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Corn+Soybean Digest

Blight Again?

Corn leaf blight last reared its ugly head in the Midsouth in 2004. Whether or not it returns in 2008 depends on Mother Nature and producers' planting decisions.

Planting one of the more susceptible hybrids in a corn-on-corn cropping system can cost growers as much as 40-50 bu./acre, according to Mississippi State University Extension corn specialist Erick Larson.

“It's a seasonal issue. Corn leaf blight was a tremendous problem for many growers in 2004, but we haven't experienced the same environmental conditions since then,” says Larson. “Disease incidences in corn vary tremendously based on environmental conditions, and conditions the last three years have not been very conducive to the development of foliar diseases.”

In addition, traditional corn growers have begun taking steps to limit their risk. “We rotated more acres of corn with cotton or soybeans, instead of growing continuous corn. And, we no longer plant those hybrids that we discovered were extremely susceptible to these diseases,” explains Larson.

This year could test those preventative measures. “We saw more disease incidences during the 2007 cropping season in continuous-corn production fields,” Larson says. “With the market trends we've seen the last 18 months, we expect this year to see a lot more corn following corn than is normally seen in this region.”

THE SAME WAS TRUE IN 2004. While it's extremely abnormal for corn leaf blight to cause production problems in the Midsouth, an abundance of mid-season cloudy, rainy days promoted an abnormally high occurrence of both the northern and southern versions of the disease in 2004.

Larson explains, “Leaf blight prematurely kills a section of the leaf tissue in a shape that we refer to as a lesion, which limits the photosynthetic capacity of that leaf. The affected plant then doesn't produce as much energy as normal to produce grain.”

He adds, “Before 2004, I had only seen one significant case of either disease in more than 10 years. Corn leaf blight-infected fields were readily apparent in 2004 in both the Delta and the hill region.”

The amount of yield reduction is largely related to the species of corn blight, hybrid susceptibility and whether corn is planted behind corn. Northern leaf blight generally infected corn more rapidly and more extensively, causing substantially more yield loss than corn infected with southern leaf blight.

“The lack of crop rotation promotes the development of these diseases. Corn grown following another crop is generally not infected as readily,” he says. “The largest yield reductions reported in 2004 resulted from northern leaf blight infections on extremely susceptible hybrids grown in fields where corn was produced the year before.”

NORTHERN CORN LEAF blight and southern corn leaf blight are fairly similar in appearance, although the northern version of the disease typically has a wider and longer lesion. Both diseases overwinter in plant debris and seed, and are spread rapidly through wind and splashing water.

With favorable weather conditions and a susceptible corn plant, corn leaf blight fungus spores need only six to nine hours to germinate. Corn specialists also suspect that weakened plants may be more susceptible to these leaf spots.

Extension plant pathologist Alan Henn at Mississippi State University in Starkville, MS, says both diseases were previously referred to by scientists as Helminthsporium. The scientific group has since been split, and now northern and southern corn leaf blight are members of different genera. The southern corn leaf blight is now Bipolaris, meaning it germinates out of both ends of the spore, and northern corn leaf blight is a member of the Exserohilum genera.

“They look very different under a microscope, and lesions on corn plants are very different,” Henn says. “Southern lesions tend to be spindle-shaped and much smaller, only reaching 1-3 cm. in length. On rare occasions the legions may be as long as 6 cm. Northern lesions are wider, and are often 2-15 cm. in length.”

Mississippi disease specialist Billy Moore first discovered Race T of southern corn leaf blight in Mississippi in 1970, and was again called out to the state's cornfields in 2004. However, he says the corn leaf blight threatening yields today is a different animal than the disease that decimated Mississippi's corn crop almost four decades ago.

“In 1970 we were dealing with a different race of corn blight,” Moore says. “The seed companies used to detassel corn during breeding. Then they discovered a gene for male sterility that reduced the amount of money needed to breed corn through detasseling. In 1970 more than 70% of corn hybrids planted in the U.S. contained Texas male sterile cytoplasm, which resulted in hybrids highly susceptible to Race T of southern corn leaf blight.”

THAT YEAR, Moore says a severe outbreak of corn blight quickly killed off corn plants across the country. “It developed very quickly, and it was disseminated by the wind. By June 18 of that year, corn leaf blight had spread from the Gulf Coast to Illinois. Before the summer was over, it had caused $1 billion in damage in the U.S. and destroyed more than 40% of Mississippi's corn crop,” he says.

After the 1970 disease outbreak, corn breeders began using other sources of male sterility. Beginning in 1971, southern corn growers planted what resistant corn seed was available, and by 1972 the threat of Race T corn leaf blight was history.

“In 1970, entire leaves were plastered with the disease and entire plants were dead,” Moore says. “Today we are dealing with a pathogen that has some resistance. Apparently, what we have now is something that will occur off and on if the weather conditions are favorable for the pathogen to develop and susceptible hybrids are planted. It's a different situation than we were dealing with in 1970.”

Henn agrees, saying, “Even though they are diseases with the same name, it's a completely different race we're dealing with today. It is not nearly as virulent, and not nearly as aggressive as the corn leaf blight that hit the state in 1970.”

Whether corn leaf blight will make an encore appearance this year or next is anyone's guess, he says. “No one can predict with any accuracy what will happen from year to year. It will depend greatly on climatic conditions.

“The situation will probably be worse in fields continuously planted to corn; cool, wet conditions and in those cultivars that are susceptible to either disease,” Henn says.

“Since inoculum is already present in some fields, the inoculum potential will be quite high,” he adds. “However, the disease isn't likely to develop without favorable conditions.”

Because corn leaf blight can overwinter in states such as Mississippi, Larson says the chance for another disease outbreak is more likely than growers would assume. He recommends planting resistant corn hybrids and adopting crop rotation practices.

“The disease is windborne so limiting our management to only crop rotation is not going to eliminate the problem if we have weather conditions needed to promote the disease,” he says.

NUMEROUS GROWERS AND consultants have inquired about fungicide treatments. One problem, Larson says, is that nobody can predict if and when disease development will occur, since it is primarily dependent upon the weather. Cool, wet humid weather favors leaf blight development.

“Furthermore, corn grain yield can be limited over a much longer period than what a single fungicide application can protect. Accordingly, proper fungicide application timing can greatly affect crop response. This is where consistent field scouting could pay big dividends — by spotting disease infection early, before it becomes too developed to control,” Larson says. “Many corn hybrids have disease resistance, as well, so fungicide use would not help these hybrids.”

Moore says, “Fungicides are preventative. You can't wait until you see advanced development of disease to treat, because it's too late. At that point in the season it's difficult to penetrate through those leaves to get to the lower foliage where the disease is developing.”

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