Agriculture: The Best Banana Bred
New hybrids may save the banana industry from destructive diseases
LAST YEAR the United States imported more than 6.3 billion pounds of bananas. That’s a banana a week for every man, woman, and child in this country, or over twenty-four pounds a year each, or three pounds more than our annual per capita consumption of apples, the next most popular fruit. Nearly all the bananas we eat are grown in Honduras, Costa Rica, Colombia, and Ecuador (they aren’t called banana republics for nothing). For example, Ecuador, the world’s leading exporter of the fruit, shipped nearly 1.6 billion pounds of bananas, worth $158 million, to the United States last year. Honduras shipped $190 million worth. Bananas account for between 35 and 40 percent of export earnings in Honduras and around 20 percent in Costa Rica.
Yet despite the enormous demand for its produce, the multibillion-dollar banana industry is in trouble. In 1972 plantation workers in Honduras discovered a fungus called black sigatoka, which attacks the banana plant’s leaves, preventing photosynthesis. Within a decade the disease had spread throughout Central America, and by 1979 it had reached Africa as well. Central and South American growers spend $60 million annually fighting black sigatoka, and in Honduras, where the infection has the strongest hold, battling the fungus generates the largest single cost of growing bananas. Worse still, the fungus also attacks the plantain, a starchy cousin of the banana which is a staple in many parts of the world. In Africa, where millions of people depend on bananas and plantains for the equivalent of their daily bread, black sigatoka has the potential to cause widespread famine.
Banana-eaters around the world may have a savior, however, in Phillip Rowe, an American plant breeder working in Honduras. Rowe, fifty, has spent the past two decades at a research station in La Lima, on the lush Caribbean coast of Honduras, breeding the world’s first disease-resistant hybrid banana. It is also certainly the world’s most expensive, having cost many millions of dollars thus far. “We have a banana now that would do in an emergency,” Rowe says. “By the 1990s people will be eating a whole new banana, and most of them will never know the difference.”
Since the late nineteenth century, when norteamericanos first went bananas for bananas, Latin American plantations have grown just two varieties of the fruit. The first was the Gros Michel, a native of Southeast Asia, which was brought by the French to the Caribbean more than two hundred years ago. The Gros Michel was big, sweet, and, most important, capable of withstanding a two-week voyage in the hold of a ship. Although other bananas actually taste better, the Gros Michel and the later commercial variety, the Cavendish, are prized by exporters because they are large and easy to ship; one of their best features is that all the “fingers” on a stalk ripen at once, about three weeks after they are picked. As they mature, bananas exude ethylene gas, which triggers ripening in fruit nearby. With the commercial varieties, it’s less likely that one banana in a box will ripen early and spoil the lot.
The Gros Michel had one drawback, however. The plant was plagued by two diseases: yellow sigatoka, first detected in 1904 in the Sigatoka Valley, in Fiji, and a soil-borne fungus called fusarial wilt, or Panama disease. Panama disease was by far the more serious of the two. Yellow sigatoka could be controlled by spraying, but no cure is known for Panama disease, which spreads through irrigation water and kills the plant. United Fruit, the largest of the three American conglomerates that dominate the banana business, managed to stay one step ahead of the fungus, almost literally, by moving its plantations to uninfected lands. By the early 1960s, however, no more land was left, and growers were forced to replant their fields with the Panamadisease-resistant Cavendish variety. The Cavendish, which looks, tastes, and ships much the way the Gros Michel does, is virtually the only type of banana that appears in American grocery stores today.
Now the Cavendish is threatened too. Black sigatoka is far more virulent than its predecessor. It attacks the plant’s leaves, creating spots of blackened, dead tissue. The spots spread, eventually killing the leaf, while an unknown by-product of the fungus causes the fruit to ripen prematurely. Left unchecked, the fungus would reduce the yield from each plant by 30 to 50 percent, and growers can control outbreaks of black sigatoka only by aerial spraying.
