Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
Southern rust of corn has been confirmed in Wisconsin by the University of Wisconsin-Madison Plant Disease Diagnostics Clinic. The sample came in from Kenosha Co. on August 18, 2017. This find is not entirely surprising considering the rapid movement of the southern rust pathogen from the southern U.S. to the northern U.S. this season. The latest southern rust map can be found at http://ext.ipipe.org.
For more information on southern rust, please see my previous post on the subject. You also should visit the Crop Protection Network Southern Rust information page where you can download a brand new fact sheet about southern rust on corn and management of the disease. Note that late planted corn will be more vulnerable to yield loss from the disease. Corn that is still silking (R1 growth stage) to milk (R3 growth stage) is vulnerable to yield loss by southern rust. Corn that is at the R4 (dough) growth stage or later is not as vulnerable and will likely not respond to a fungicide application. Even if corn is at a vulnerable growth stage, remember that we also have to have conducive weather for the pathogen. Extremely dry weather is not conducive for disease. High humidity and temperatures in the 80s favor disease increase.
Continue to scout and growth-stage your corn. If you find evidence of what you think is southern rust, I would encourage you to send it to the Plant Disease Diagnostics Clinic for confirmation.