He says dryland corn appears to be less susceptible to rust, perhaps because pivot irrigation better maintains a moist leaf environment.

According to Isakeit the symptoms of southern rust, caused by the fungus Puccinia polysora, are slightly raised, circular, orange pustules that are mainly on the upper sides of leaves. Pustules can also occur on stalks and husks. Early in the season, pustules are on the lower foliage and they progress to foliage in the upper canopy during the growing season.

“Applied early enough, fungicides have proven successful in helping to fight the fungus. There are no thresholds for economical use of a fungicide at this stage. The benefits of an application will be affected by hybrid susceptibility, timing of infection, and environmental conditions that support disease development,” he warns.

He says producers need for the leaves to dry out until the corn can be harvested. If this happens, then yield loss will be held to a minimum. But the more rain that falls, the more costly the yield damage will be.

“Since the turn-around time from infection to new spores with this fungus ranges from 9 to 12 days, my guess is that a fungicide application at silking may have had the greatest impact on reducing disease development.”

In a report issued by Isakeit concerning Southern Rust along the Upper Coast, the pathologist says leaf wetness (rain or dew) is necessary for infection by wind-blown spores and the optimal conditions for infection are 16 hours of dew at 80 degrees. But he warns the fungus can infect corn over a temperature range of 54 degrees to 97 degrees. The good news is that the fungus has a limited host range (corn) and can’t survive without a living host, so it does not overwinter on crop debris nor does it remain present in the soil.