In 1969, after Panama disease had forced gnawers to switch to the Cavendish but before the onslaught of black sigatoka, Rowe, who has a Ph.D. from Michigan State University, arrived in Honduras with his wife to work for United Fruit. “The company had the good sense to realize that if disease could appear once, it could happen again,”he says. “Nature had provided us only two varieties suitable for export—the Gros Michel and the Cavendish—and if we had to change again, we would have to breed new ones.”
Rowe set to work searching for natural resistance among nearly 850 varieties of bananas, wild and cultivated, that United Fruit had gathered around the world from 1959 to 1961. Most of them had originated in Southeast Asia, including the Cavendish, which had been discovered in a botanical garden in Saigon. Once Rowe had selected the disease-resistant plants, he began crossbreeding them to develop a hybrid suitable for commercial use.
By the early 1980s United Fruit had spent upwards of $10 million on the project, and no superbanana was in sight. The company decided to withdraw when it realized, belatedly, that it could not hope to patent the eventual fruits of Rowe’s labor, and thereby keep them from competitors. United Fruit donated its botanical collection and its research facilities to the Honduran Foundation for Agricultural Research (FHIA), a private scientific organization established in 1984 with the help of a ten-year, $20 million grant from the U.S. Agency for International Development (AID). “They also donated their plant breeder,” Rowe says. He now supervises about forty people as the head of FHIA’s banana-breeding program, which is considerably larger and further along than any of the three other banana-breeding projects in the world.
For plantation owners Rowe’s new banana will be ready none too soon. Black sigatoka is growing immune to one fungicide after another—thus far growers have gone through three. (The fungicides act only on the leaves and do not reach the fruit.) But growers are helpless in the face of a new variety of Panama disease. The original form of the fungus, called race one, has mutated in some parts of the world, giving rise to three new races. Race four, the most virulent of them, is now attacking banana crops in Taiwan, South Africa, and Australia.
“Panama disease has no respect for anything,”says Christopher Millensted, one of FHIA’s founders and currently its director of development. “Soils in Latin America are loaded with it, and once it mutates, it could attack any banana variety. If the mutation that has already happened in Taiwan occurs here, the Cavendish is in grave jeopardy, and I wouldn’t put a buck on the table and say that it’s not here already.” Fortunately, two years ago Rowe found a race-four-resistant banana in the botanical collection. This banana’s genes will have to be incorporated into his hybrid.
As IMPORTANT AS Rowe’s work is to the banana industry, it is vital to the millions of people who depend on bananas and plantains for food. World production of bananas and plantains is about 68 million metric tons a year. The total world export trade in bananas is only seven million tons, and trade in plantains is a fraction of that, which means that both fruits are much more important as local food crops than as export commodities.
Black sigatoka has devastated the plantain in Latin America. AID estimates that the area planted in plantains in Central America has decreased 50 percent since black sigatoka spread throughout the isthmus. Production of plantains has fallen off by more than half, and a third of producers have abandoned their fields. Plantain farmers could control the fungus by spraying, Rowe says, “but most of them can’t afford it.”
Worse still, black sigatoka has spread to Africa, where virtually all of the 24 million metric tons of bananas and plantains produced are consumed locally. The telltale spots first appeared on plantain leaves in Gabon in 1979. The fungus quickly swept across the continent, its spores blowing from plant to plant in the wind; it is now found in every equatorial nation except Uganda. “What frightens me more than anything is what is going to happen to East Africa if we don’t get a black-sigatoka-resistant variety soon.” Rowe says. In tropical Africa the banana and the plantain are dietary staples for millions of people. Seven to eight million Ethiopians eat the starchy center of the stem of the ensete, or Abyssinian banana (whose fruit is inedible). In West Africa millions of people eat plantains. Twenty million East and Central Africans each consume as much as 500 pounds of bananas a year. They pick their bananas green, when the starch has not yet turned to sugar, and roast or steam them.
