There are currently 41 counties in Indiana with a positive confirmation of tar spot. As we are learning this season when we have had favorable environmental conditions, there may be pockets of tar spot even in areas that have not seen the significant yield impacts previously. In the map above, all gray counties indicate that tar spot was found in the county in previous seasons.
While tar spot continues to be documented across the state, Southern corn rust has now been confirmed in four counties (Knox, Dubois, Warrick, and Shelby) last week in Indiana. I suggest if you have not gotten out and looked for these diseases now is the time. Even if your corn is approaching black layer it will be important to document tar spot in your fields for the future disease management decisions.
I have been getting many questions on if a late season fungicide 4 or 2nd fungicide application should be applied. Here are my thoughts on what to consider to make that decision:
- What growth stage is the corn? If at dent (R5) or beyond I would not recommend an application.
- What has been the history of tar spot in your field? How much disease do you see currently? Where is the disease in the canopy? Just lower leaves or has it moved up to the ear leaf?
- When was a fungicide applied? If more than 3 weeks then the effective period for many of our fungicides has run out.
- Has the environment be favorable for tar spot development – there’s an app you can use to check your field risk.
- If you do decide to make a fungicide application at this point in the season, leave check strips to determine if the fungicide gave you a return on your investment.
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 for future risk assessments and 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 or
email me a photo of the leaf at: dtelenko@purdue.edu.
Written by: Dr. Darcy Telenko, Associate Professor and Field Crop Plant Pathologist in the Department of Botany and Plant Pathology at Purdue University.
