As many are deciding whether to put out fungicide on their corn acres, it’s very important to correctly identify what leaf spots you might be seeing and sometimes knowing what it is not – is just as important.
Southern corn rust has one of the greatest yield robbing potentials of corn diseases, but if it is not present and/or comes in late enough in the season/growth stage, it will not be able to impact yield. It is best to use trusted sources to know where Southern corn rust has been confirmed and one such source is the Crop Protection Network – https://cropprotectionnetwork.org/maps/southern-corn-rust . As of now, I have not confirmed any Southern rust in TN, but there are confirmed reports in AR and KY. If you suspect you have Southern corn rust contact your local county agent or state specialist to confirm.
While Southern rust can be confused with common rust, there are differences between the two rusts from color, shape, and location of lesions (see table and images below).
| Pustule Characteristics: | Common Rust | Southern Rust |
| Location | upper and lower leaf surfaces | primarily upper leaf surface |
| Color | brownish, red | orange to light brown |
| Shape | elongated | round |
| Distribution | scattered | densely packed |
| Other differences: | ||
| Optimal Temperature | 61-77 °F | 77-82 °F |
| Probability of Effecting Yield | low | moderate-high |
Additionally, other leaf spots that do not pose a threat to yield or warrant a fungicide application but could be confused with Southern corn rust include: Curvularia, Physoderma brown spot, and other physiological leaf spotting/damage (see example images below).
