The following is from John E. Woodmansee, Extension educator, Agriculture/Natural Resources.
Purdue experts have confirmed southern rust of corn in several Indiana counties, and the list of affected counties is likely to increase, if trends continue.
Recently, Kiersten Wise, former Purdue University plant pathologist, now with University of Kentucky, and Tom Cresswell, Director of Purdue's Plant and Pest Diagnostic Lab, wrote that southern rust of corn, caused by the fungus Puccinia polysora, has been detected in several Indiana counties. "The detection of southern rust is earlier than normal, and has caused concern because there are many acres of late planted or re-planted corn in the state in 2017 that have the potential to be impacted by the disease," they said. "However, whether or not southern rust will develop into a disease of concern will largely depend on the weather."
Wise and Cresswell said that southern rust can be a damaging disease, and foliar fungicide applications between tassel (VT) and milk (R3) can help protect plants from infection and disease development.
"It is important to consider corn growth stage and weather conditions before deciding on a fungicide application," they said. "If the weather is hot, (mid-80s and above), humid, and there are heavy dews and rainfall, then southern rust may develop and spread more quickly. Cooler, less humid weather will cause the disease to develop and spread slowly."
Yield potential should also be a consideration, and the experts said high-yield potential fields should be prioritized when making management decisions. Farmers trying to decide if a fungicide application is warranted can scout fields carefully and frequently and watch the weather to determine if and when a fungicide application is needed.
"There are several fungicides available that provide very good control of southern rust," they said. Fungicide efficacy of specific fungicide products for corn diseases are described in the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group: www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf.
"Just remember that common rust, caused by Puccinia sorghi, and southern rust are easy diseases to confuse, and any suspected samples should be sent to the Purdue Plant and Pest Diagnostic Laboratory for confirmation before deciding on a management tactic," they said. "Weather conditions have favored continued development of common rust, but fungicide applications for common rust in hybrid corn are unlikely to be economically beneficial."
The Purdue Extension publication BP-82-W, "Diseases of Corn: Common and Southern Rust," has more information on distinguishing between common and southern rust, and additional details on both diseases. It is available online from Purdue's Education Store at: https://mdc.itap.purdue.edu/.
Finally, Cresswell and Gail Ruhl, Purdue plant disease diagnostician, recently named Indiana counties affected by southern rust so far, as confirmed by Purdue's Plant and Pest Diagnostic Laboratory. The counties include: Parke, Pike, Dubois, Bartholomew, Perry, Washington, Decatur, Shelby, Jennings, Clay, Jackson, Spencer, Orange and Warrick (current as of Aug. 2). As you can see, these are all southern Indiana counties. Here in northeastern Indiana, it's time to scout fields for symptoms. If you suspect southern rust, follow instructions at https://ag.purdue.edu/btny/ppdl/ to send a sample to Purdue PPDL, or request help on submitting a sample from your local Purdue Extension office.
For more detailed information on southern rust, Ruhl also called attention to a publication on southern rust available from the Crop Protection Network website: http://cropprotec tionnetwork.org/corn/southern-rust-cpn-2009/.