Southern corn rust has now been confirmed in two counties (Knox and Warrick) and we are awaiting samples from Porter, Shelby, Bartholomew, Decatur, Jennings that are highly probable for this week in Indiana (Figure 1). While tar spot continues to be documented across the state (Figure 2). I suggest if you have not gotten out and looked for these diseases now is the time, since much of the corn is at tassel or silk or beyond across the state.
Figure 1. July 24, 2025 map of southern rust (Puccinia polysora).
There are currently 25 counties with a positive confirmation of tar spot (Figure 2). I have been getting many questions on if a fungicide application should be made and now is the time to get out scout to see if foliar disease is active in your fields. I have also observed gray leaf spot in many of our research sites.
Figure 2. July 24, 2025 map of tar spot (Phyllachora maydis) and image of an infected leaf.
Here are my thoughts on what to consider to make a fungicide application decision.
- Field history/previous crop – what diseases have been an issue in previous years?
- Amount of disease present in the field – what diseases do you find? Where are they in the canopy? Is the disease active in your county or surrounding counties?
- Hybrid/variety susceptibility to diseases
- Current weather conditions (use the forecasting tools https://cropprotectionnetwork.org/crop-disease-forecasting )
- The value of the crop and cost of fungicide application.
A fungicide application can be effective at reducing disease and protecting yield, but there are a number of factors that need to consider. If you are worried about tar spot or southern rust See figures 3 and 4 for handy tables to assist in making that decision.
Figure 3. A decision table for tar spot outlining possible benefits from applying fungicides based on when disease is first detected in a field and crop growth stage. Source: Crop Protection Network https://cropprotectionnetwork.org/maps/tar-spot-of-corn
Figure 4. A decision table for southern rust outlining possible benefits from applying fungicides based on when disease is first detected in a field and crop growth stage. Source: Crop Protection Network https://cropprotectionnetwork.org/maps/southern-corn-rust
We are still documenting tar spot and southern rust as it is important to understand the disease distribution and severity across Indiana. It is extremely important to know if this disease is present in your fields to implement disease management tools if necessary. If you observe tar spot in a county that has not reported this season or would like to share what you have been seeing on your farm, then please send a sample to the Purdue Plant Pest Diagnostic Lab (PPDL) https://ag.purdue.edu/btny/ppdl/Pages/Submit-A-Sample.aspx