According to Stephen Carr, the principal agriculturalist of the Africa Technical Department of the World Bank. East Africa will be the region hardest hit by black sigatoka. “We think black sigatoka is really very serious,”he says. “The areas where it’s advancing in East Africa are extremely densely populated. People are squeezed onto pocket-handkerchief-sized pieces of land. For fifty percent of the population of Uganda, bananas are the dominant staple. The same is true for parts of Rwanda, Burundi, Zaire, Kenya, and Tanzania. In all of these areas a substantial part of the population is dependent upon bananas for eighty percent of its carbohydrates. Bananas are a good perennial crop, they don’t deplete the soil, and an acre and a half of bananas will feed an entire family. If you had to replace that with an annual grain crop, you’d be in real trouble.”
All told, AID and the World Bank estimate, anywhere from 50 to 100 million Africans will be affected to some degree by the encroachment of black sigatoka. “Black sigatoka is devastating once it becomes uniformly spread,” Rowe says. “If it spreads as projected in the areas where it has already appeared, it’s going to cause famine in three to five years.”
Rowe’s work on a hybrid commercial banana could well save both the Latin American banana industry and the populations of parts of equatorial Africa. Rowe has already made considerable progress in crossing plantains with the disease-resistant bananas that he has spent the past two decades developing. “It’s a little like breeding elephants to improve giraffe stock,” Millensted says, admiringly. Rowe has recently begun working with the East African cooking banana, which grows at altitudes of 3,000 to 9,000 feet, to incorporate it into his program.
BREEDING A NEW banana, however, is a lot more difficult than breeding a new hybrid of wheat, say, or a tomato, because the genetics of edible bananas are quite peculiar. Unlike most bananas—and, in fact, unlike most other organisms on Earth—the Gros Michel and the Cavendish are triploids, which means they have three sets of chromosomes instead of the usual two.
This condition, known as triploidy, is what makes edible bananas edible and the Gros Michel and the Cavendish so attractive commercially, because it causes their fruit to be large, hardy, and seedless. (Those tiny dark specks in the center of a banana from the supermarket are infertile vestigial seeds.) Diploid bananas, those with two sets of chromosomes, contain scores of seeds as big as half a pencil eraser and hard enough to break your teeth.
Seedlessness poses no problem for growers. All bananas can be propagated by rhizomes, which are shoots that the banana plant sends out beneath the ground; new plants grow up from the rhizomes around the parent stalk. Growers can uproot the sprouting rhizomes and replant them, or they can cut a tiny piece from a rhizome’s growing tip and clone dozens of new banana plants in a petri dish.
Seedlessness does, however, make difficult the job of the plant breeder. Since the Gros Michel and the Cavendish have no seeds, they are technically sterile, and an organism that cannot reproduce cannot be crossbred.
Fortunately, certain subvarieties of the Gros Michel can be forced to produce seeds if their female flowers are fertilized by hand with pollen from diploid bananas. But this laborious process yields very few seeds, only two or three a bunch, and in breeding for some characteristics Rowe needs as many as 10,000 bunches to produce a single seedling. “You have to go through a lot of bananas to get a few seeds, and we need a lot of seeds,” he says. “We plant from five to ten thousand plants a year for each subsequent pollination, and it takes about three years from seed to seed.”
To complicate matters, when Rowe crosses the Gros Michel with his disease-resistant diploid bananas, the offspring he creates are tetraploids— bananas with four sets of chromosomes. Tetraploids, like diploids, make seeds. To get a commercially suitable triploid, Rowe must now back-cross a tetraploid to a diploid. Recently he has come up with tetraploids and diploids that he considers good enough to use in producing the ultimate banana.
One tetraploid in particular looks very promising. In its pedigree is a banana from Burma—and ten other types. It’s completely resistant to black sigatoka as well as to a number of other banana blights. Unfortunately, it is also full of seeds, is susceptible to Panama disease, and has a flavor very different from that of the familiar Cavendish.
Still, in banana-breeding terms, success is just around the corner. “We fully expect to get a triploid that will be a good export banana in the next five years,” Rowe says. “You have to understand, that’s a very short time— only about two or three more crosses. The beautiful thing about the banana is, once you get a plant with the characteristics you need, you can then produce it vegetatively, by rhizomes. And we expect it to taste as good as or better than the Cavendish.”
—Shannon Brownlee