Farm Progress

Rust on corn more prevalent this summer

A fungicide could stop 75% to 90% of a field’s rust problem, but spraying it can cost up to $35 an acre.

August 17, 2017

3 Min Read
RUST WOES: Every year, some Ohio farmers find southern or common rust on their corn plants, but this year both diseases have been more prevalent. Southern rust spreads quickly from one leaf to another, and from one plant to the next.

Resembling rust on a pickup, a fungal disease that can afflict corn has been confirmed in a more cornfields than usual 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, says Pierce Paul, an Ohio State University Extension corn and small-grain specialist.

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 says. 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% yield losses, particularly on susceptible corn varieties, Paul adds.

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 says.

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 diseases of corn, says Peter Thomison, an OSU Extension agronomist.

Despite the heavier-than-usual presence of southern and common rusts, neither is nearly as frequent as two other fungal diseases — gray leaf spot and northern corn leaf blight, Thomison says. 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 seem as widespread as it is, Paul says. 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 says. A fungicide, if applied at the first signs of disease, could stop 75% to 90% of a field’s rust problem, but spraying it can cost up to $35 an acre, he says.

“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 says.

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-orange 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 says. So common rust that can affect corn does nothing to soybeans, and soybean rust won’t harm corn.

Source: OSU

 

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