BLIGHT IN THE CORN BELT
But in 1969, a few farmers and scientists noticed the same problem recurring in midwestern seed fields and hybrid corn test plots.. One account noted: "In the late summer and early fall of 1969, a few corn fields in southern Iowa began behaving erratically. Ears rotted inside husks. Stalks fell to the ground. Shortly, the same thing happened in isolated fields in Illinois and Indiana." This same scientist noticed that only certain hybrid corn varieties were susceptible to the disease. In Florida, too, a few seedsmen found that hybrid corn varieties growing there were particularly vulnerable. Yet there was no adequate scientific explanation for the new disease. Scientists knew it was a fungus, but they didn't know what kind or how it worked.
In 1970, the disease was first reported in February from southern Florida, near Belle Glade. Between May 5 and May 20, heavy infestations were cited in southern Alabama and Mississippi. By June 18, the disease covered the entire state of Florida, lower Alabama, and most of Mississippi. The lower third of Louisiana and coastal Texas were also infected.
Reproducing rapidly in the unusually warm and moist weather of 1970, its spores carried on the wind, the new disease began moving northward toward a full-scale invasion of America's vast corn empire. Later to be identified as "race T" of the fungus Helminthosporium maydis, it soon became known as the Southern Corn Leaf Blight. [Since reclassified as Bipolaris maydis.
The new fungus moved like wildfire through one corn field after another. In some cases it would wipe out an entire stand of corn in ten days. Moisture was a key factor; a thin film on leaves, stalks, or husks was all the organism needed to gain entry to the plant. Within twenty-four hours it would start making tan, spindle-shaped lesions about an inch long on plant leaves, and in advanced form would attack the stalk, ear shank, husk, kernels, and cob. In extreme infections, whole ears of corn would fall to the ground and crumble at the touch.
The fungus moved swiftly through Georgia, Alabama, and Kentucky, and by June its airborne spores were headed straight for the nation's Corn Belt, where 85 percent of all American corn is grown. By this time, however, unsuspecting Corn Belt farmers had already planted their crops and were largely unaware of the bitter harvest headed their way.
The fungus could begin reproducing within sixty hours of landing on a corn plant-yielding a new generation of its own kind every ten days-and its spores could survive temperatures of 20 degrees below zero and still germinate, which meant they could linger in fields and plant remnants through the winter. In some cases, the fungus could even penetrate corn seed, causing it to fail or produce blighted seedlings.
In its wake, the Southern Corn Leaf Blight left ravaged corn fields with withered plants, broken stalks, and malformed or completely rotten cobs covered with a grayish powder. When farmers harvested what they could, clouds of spores were thrown up into the air behind their combines, spreading the disease even farther.
In just four months-from May to September 1970—the disease had spread as far north as Minnesota and Wisconsin (it later entered Canada), and as far west as Kansas and the Oklahoma panhandle. The nation's corn farmers were facing a full-blown crisis.
What really panicked commodity traders and government officials was the blight's penetration of the Corn Belt; just three midwestern states-Illinois, Indiana, and Iowa-accounted for half the nation's total corn production. "There has always been blight in the South," said former Chicago Board of Trade Chairman William Mallers, "but when you get [blight] in the Corn Belt, you're really talking." In August 1970, Illinois Secretary of Agriculture John W. Lewis was estimating that 25 percent of his state's corn crop was already lost to the blight. Just one year earlier, Illinois had been the nation's top corn producer, accounting for more than one-fifth of the crop.
THE RIPPLE EFFECT
Adding to the reality of the disease itself were rumors that any blighted grain would be toxic to humans and animals. Further questions emerged about "secondary organisms" that might invade the grain, causing still other kinds of toxic problems. In fact, pathologists at the University of Illinois did discover "secondary fungi"-capable of producing the potent poisons known as aflatoxins-growing on blighted corn stalks, husks, and ears. But no toxic effects were reported in livestock or humans.
