The best management tool for Southern rust is to plant hybrids with genetically built-in resistance to the variety. However, in the past few years new races of the disease have shown up in the Southeast, such as the one that popped up in 2008-2009 that can attack once rust-resistant corn varieties with the Rpp 9 gene. 

For this year’s crop, the best hope is to treat with a preventative fungicide, but that too hasn’t always been a good management tool, Heiniger says.

A number of the strobilurin fungicides can do a good job of buying a grower some time, if he gets it on the crop in time, he adds.

If a grower waits until he sees the symptoms of the disease in a corn field, it’s too late for fungicides.

“A couple of years back, we had a lot of Southern rust in some of corn plots, and we tried everything we could find, but nothing did much good after the disease got going,” Heiniger says.

Southern rust starts in a corn field with a circular pattern of yellow corn plants. In bad cases, the disease will spread through a field and across a whole farm in a few days.

But basically, if you see that distinctive yellow circular pattern in a corn field, it’s too late to treat with a fungicide, the North Carolina corn specialist says.

Triazole fungicides and strobilurin fungicides all have good activity against Southern corn rust. The triazoles will have greater systemic activity and strobilurins a longer protective window.

At best, the protective window provided with strobilurin fungicides is three weeks.

Growers with high yield potential and with high likelihood of re-infections may need to spray more than once to get corn past the danger stage from Southern rust.

With the combination of a high yielding crop and a high value crop, it is critical for growers to do everything they can to protect the extra corn yield expected for the 2012 crop in the Upper Southeast.

Growers will likely have to make some difficult decisions on the value of the crop versus the cost of fungicides, and possibly multiple applications.

In addition to high value, with corn prices topping $7 in June, this year’s corn crop in one of the highest risk crops ever.

The high seed cost, combined with technology fees and high fertilizer and crop protection costs, make it essential for most growers to get everything possible from their corn crop.

rroberson@farmpress.com