The American corn blight
Farmers, food manufacturers, and consumers in the United States found out in 1970 just how important genetic diversity is. That was the year the Southern corn blight came to visit.
 |
The
American corn blight of the 1970s showed how vulnerable single-crop
systems (“monocultures”) are to pests and
diseases. This corn suffered from a blight of a different sort
— a summer-long drought in 1993. (Fred
Powledge) |
Farmers are always on the lookout for ways to grow more productive crops on the same amount of land. For many years, plant breeders (who typically worked for seed companies, state universities, or the U.S. Department of Agriculture) had been using artificial means to combine (“cross”) corn and other plants that would not normally breed together. And not just plants: A mule is the result of a cross between a horse and a donkey; mules cannot ordinarily reproduce by themselves.
The results of these artificial joinings of similar, but not identical, species are called hybrids. Hybridization is used extensively in agriculture to force plants and animals to develop characteristics that humans want. Thanksgiving turkeys, for example, are the result of human-controlled breeding to produce abnormally large breasts because that is what people like to eat. The result is a strange bird that can hardly walk. Artificial breeding is used widely in livestock. The semen of a single male (a bull, or a ram, for example) can be collected, frozen, and used to produce thousands of offspring that exhibit some of the traits of the “father” they will never meet.
With artificial breeding so widely accepted by the agricultural community, it came as no surprise that when a hybrid corn (called maize in most of the world) turned out to have many desirable characteristics, it would be adopted widely by farmers. Fields that once produced many different kinds of corn (that is, that were the home of a great deal of corn diversity) now were planted with a single species. The term for this is monoculture — the use of a single sort of organism, rather than a variety of organisms.
The corn that brought so much cheer to farmers had a problem, though. A fungus called Bipolaris maydis had showed up on the maize in the Philippines in the 1960s. Like many fungi that attach themselves to plants, this one sapped part of the corn’s energy. The fungus had become an enemy to the growers of corn in the Philippines. Farmers in the U.S. paid little attention, however. Like most diseases of the Third World, there was little notice or consternation in the richer nations. Then, however, the fungus moved to Mexico, right next door to the U.S.
By 1970, the disease was showing up in Florida, where a lot of corn is grown. When the fungus started infecting corn plants and making them unsuitable for marketing, the price of those that survived rose steeply. Farmers attempted to control the fungus by spraying their fields with fungicide, but that was expensive and only partly successful. By the time the disease had spread through the American South and into the Great Plains, it had caused a full-blown crisis.
The Southern corn leaf blight, as it became known, cut production of the essential food by 15 percent. It cost U.S. farmers and consumers hundreds of millions of dollars. And, because cattle are fed a diet heavy in corn and the cost of that corn went up, too, the price of beef rose steeply.
Who's the culprit?
So who’s the bad guy in
the corn crisis? You might think it was Bipolaris maydis, the
mean old fungus that crossed an ocean and several national borders to
satisfy its craving for corn. But it
wasn’t.
The fungus was just doing what all organisms, including humans, do — trying
to stay alive by finding something to eat. The real culprit was monoculture:
the decision by plant breeders and farmers to plant just one variety
of corn. In doing that, they eliminated diversity from their fields.
If
there had been a diversity of corn types, some of them quite likely
would have been resistant to the fungus and some of the crop would have
been
saved.
As
it was, the fungus swept through the fields like an ocean wave. The culprit
was what a U.S. National Research Council report later called “genetic
vulnerability.”
The vulnerability was not limited to corn. In 1991, the genetic similarity of Brazil’s orange trees made them easy targets for the worst outbreak of a citrus disease in the country’s history. In farming communities around the world, important crops are highly vulnerable. In Bangladesh, some 62 percent of the rice varieties come from a single mother plant. In Indonesia it’s 74 percent, and in Sri Lanka the figure is 75 percent.
The Southern corn leaf blight and Brazilian citrus disease were bad, but they were nothing compared to the tragedy that swept across Ireland 130 years before. Click the link that follows to learn about the Irish Potato Famine.
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