Foliar Disease Scouting Tips: What to Look for in Corn

Darcy Telenko, Extension Field Crop Pathologist

Tar spot confirmed and active in Indiana this week

Corn growth stages are quite variable across Indiana but a number of areas with early planted corn about waist high. We just scouted our research plots in northern Indiana at the Pinney Purdue Ag Center and have found tar spot in our first planted corn April 25 which is at V4/V5 and at another site where the corn is V7.  Active tar spot has also been found in Iowa and Kansas which means that we need to start to monitoring for disease in Indiana to make an informed decision if a fungicide is necessary as the crop reaches reproductive stages. Recent weather may have made conditions conducive for foliar diseases, therefore it is now time to start to get out and scout that lower canopy.  As a reminder for disease to occur, three things need to be present 1. Pathogen, 2. Host, and 3. Favorable Environment. The major diseases we monitor in Indiana such as gray leaf spot, northern corn leaf blight, southern rust and tar spot all might start to make an appearance (Figures 1 and 4).

Figure 1. Tar spot stromata found in lower leaf, June 10, 2024. Photo credit: Morgan Goodnight, Purdue University.

A few questions to think about when scouting and looking for disease:

1. What is the disease history in the field? How much residue is still present? (What happened in previous years?)

2. What growth stage is the field? Early planting vs. late

3. Is irrigation being applied? How much and how often? If water is being applied, it can change the environmental conditions and disease risk in a field.

Tar spot continues to be a concern this season. Tar spot may be hard to find early Figure 2 shows examples on how small the initial tar spot lesions (stromata) will be. Previously, we usually find the stromata in fully-expanded leaves knee to hip height in the canopy. We will continue to monitor for disease and keep you updated. Again, the recent favorable weather has helped to promote tar spot.

I would like to make a few recommendations when using the Tarspotter App once corn is at V8.

  • If you have a history of the tar spot it is time to keep an eye out and make an informed management decision. 
  • What to look for: Small, black, raised spots (circular or oval) develop on infected plants, and may appear on one or both sides of the leaves, leaf sheaths, and husks. Spots may be found on both healthy (green) and dying (brown) tissue.
  • Use the App initially to tell you to get out and scout – we have time to apply fungicides if we find tar spot in the lower canopy.  We don’t want to apply fungicides before growth stages of V8.
  • Research has shown the best return on investment in making a fungicide application in corn occurs when the fungal diseases are active in the corn canopy between VT to R3. You just need to be scouting for those early lesions to help inform your decision making.
  • But, don’t wait too long, if significant disease develops in the upper canopy, then a fungicide application may be too late at that time. 
Figure 2. Examples of corn leaves infected by tar spot. The spots (stromata, in black squares) will be embedded in the leaf, raised (bumpy to the touch), and will not rub/wash off. In addition, they may be surround by a slight halo. Photo Credits: Darcy Telenko, Purdue University.
Figure 3. Tar spot map for June 10, 2024 (source ).

When should I be putting out a fungicide?

A well-timed, informed fungicide application will be important to reduced disease severity when it is needed, and we recommend holding off until the diseases is active in your field and corn is at least nearing VT/R1 (tassel/silk) or even R2 (blister). Scouting will be especially important if the recent rains we have seen continue.

We are working hard to try to understand this new disease to minimize losses. The good news is that there are a number of fungicides are highly efficacious against tar spot here in Indiana when applied from tassel (VT) to R3 (milk). I would recommend picking a product with multiple modes of action. The national Corn Disease Working Group has developed a very useful fungicide efficacy table for corn diseases (see link).

We will continue keeping a close eye on tar spot.

Figure 4. Examples of A- gray leaf spot lesions, B-northern corn leaf blight lesion, and C- southern corn rust pustules on a corn leaf. Photo Credits: Darcy Telenko

Please help us track tar spot and southern rust, contact me if you suspect a field has tar spot and/or send a sample to the Purdue PPDL for confirmation. Research funding from the Indiana Corn Marketing Council is supporting sample processing, therefore there will be no charge for corn tar spot or southern rust samples submitted to the clinic.

I want to ask before you submit a sample you do a quick and dirty “scratch test” to see if you can rub the spot off the leaf, especially if you have leaves with just a few small spots. I have been successful in detecting these false spots by using my nail to scratch as the suspect lesion. This is a quick way to check, but as always if you are unsure send an image or the sample to the Purdue Plant Pest Diagnostic Lab. Please collect several leaves showing the symptoms and send them with a PPDL form

Please wrap the leaves in newspaper, ship in a large envelope, and ship early in the week. If you are sending samples from multiple locations, please label them and provide the date collected, hybrid if known, field zip code or county, and previous crop (see article on Field Crop Disease Samples Needed in Indiana).

Mail to: Plant and Pest Diagnostic Laboratory, LSPS-Room 116, Purdue University, 915 Mitch Daniels Blvd, West Lafayette, Indiana 47907-2054. The lab is operating and the building is open. If dropping off a sample is more convenient than shipping, please call or email the lab prior to stopping by Phone – 765-494-7071 or Email –

In addition, the 2024 tar spot and southern rust maps are live that will be updated when a positive county confirmation is detected. If you are interested in up-to-date information on the current detection of these diseases, the maps are available on the front page of our Extension website

If you have any question please contact Darcy Telenko ( or PPDL (

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