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To Understand the Future Risk on Your Farm Now is Time to Scout for Tar Spot

Tar spot of corn has been a concern this season after the localized epidemics we experienced last year in Indiana. At this point in the field season, there are a number of areas with active tar spot.We again would like to document tar spot disease distribution and severity in Indiana. Our research trials are in full swing and optimistically we will have some positive results about management this winter, but we still need your help collecting samples and documenting this disease. It is not only important to understand tar spot distribution statewide, but 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.

We have currently confirmed active tar spot fields in 20 counties in 2019. In 2018, we confirmed tar spot in 41 counties (Figure 1). 

Figure 1. Distribution of tar spot in Indiana in 2018 and current distribution as of Oct 1, 2019. (source)

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. Often, the black spots are surrounded by a tan or brown halo; this is especially obvious on healthy leaves (see Figure 2). 

Figure 2. Corn leaves infected by tar spot. Infection can range from severe to mild on a leaf. The spots will be raised (bumpy to the touch) and will not rub off. In addition, they be surround by a tan or brown halo. Photo credit: Darcy Telenko

We greatly appreciate the samples that we have received this season and want you to keep it up. If you have (or think you have) corn tar spot, please collect several leaves showing the symptoms and sed them with a PPDL form.

Please wrap the leaves in newspaper and ship in a large envelope. Please ship early in the week. If you are sending samples from multiple locations please label them and provide the date collected, variety of corn, field zip code or county, and previous crop.

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.

Mail to: Plant and Pest Diagnostic Laboratory
LSPS-Room 116, Purdue University
915 W. State Street, West Lafayette, Indiana 47907-2054

Question please contact Darcy Telenko ( or PPDL (

map Figure 2. Map of counties confirmed for tar spot in Indiana as of August 15, 2019.
leaf with tar spot Figure 3. An example of a leaf with just on tar spot stroma (left) and a leaf with multiple stroma (right). Photo credit: Darcy Telenko


We are starting to find a few foliar diseases in soybean in Indiana.  

Downy mildew caused by Peronospora manshurcia is commonly found on soybean but rarely causes serious yield loss. Symptoms appear as irregularly shaped, pale green to light yellow spots. If you flip the leaf over you will likely see the distinctive sporulation on the underside, (see figure 4). Older lesions will turn brown with yellow-green margins.

Figure 4. Downy mildew symptoms on upper leaf – pale green to light yellow spots, and the underside showing the sporulation. Photo credit: Darcy Telenko

Frogeye leaf spot caused by Cercospora sojina is a foliar disease can cause significant yield losses. We have started to find a few frogeye lesions in our soybean sentinel and research trials locations (see figure 5).

Figure 5. Frogeye lesions on soybean. Photo credit: Darcy Telenko

Management practices for frogeye are aimed reducing soybean susceptibility and inoculum availability. Infected debris from previous crops is the primary source of inoculum for this disease. Any practice that helps reduced or bury the infected residue will help reduced inoculum in a field such as fall tillage or soybean- corn crop rotation. There are a number of varieties available with frogeye resistance. Fungicide spray application after growth stage R1 can reduced severity, while applications made at R3 are considered most effective for frogeye. There are number of fungicides available to use for frogeye management. The North Central Regional Committee on Soybean Diseases (NCERA-137) annually releases foliar fungicide efficacy tables for control of the major foliar soybean diseases in the US. Click here.

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