Understand the conditions that make this disease endanger yield
Flying in on tropical storms, southern rust can steal yield and cause headaches for corn farmers. The nasty disease can attack quickly and do a lot of damage in a hurry, especially in warm and humid conditions.
AN AGGRESSIVE FUNGUS
“We have two rust diseases that can affect corn in the U.S.; they are common rust and southern rust,” says Tamra Jackson, University of Nebraska Extension plant pathologist. “Southern rust is caused by an aggressive fungus that causes much greater yield loss.”
Southern corn rust, caused by the fungus Puccinia polysora, creates round, light brown to orange spots scattered mainly on the upper leaf surface. They might also appear on husks, stalks, lower leaf surface and ear shanks. The disease is easily confused with common rust, which produces pustules (fuzzy, raised structures) on both sides of the leaf.
Warm temperatures (70° and above) during the day or at night favor southern rust development, Jackson says. It also needs high humidity, rainfall or irrigation to spread. Because it doesn’t overwinter on residue, it spreads with summer storms, moving from the south to the north.
Timely management is key for controlling the impact of southern rust, says Kiersten Wise, University of Kentucky Extension plant pathologist. The disease can build up in cornfields quickly with the right weather conditions.
“If it comes at the right time and we aren’t protecting our crop, it can do some really significant damage to yield,” she says.
Foliar fungicides can manage the disease, but timing is critical. Research shows fungicide applications need to go on crop from tas-seling through the milk stage (VT to R3), Wise says.
“If you have good yield potential, it’s likely worth a fungicide application to protect the crop,” she says.
Other management practices include planting resistant hybrids and avoiding late planting.
Fast Facts About Southern Rust
- Southern rust spores can travel miles on air currents.
- Once infected, symptoms can be observed within three to four days if the weather is favorable.
- Within seven to 10 days, spores are produced and can travel to other plants.
- Favorable southern rust conditions include hot temperatures and at least four hours of consecutive leaf wetness.
- Research shows southern rust can continue even with temperatures around 110°F.
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