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Corn epidemic proves need for diversity
Where grass is greener
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Last time, I outlined how we improved corn yields and lowered production costs by backcrossing a gene that conferred pollen sterility to hybrid corn varieties. By 1956, just 12 years after the trait was discovered, 85 percent of all the corn grown in the United States and Canada carried the Texas cytoplasm male sterile (Tcms) gene.
Another way to look at that is 85 percent of all the corn in North America had that one Texas corn plant discovered in 1944 as its maternal ancestor. This represents the same risk as having all your investments in one stock. As long as that stock grows and yields well, everybody is happy; but if that stock tanks, so does your entire portfolio – because it contains only that one stock.
And so it went with hybrid corn from 1950, when the hybrids first were released to growers, until 1970. Yields soared, production costs remained low and the effects of bad weather were buffered. We were able to feed millions more people around the globe because of the beneficial exploitation of this trait.
Twenty years of good luck led to complacency. All this time, the virtual monoculture of Tcms corn left the North American corn crop vulnerable. Even though the nucleic genes were diverse, 85 percent of all the corn planted carried a cytoplasmic gene from that single corn plant discovered in Texas in 1944.
Any pest that could find a way to use that trait as a key to unlock a back door past the corn plant’s defense systems would have the North American corn crop at its feet. That pest made its presence known in the summer of 1970. A leaf fungus that was a minor problem had undergone a shift in its gene pool to a race that was wickedly virulent on corn.
Tropical Storm Becky and Hurricane Celia produced subtropical conditions all the way up into Iowa, providing perfect conditions for the disease to develop. Starting in Florida and sweeping north into the southern states and then into the Midwest and West, the fungus wreaked havoc upon the corn crop. Corn farmers lost 50-100 percent of their crop.
That September was my first quarter of college at the University of Tennessee. The halls of the Crop Science building were lined with small shocks of corn brought in by distraught farmers. The plant pathologists knew what the problem was within six weeks of its explosion in Florida. Satellite infrared photos showed very clearly the isolated patches of healthy corn – all of which carried the Normal cytoplasm gene (N-gene).
Knowing did not help the farmers in 1970. There was nothing to be done to stop the blight. It had to be a lonely feeling to be a farmer in Iowa standing in your field of healthy corn as far as the eye can see, hearing the reports from the South and knowing that the blight is headed for you.
How do you face your wife and family who trusted you to provide for them? How do you pay back the half million dollars you borrowed to put the crop in? Waiting for the blow to land that you cannot dodge can be almost as stressful as the blow itself.
It takes strong faith to be a farmer, and 1970 tested the faith of many. Over $1 billion of corn, in 1970 dollars, was lost. This was worse than the Irish potato famine or any other plant-disease-caused famine. This made the Southern Corn Leaf Blight epidemic of 1970 in North America the largest single loss of a single crop in the history of the human race. (The Great Bengal Famine of India in 1943-45 was not caused by plant disease. It was caused by the British).
But we bounced back the very next year. Plant diseases are very finicky about environmental conditions, which is why we do not particularly worry about them as a terrorist tool. Weather conditions in 1971 did not favor the disease, and a bumper crop of corn was the result.
But the real success came throughout the 1970s and 1980s. Plant breeders and plant pathologists realized the problem was caused by having all our eggs in one basket, genetically speaking. All major crops were analyzed for genetic lineage, and we found that many crops had too much genetic uniformity. For instance, all our soybean cultivars had nine parent lines in common. This led to feverish breeding programs to expand parental diversity in all our crops.
Today, we celebrate diversity in agricultural crops. We heeded the warning and learned from our mistake, and so far it has not been repeated. All this happened and you probably never noticed. That’s the way it is with human progress. Building takes the concentrated and durable effort of many, while destruction takes little effort and gets all the headlines.
Next time, a step closer to designer genes – naturally occurring transgenic plants.

Gardner is the extension agent for Bryan County. He can be reached at

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