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

Yes, we have found active tar spot in Indiana.

Early planted corn in Indiana is reaching mid- to late-vegetative stages and some tasseling in the south. Therefore, it is time to start monitoring for diseases to make an informed decision if a fungicide is necessary. As a reminder for disease to occur, three things need to be present 1. Virulent Pathogen, 2. Susceptible Host, and 3. Favorable Environment. Recent rains have made conditions conducive for many of the foliar diseases in corn therefore it is now time to get out and scout. 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. This week we have found a low incidence of tar spot, gray leaf spot, northern corn leaf blight, and common rust in the lower canopy (figures 3 and 4).

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 of corn continues to be a concern this season after the localized epidemics we have experienced last few years in Indiana. In our scouting rounds this week, we did find tar spot at a low incidence in our field trial locations. The disease is just starting and the wet conditions last week probably kicked it off. The fields were located in Porter and LaPorte Counties in Indiana (see map Figure 3). Michigan has also had positive confirmations in Allegen and Eaton Counties and Iowa in Muscantine County. In the Indiana fields, the corn was between V5 and V8, had a history of tar spot. The disease was found in some of the lowest leaves in the canopy. We will continue to monitor the disease progression in these fields and will provide updates on any significant spread in the field or increases in disease severity. Again, the recent rains have favored tar spot. See the forecast from the Tarspotter App all of Indiana was red last week, whereas as of today, July 6, 2021, most of the state is still at high risk except in southeast Indiana (Figure 4). If you have a history of the disease it is time to keep an eye out and make an informed management decision (Figure 4).  

Tar spot in corn map (in gray color the most affected states: Indiana, Illinois, Wisconsin, Iowa, Ohio and Michigan) Figure 1. Tar spot map for July 6, 2021. (Source: EddMaps at
Map of Indiana with red marks that indicate favorable environmental conditions for tar spot Figure 2. Tarspotter App forecast from July 6, 2021. Red color indicates favorable environmental conditions for tar spot if corn at V8 or larger, blue color indicates unfavorable environmental conditions for tar spot. Source: Tarspotter App v. 0.52 Smith, D., et al. ©2021 Board of Regents of the University of Wisconsin System.
Corn leaves infected by tar spot. Infection can range from mild to severe on a leaf.
Figure 3. Corn leaves infected by tar spot. Infection can range from mild to severe on a leaf. The spots will be raised (bumpy to the touch) and will not rub off. In addition, they may be surround by a tan or brown halo, and high severity can lead to a rapid blighting of the leaf. Photo Credits: Darcy Telenko

After hearing this news we 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 week’s rain.

Based on our recent research, to minimize the impact of tar spot on crop yield we need to be protecting the ear leaf and above until the corn reaches black layer. In our fungicide timing trials applications made at VT/R1 (tassel /silk) did a good job controlling tar spot, but we did see that once the fungicide ran out of steam (3-week window) tar spot began to pick up. 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 become active in your field and corn is at least at V8 or nearing VT/R1 (tassel/silk) or even R2 (blister) if we move back to drier conditions. Scouting will be especially important if the recent rains we have seen continue.

If you suspect tar spot in your fields, please consider submitting samples for confirmation. We are still documenting the disease in Indiana, similar to last few years. 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. Sometime, the black spots may be surrounded by a tan or brown halo; this is especially obvious on healthy leaves (see Figure 2). 

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 abel them and provide the date collected, variety of corn, 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 –

In addition, the 2021 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  or at CornipmPIPE.

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

 from left to right examples of A- gray leaf spot, B-northern corn leaf blight, and C-northern corn leaf spot lesions on a corn leaf
Figure 4. 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
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