Banana twilight

A widely read and frantically e-mailed New York Times story has opened many new eyes to a horticultural disaster anticipated for many years: the commercial extinction of the Cavendish banana. Dan Koeppel's warning is right on target:

By sticking to [a] single variety, the banana industry ensures that all the bananas in a shipment ripen at the same rate, creating huge economies of scale. The Cavendish is the fruit equivalent of a fast-food hamburger: efficient to produce, uniform in quality and universally affordable.

But there’s a difference between a banana and a Big Mac: The banana is a living organism. It can get sick, and since bananas all come from the same gene pool, a virulent enough malady could wipe out the world’s commercial banana crop in a matter of years.

This has happened before. Our great-grandparents grew up eating not the Cavendish but the Gros Michel banana, a variety that everyone agreed was tastier. But starting in the early 1900s, banana plantations were invaded by a fungus called Panama disease and vanished one by one. Forest would be cleared for new banana fields, and healthy fruit would grow there for a while, but eventually succumb.

By 1960, the Gros Michel was essentially extinct and the banana industry nearly bankrupt. It was saved at the last minute by the Cavendish, a Chinese variety that had been considered something close to junk: inferior in taste, easy to bruise (and therefore hard to ship) and too small to appeal to consumers. But it did resist the blight.

Over the past decade, however, a new, more virulent strain of Panama disease has begun to spread across the world, and this time the Cavendish is not immune. The fungus is expected to reach Latin America in 5 to 10 years, maybe 20. The big banana companies have been slow to finance efforts to find either a cure for the fungus or a banana that resists it. Nor has enough been done to aid efforts to diversify the world’s banana crop by preserving little-known varieties of the fruit that grow in Africa and Asia.

The problem is straightforward. Commercial bananas are sterile mutants, whose separation from sexual reproduction prevents the Cavendish cultivar from evolving resistance to Fusarium oxysporum (the fungal agent responsible for Panama disease), black sigatoka, and any number of other fungal or bacterial threats. The technical details regarding the use of asexual vegetative reproduction to propagate bananas are fascinating:

Photo: Steve Hopson, © 2007.

The cultivated banana is often listed in botanical references as Musa x paradisiaca (Musaceae), although it is actually a complex hybrid derived from two diploid Asian species, M. acuminata and M. balbisiana. Common cultivated bananas are usually triploid (3n) with three sets of chromosomes. [Note: The word "set" is defined here as one haploid set of chromosomes.] If A represents one set of chromosomes from diploid M. acuminata (AA) and B represents one set of chromosomes from diploid M. balbisiana (BB), then hybrid bananas have three sets of chromosomes represented by AAB, ABB or another 3-letter (triploid) combination of A's and B's. Like seedless watermelons and red grapes, bananas are sterile and do not produce mature seeds. . . . Bananas are sterile and seedless because they are odd polyploids in which one set of chromosomes (A or B) has no homologous set to pair up with during synapsis of meiosis. Therefore meiosis does not proceed normally, and viable gametes (sex cells) are not produced. Since banana fruits (technically berrylike ripened ovaries) develop without fertilization they are termed parthenocarpic. Without viable seeds, banana plants must be propagated vegetatively (asexually) by planting corms, pieces of corms or sucker sprouts.

The solutions are obvious and expensive. Organizations preserving diverse cultivars and wild varieties need all the help they can get. One of those varieties — if we haven't already destroyed it in our ongoing, careless elimination of natural and agricultural biodiversity — may offer temporary relief against Fusarium oxysporum and Panama disease. And the commercial banana, snared in the sterile trap of mass propagation, needs for the first time in decades to have sex.