A combination of wet weather in early July and cooler than normal temperatures in the middle of the month has worked to increase the level of Southern corn rust in North Carolina.

The disease was reported in mid-July in in at least 10 North Carolina counties – Beaufort, Craven, Greene, Hyde, Jones, Lenoir, Pamlico, Tyrrell, Wayne and Wilson.  

Ron Heiniger and Steve Koenning, North Carolina State University Extension corn specialists, have stressed that is important to spray before the disease becomes established. Once it is established, it is harder to stop or control. Farmers need to determine the maturity level of their corn and then decide if they need to treat it with a fungicide.

If Southern rust attacks prior to dent stage, North Carolina Extension agents advise farmers to aggressively treat infected fields with the best fungicides available at the highest rates. Once corn has reached the dent stage, minimum damage results from the disease and treatment isn’t necessary. Until such time, severe yield loss may result if untreated.

Ordinarily, Southern rust is not a problem for North Carolina farmers because spores arrive so late in the season that little or no impact on yield occurs, but this year conditions have been ideal for Southern rust in the Tar Heel State.

Early warm temperatures helped the disease move toward North Carolina earlier than usual. The movement was aided by Hurricane Arthur while moist conditions in July created an ideal environment for the disease in North Carolina. Paradoxically, conditions became more conducive to corn rust when daytime temperatures fell into the 80s and nighttime temperatures dropped into the 60s in mid-July.

“Conditions are better for Southern rust to develop when temperatures are in the 80s rather than in the 90s,” Koenning said. “Temperatures in the high 90s put a kibosh to Southern rust because it doesn’t like really high temperatures.”

Koenning said farmers with susceptible hybrids (most hybrids are probably susceptible) need to make a fungicide application as soon as possible. If temperatures get back into the high 90s and there is less rainfall, the need for fungicides decreases.

Southern rust can cause severe yield loss and can spread rapidly. It is very aggressive and takes just five to 10 days to go from the first signs of rust on the leaves to complete leaf loss. It is a fungal disease of corn that is well adapted to warm, humid or wet environments.

“Southern rust can be recognized by the bright orange or golden brown, circular to oval pustules that give leaves a rusty appearance. The pustules are about the size of a pin head and are filled with powdery masses of orange spores that are readily dislodged and blown in the wind,” Koenning said. “Thanks to these spores, Southern rust can spread quickly.”

Southern rust is rare in North Carolina. The last time the disease hit the state and caused problems was in 2003. “Experiences fighting Southern rust in 2003 showed that once the fungus infects the leaf and starts producing large numbers of spores it is very difficult to stop this disease with fungicides,” Koenning and Heiniger wrote in a July bulletin  on Southern rust.

“Maximum rates of Tilt and Quadris only slowed the infestation down for a few days. While the number of fungicides for corn have increase substantially since 2003 there is no evidence that these newer fungicides will be any more effective at stopping or reversing an infestation that has already begun,” Koenning and Heiniger wrote.

“The key to control of Southern rust is to prevent the early spores from infecting the leaf thus avoiding further producing of spores. In other words the same strategies that are being promoted for Soybean Rust apply to Southern Rust in corn. It is important to treat PRIOR TO the first spores reaching the field.”