last few years, multiple organizations have joined the hunt, but no new plants, yet.
Genetics to the rescue
Earlier this year (2016), a trio of researchers from the University of California at Davis, the University of Queensland, and the University of Wageningen announced a stunning discovery. Using pieces of the genetic code for both the Cavendish banana and two of the three most serious banana-attacking fungi, Ioannis Stergiopoulos (UC Davis), André Drenth (Queensland), and Gert Kema (Wageningen) demonstrated how the fungus appropriates the banana’s metabolic pathways and uses its nutrients. Stergiopoulos characterized this discovery’s significance: “This parallel change in metabolism of the pathogen and the host plant has been overlooked until now and may represent a ‘molecular fingerprint’ of the adaption process.” Fungi adapt their metabolism to match that of the host plant, enabling the fungi’s efficient and swift conquest of the host. Understanding this mechanism should lead to development both of more resistant plants and more effective fungicides.
This research looked at the Sigatoka complex of three fungal diseases caused by three species of the Pseudocercospora fungus, not Panama disease caused by TR4. However, as Steriopoulos pointed out, researchers can parlay this understanding of the importance of the metabolic takeover process into solutions for other fungal diseases, not just of bananas but also other crops.
Consequences of a Bananapocalypse
Bananas are the world’s favorite fruit and probably our oldest fruit. The 100 million bananas eaten yearly make this the world’s fourth most important food crop. In the
United States and Europe, bananas are a sweet treat. In much of the world, though, the fruit is a diet staple, providing up to 30 percent of daily calories consumed. In addition to providing calories, banana crops are critical economic assets to growers and to the countries that produce them. If the Cavendish banana goes extinct, banana lovers won’t be the only ones affected; millions of people in developing nations will suffer as well. This promising genetic research could save much more than just a popular fruit.
Resources
"Experts Race to Stop Bananapocalypse from Hitting Latin American Producers." The Guardian 21 April 2016. Retrieved November 9, 2016.
"Fungi Could Wipe Out Bananas in 5 to 10 Years." Futurity 15 August 2016. Retrieved November 7, 2016.
"Panama Disease." Retrieved November 10, 2016.
"With the Familiar Cavendish Banana in Danger, Can Science Help It Survive?" The Conversation 23 October 2016. Retrieved November 7, 2016.
Images credits:
Banana plant and fruit. Tun Institute of Learning
Banana varieties. Wikimedia Commons.
Australian banana plantation ca. 1907. Wikimedia Commons.
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