However, it was learned that the blight itself could be transmitted in corn seed. And that fed speculation that the blight was being exported to foreign countries through American corn seed. By early 1971, the corn blight was reported in Japan, the Philippines, Africa, and Latin America, and some importers of corn seed, such as Australia and New Zealand, were wondering if the problem didn't originate with American seed. Addressing the question, Ramparts magazine, in a March 1971 editorial, wrote, "There is considerable speculation as to whether through our exports of diseased corn. We are spreading the blight around the world." At that time the United States was exporting some 46.8 million pounds of corn seed to all parts of the world, worth about $5 million annually. Yet proving that blight in other countries originated in U.S. seed was difficult when the importing countries weren't looking for it in the imported seed.
Commenting on that discovery in 1971, pathologist A. L. Hooker noted that it was "most unusual" that the cytoplasm of corn plant cells played a major role in determining the disease reaction, since in almost all other diseases, genetic factors in the nucleus of the cell determined disease resistance or susceptibility. Because of this, explained Hooker, corn breeders and seedsmen had no reason to suspect that uniformity in the corn crop would pose any problem. But it did.
The cytoplasm found common in most hybrid corn at that time was called "Texas male-sterile cytoplasm," or "T-cytoplasm," after a Texas variety of corn in which it was discovered. For twenty years preceding the blight, T-cytoplasm was used by plant breeders and seed companies to simplify the process of hybrid corn seed production. Male-sterile cytoplasm produced tassels on corn plants that bore impotent pollen, which-in combination with a fertility-restoring gene in the hybrid cross-enabled scientists to crossbreed and pollinate large numbers of plants more easily. T-cytoplasm thus eliminated the time-consuming, labor-intensive, and economically expensive step of hand detasseling corn plants. It was a revolutionary invention in plant breeding. But what scientists didn't know then about T-cytoplasm was that it also carried a gene in the mitochondria (an organelle of the cell that produces chemical energy for the cell) which enabled the new strain of the corn blight fungus to do its damage.
T-cytoplasm was a man-made change in corn plants used to foster the quick and profitable production of high-yielding, hybrid corn seed. It was a change accomplished and advanced by science and commerce without full knowledge of the potential consequences. The new strain of corn blight fungus, Helminthosporium maydis, was a mutation perfectly keyed to a gene in that cytoplasm.
At least 80 percent of the hybrid corn in America in 1970 contained T-cytoplasm, which is why "race T" of Helminthosporium maydis laid waste to 15 percent of the nation's corn crop. "The USA in 1970 had 46 million acres of corn with Texas male sterile cytoplasm," wrote Iowa State University Pathologist J. Artie Browning in 1972. "Such an extensive, homogenous acreage of plants…is like a tinder-dry prairie waiting for a spark to ignite it. Race T was the spark...."
The official scientific response to the corn blight came in August 1972, with the release of the National Academy of Sciences study Genetic Vulnerability of Major Crops. The corn crop fell victim to the epidemic," said the Academy's report, "because of a quirk in the technology that had redesigned the corn plants of America until, in one sense, they had become as alike as identical twins. Whatever made one plant susceptible made them all susceptible." The Southern Corn Leaf Blight, said the NAS study, was genetically based - key finding.*
*In a 1976 paper entitled "An Evaluation of Special Grant Research on Southern Corn Leaf Blight," the USDA also acknowledged the genetic uniformity in the nation's corn crop as one of the primary causes of the 1970 Corn Leaf Blight. "In the [1960s], it became clear that relatively few corn breeding parents were being used to produce the bulk of American hybrid corn varieties," said the report. "This narrowness of germplasm set the stage for potential vulnerability to diseases, insects and other stresses. In early 1970, environmental conditions in Southern and Northcentral corn producing regions were favorable for easy disease establishment and spread among vast plantings of highly uniform varieties. The [Southern Corn Leaf Blight] epidemic became of national and international significance."
Looking beyond corn, the Academy also warned that most other crops were "impressively uniform genetically and impressively vulnerable." Moreover, the study added, "this uniformity derives from powerful economic and legislative forces," such as food company preferences for one kind of crop and government marketing orders requiring specific kinds of fruits and vegetables. But despite these warnings, not much has changed since 1972. Corn is less vulnerable, but 43 percent of the nation's corn acreage is planted to varieties derived from 6 inbred lines. Other crops are even more vulnerable. And cytoplasmic breeding systems are still being used in a number of crops, including corn.
|Exercpted from: http://www.victoryseeds.com/information/corn_panic.html|