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Rust on Corn More Prevalent This Summer
Ohio Ag Connection - 08/09/2017

Resembling rust on a pickup, a fungal disease that can afflict corn has been confirmed in a higher than usual number of cornfields in southern Ohio.

Every year, some Ohio farmers find southern or common rust on their corn plants, but this year both diseases have been more prevalent, said Pierce Paul, an Ohio State University Extension corn and small grain specialist. OSU Extension is the outreach arm of the College of Food, Agricultural, and Environmental Sciences (CFAES) at The Ohio State University.

Typically, rust fungi arrive late in the growing stage and do limited damage. However, this year farmers had to replant several times, and the younger corn plants are at greater risk for damage, Paul said. Common and southern rusts are most problematic when they infect the plant before it produces silk or a tassel.

Southern and common rusts form orangish and brownish raised spots, respectively, on the leaves of corn plants and can cause as much as 30 to 40 percent yield losses, particularly on susceptible corn varieties, Paul said. Under hot, wet weather conditions, southern rust spreads quickly on a leaf, from one leaf to another, and from one plant to the next.

"I haven't gone a day over the past two weeks without an email or a call or a box of (corn plant) samples to test," Paul said.

This summer's heavy rainfall and humidity may have created conditions that spurred the spread of southern and common rust, as well as other fungal disease on corn, said Peter Thomison, an OSU Extension agronomist. Despite the heavier than usual presence of southern and common rusts, neither are nearly as frequent as two other fungal diseases, gray leaf spot and northern corn leaf blight, Thomison said. Gray leaf spot and northern corn leaf blight are beginning to show up, and the mild, wet conditions experienced over the last few days will certainly favor the latter.

Farmers finding a lot of rust or another type of fungal disease in their fields have to decide whether to fight it. For all four diseases (the two rusts, gray leaf spot and northern corn leaf blight), farmers need to scout fields before applying a fungicide because, from the road, the disease may not seen as widespread as it is, Paul said. He advised paying particular attention to the late-planted fields as they are the most vulnerable.

Since the price of corn is relatively low, it may not be economical for some farmers to invest in fungicide to fend off a disease, Paul said. A fungicide, if applied at the first signs of disease, could stop 75 to 90 percent of a field's rust problem, but spraying it can cost up to $35 an acre, he said.

"Farmers don't want it to reduce their yields. But they're running the math in their heads to try to see if they'll have a return on their investment," he said.

Southern rust typically causes more damage than common rust. More prevalent in warmer climates, southern rust favors southern Ohio, while common rust flourishes in the more temperate climate of central and northern Ohio. The two rusts look slightly different. Southern rust is characterized by small, circular, light-orangish bumps predominantly on the upper surface of the leaves, whereas common rust produces larger, more elongated and darker, cinnamon-brown bumps on both leaf surfaces.

Rust fungi are not unique to corn. Several crops have a rust fungus that can attack them. Soybeans have one major rust fungus, corn and wheat have three, and each is specific to the plant, Paul said. So common rust that can affect corn does nothing to soybeans, and a soybean rust won't harm corn.

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