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Understand Hybrids' Resistance to Fungal Diseases Before Spraying

Economic threshold levels for fungal diseases vary, and growers are urged to consult with their seed dealers.

Word has it that fungal diseases are threatening Eastern Corn Belt crops. The chatter could mislead farmers into making unnecessary fungicide applications, says Greg Shaner, Purdue University Extension plant pathologist.

While it is true that fungal pathogens are present in certain corn and soybean crops, producers should not rush to use their spraying equipment, he says.

"There's a lot of talk around about whether it's worth applying fungicides to both hybrid commodity corn and soybean," Shaner says. "I think the decision about whether to use a fungicide on corn first hinges on the susceptibility of the hybrid to gray leaf spot, northern corn leaf blight and, possibly, rust.

"That information is available from seed company Web sites and catalogues. If a hybrid has poor resistance, that would be one factor in considering whether to use a fungicide. Unfortunately, seed companies use different systems of rating, but most use a numerical scale."

Severe cases of corn rust, leaf blight and gray leaf spot can reduce yields by as much as 20%, Shaner says. Such losses are rare with today's crop genetics. But that doesn't mean farmers won't see some infection when they scout fields, Shaner says.

"Many corn hybrids have what we call a 'partial' resistance to these leaf diseases," he says. "The fungus does infect the plant and it does produce lesions, but it takes longer for these lesions to develop and they don't produce as many spores. Therefore, the epidemic doesn't progress that rapidly.

"So the fact that you see two or three lesions per plant in a hybrid cornfield doesn't mean the disease is going to become severe enough to justify a treatment with a fungicide. That's why I say it is really important to find out as accurately as possible what the degree of susceptibility is to any one of these diseases."

The northern corn leaf blight and gray leaf spot fungi destroy leaf tissue through the formation of large gray-green or tan lesions. Both fungi survive the winter in corn residue, infecting new crops when spores are carried by spring winds.

The corn rust fungus requires living plant tissue to grow and produce spores. The fungus forms small orange pustules on leaves. Those pustules later turn leaf tissue brown, causing leaves to wither and die.

Soybean producers need to be on the lookout for two fungal diseases, Shaner says.

"Right now the disease we see most often on foliage of soybean is brown spot. That's one disease that is very common," he says. "Typically, that disease remains low in the canopy throughout much of the growing season so, essentially, the soybean plant is outgrowing it. As the plants approach maturity - and if conditions remain wet - then the disease will move up the plant. But brown spot is a disease that we've never considered to be that much of a problem. It just doesn't do that much damage." Frogeye leaf spot is another concern for soybean growers, Shaner says. Reddish-brown spots with tan centers develop on leaves. Over time, the infection can lead to premature defoliation.

"Frogeye leaf spot is another residue survivor," Shaner says. "We've seen a little bit of the disease already this season and, with this warm, humid weather, we'll probably see more."

Economic threshold levels for fungal diseases vary, and growers are urged to consult with their seed dealers. Should fungicide treatments be necessary, application timing is critical, Shaner says.

"It is important to get fungicide on very early, when the disease is first showing up," he says. "But, again, for corn hybrids that have partial resistance, a few early lesions doesn't necessarily mean the disease is going to become severe."

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