Common rust on corn leaves is, well, common. It's not unusual to find some there, especially later in the season. However, southern rust doesn't appear every year. There are confirmed cased of it now in the southern counties where you might expect it- Vanderburgh, Posey, Jennings and Knox, but also in Tippecanoe County, where you wouldn't necessarily expect to find it. Thee reports are confirmed by Kiersten Wise, Purdue University Extension disease specialist.
Southern rust is usually found only on the upper surface of corn leaves, while common rust will infect both the top and lower sides of leaves. There are also differences in color of the pustules. Southern rust pustules tend to be more densely packed with infectious material, waiting to spread to other leaves, carrying a deeper orange color.
There is actually good news in this story. While southern rust can cause yield loss if it comes into an area early enough, it did not arrive early enough in most of Indiana to cause yield impact this season, Wise says. It's more of a curiosity and something not to worry about rather than something that will be a major driver on corn yields this fall. The same is true for common rust. It didn't invade quick enough to be a yield threat in most cases.
The other good news is that just because the rust is here this here, either southern or common, it doesn't follow that it will be back next year. It doesn't over winter here, but instead has to be brought up by Gulf Winds blowing northward. The winds blow both forms of the disease up from the Gulf region as spores. Then when conditions are right they can infect corn leaves.
To make matters more confusing if you're trying to determine if you have rust on leaves and which one you have, both types can occur on the same leaf. Just remember that southern rust is more likely to be found only on the top side of laves. Refer to the Purdue University Diagnostic Training Center publication, the Purdue Extension Corn & Soybean Field Guide, for more help in identifying which form you have.
Of all possible problems that could befall this year's crop, leaf rust is not likely a major one to worry about. It won't impact yield coming in this late, and that's the bottom line.