Southern rust is now active on corn in Kansas and Nebraska (see map below). This disease normally arrives in Kansas around the first of August, but in 2015 it has likely been in the state since July 1. This combined, with very late corn planting in many areas of the state, increases the threat that this disease could cause significant yield loss problems in 2015. How severe it could become is related to the weather. As of now, the 30-day forecast would suggest that weather will be favorable for the disease. Southern rust likes 90-degree days and high humidity.
Symptoms of southern rust include pustules that are usually circular to oval, with a diameter of 0.2 to 2.0 millimeters. They typically are densely scattered on the upper leaf surface (Figures 1 and 2). Occasionally pustules can be seen on the underside of the leaf near the midrib; however they are usually confined to the top side of the leaf.
Sporulation can be so profuse that the leaf surface becomes covered with a layer of “spore dust” that transfers easily to clothing as a person walks through an infected field. Light-colored clothing will quickly take on an orange-brown color. Southern rust can sometimes be confused with common rust. For more information on identify corn rusts see K-State Research and Extension Bulletin MF3016, Corn Rust Identification and Management in Kansas.
Figure 1. Southern rust on corn. Photo courtesy of Doug Jardine, K-State Research and Extension.
Figure 2. Close-up of southern rust on corn. Photo courtesy of Tom Allen, Mississippi State University.
Fields that have been sprayed for gray leaf spot should be protected from southern rust as well for three to four weeks after application. Fields that have not received a fungicide application at tasseling should be regularly monitored for the build-up of southern rust. Fungicide applications as late as hard dough have been reported to provide economic returns in some instances. Most of the commonly used corn fungicides will provide adequate protection against southern rust.
Gray leaf spot update
Gray leaf spot continues to develop in many areas of the state. A corn field near Rossville, in Shawnee County, had gray leaf spot lesions on the ear leaf +3 and was only at the R2 stage of development. Active scouting for gray leaf spot on late-planted corn fields is encouraged. While the optimum time to apply fungicides for gray leaf spot is between tasseling and brown silk, later applications during the R2 stage of development when disease pressure is severe can still provide a good economic return on investment. The 10-day weather forecast is less favorable for gray leaf spot development, but scouting is encouraged.
Doug Jardine, Extension Plant Pathology