Southern corn rust pustules brought on by extended periods of heat and humidity are primarily found on the upper leaf surface.

AgVenture Regional Product Manager Jeff Shaner reported the beginning of southern corn rust developing in eastern states, including Indiana, Kentucky and Tennessee, during travels last week.

Southern corn rust, a fungus caused by Puccinia polysora, thrives in temperatures above 80 F with high humidity, particularly if corn has been planted late. The abundance of overnight dew and urediniospores, along with rain, can spread the fungus development.

Southern rust starts with clusters of small circular lesions up to 2 millimeters. As lesions mature, fungus erupts through the leaf surface and lesions become elongated with a yellow halo, eventually becoming rust-colored oblong pustules, usually found on the upper leaf surface. The fungi can also attack stalks and husks.

Consider fungicide applications when more than 10 percent of the leaf area is affected. Monitor weather forecasts to determine if high humidity and hot temperatures will persist. Consult your AgVenture Yield Specialist for a specific treatment plan for your fields.