Time to Scout for Diseases in Field Crops: What to Look for in Corn

Darcy Telenko, Extension Field Crop Pathologist


We have NOT found active tar spot in Indiana yet.

Corn growth stages are quite variable across Indiana but a number of areas with early planted corn about waist high. Recent news that tar spot was found in Iowa, Missouri, and Kansas means that we need to start to monitoring for disease in Indiana to make an informed decision if a fungicide is necessary. The hot and dry conditions in May and most of June kept foliar disease at bay, but recent rains may have made conditions conducive for many therefore it is now time to get out and scout.  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, northern corn leaf spot, and tar spot all might start to make an appearance in the next couple of weeks (figure 1 and 2).

As of June 26, 2023 we have not found or had any reports of tar spot in Indiana.

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.

Figure 1. Examples of A- gray leaf spot, B-northern corn leaf blight, and C-northern corn leaf spot lesions on a corn leaf. Photo Credits: Darcy Telenko

Tar spot continues to be a concern this season. In our scouting rounds last week, we did not find any. Tar spot may be hard to find early Figure 2 shows how small the initial tar spot lesions (stroma) will be. Previously, we usually find the stroma 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 rains help promote tar spot. See the forecast from the Tarspotter App for June 26 much of the state is red (Fig 3).

I would like to make a few recommendations when using the Tarspotter App.

  1. If you have a history of the tar spot it is time to keep an eye out and make an informed management decision. 
  2. In order to get it to turn on a growth stages of V8-R4 must be selected, if you make this selection and your corn is not at V8 you are cheating the model.
  3. 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.
  4. Our recent research has shown that making an application just after first detection is effective – you just need to be scouting for those early lesions.
  5. But, if you wait until we have significant disease 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 (stroma, 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
Figure 3. Tar spot map for June 27, 2023 (source https://corn.ipmpipe.org/ ) and Tarspotter App forecast from June 26, 2023. Red color indicates favorable environmental conditions for tar spot if corn at V8 or older. Source: Tarspotter App v. 0.53.3 Smith, D., et al. ©2022 Board of Regents of the University of Wisconsin System.

I know the next question – should I be putting out a fungicide?

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. Most of our corn sites across the state are quite clean and disease pressure is minimal, so far this season. It is important to keep scouting, especially after last weekend’s rain.

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 we found a number of fungicides are highly efficacious against tar spot here in Indiana when applied from tassel (VT) to R2 (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. Please help us track tar spot, 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 samples submitted to the clinic.

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.

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 https://ag.purdue.edu/department/btny/ppdl/submit-samples/_docs/ppdl-1-w.pdf

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.

Mail to: Plant and Pest Diagnostic Laboratory, LSPS-Room 116, Purdue University, 915 W. State Street, 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 – ppdl-samples@purdue.edu.

In addition, the 2022 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 https://indianafieldcroppathology.com/

If you have any question please contact Darcy Telenko (dtelenko@purdue.edu/764-496-5168) or PPDL (ppdl-samples@purdue.edu/765-494-7071)

Read More