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Southern rust and other corn diseases hitting Nebraska corn fields

LINCOLN, Neb. – Southern rust of corn has been confirmed in Thayer, Clay and Hall counties, meaning Nebraska farmers should be scouting for it and other corn diseases, a University of Nebraska-Lincoln plant pathologist said.

Although only three counties have confirmed cases, the disease most likely is more widespread than that, said Tamra Jackson, UNL Extension plant pathologist in the university’s Institute of Agriculture and Natural Resources.

“I am especially encouraging people in southern counties to scout fields regularly, paying close attention to fields that were planted late. Those are the ones that could sustain the worst damage if disease becomes severe,” Jackson said.

Rust diseases produce large amounts of spores that are easily moved by wind for long distances, Jackson said.

This is the third consecutive year southern rust has been reported in Nebraska, having caused substantial yield loss in south central Nebraska in 2006 and to a lesser extent in 2007.

“In 2006, it was a tremendous problem in south central Nebraska with yield losses up to 30 percent,” she said.

Southern rust can increase rapidly during periods of high humidity or moisture.

“Dry weather will help slow them down, but if there is dew in the morning, then there is enough moisture for this disease and others to increase,” Jackson said.

All corn fields should be monitored regularly for its spread.

The simplest and most reliable way to differentiate the diseases without a microscope is to examine both leaf surfaces for spore development, Jackson said.

“Southern rust spore production usually is limited to only the upper leaf surface and tends to be more tan/orange in color than that of common rust until the spore type changes and turns black later in the season,” she said.

Common rust also is unusually severe in parts of the state, including southwest Nebraska into northeast Colorado.

Common rust is severe enough that some farmers are applying fungicides.

“People are treating commercial corn for common rust who never had to treat before,” Jackson said. “This disease is typically more severe in the eastern corn belt and historically has only been a problem in Nebraska on seed and sweet corn.”

High heat slows down common rust, but warmer night temperatures will favor southern rust.

Unlike southern rust, common rust can produce spores on both sides of the leaf. It also tends to be more brick red/brown in color until the spore type changes and turns black later in the season.

Gray leaf spot also continues to increase in severity across the state. While this disease is a resident of Nebraska, the wet weather also has increased its appearance as well.

“Gray leaf spot requires several days for obvious lesion development after fungal infection, so the disease can advance without being immediately obvious to the observer and is probably one to two leaves higher on the plant than is apparent,” Jackson said.

To compound the problem, fungicide applications that farmers may have made several weeks ago no longer are providing protection from these fungal foliar diseases. These last only about two to three weeks.

“Any products applied more than two or three weeks ago are gone,” she said. “If disease becomes severe, some producers will need another application. These also can be expensive, averaging $20 an acre.”

In addition, farmers need to pay attention to the pre-harvest intervals (PHI) for some fungicides. Some products restrict their use after the development of brown silks, so those will not be able to be used in some fields.

“It’s important to pay close attention to the label restrictions on the most recent version of the product’s label,” Jackson said.

Nebraska farmers who apply fungicides incorrectly will not be able to sell their crop.

More information about rusts, including fungicides and detecting the disease, can be found in UNL Extension NebGuide G1680, Rust Diseases of Corn in Nebraska, available at http://www.ianrpubs.unl.edu/sendIt/g1680.pdf or from a local UNL Extension office.

Results from foliar fungicide trials conducted in Nebraska are available at the UNL Extension Plant Pathology team’s Web site, Plant Disease Central, at http://pdc.unl.edu/home.

More information about rust also is available in the Aug. 1 edition of Crop Watch, UNL Extension’s crop production newsletter, at http://cropwatch.unl.edu/archives/2008/crop18/corn_rusts.htm.


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