The photo shows tar spot on upper leaves, left, and lower in the canopy, right. Crop yields are hurt most by infections on the upper leaves, which collect the most light and are responsible for the filling of the grain on the ear. Graphic provided by Purdue Plant and Pest Diagnostic Laboratory.

By Jeff Burbrink
Ag & Natural Resources Extension Educator, Purdue Extension LaGrange County

Jeff Burbrink

LAGRANGE — There are reports of some foliar diseases beginning to appear in the lower canopy of corn around Indiana.

A few diseases seen by Purdue’s crop disease specialist Darcy Telenko or sent in to the Purdue Plant Pest Diagnostic Clinic included gray leaf spot, tar spot and common rust in corn.

On the tar spot map, you can see gray areas where Purdue has confirmed tar spot in past years. In Indiana, we have confirmed tar spot for this season in 17 counties as of July 3. That includes nearby St. Joseph, LaPorte and Porter counties in Indiana, and Branch County in Michigan. The map is continuously updated.

Provided by The Crop Protection Network, which Purdue is a partner in, shows the current counties with confirmed tar spot on July 9.

Telenko has received a few reports that tar spot is increasing in the lower leaves and moving up into the mid-canopy in some of these fields, and we suspect with the current weather conditions the disease will continue to be detected in fields in Indiana. We will continue to monitor and provide updates. Telenko encourages you to send sample to the Purdue Plant and Pest Diagnostic Lab if you suspect issues.

Telenko just released a tar spot specific article on “Fungicide Application Reminders to Optimize Management of Tar Spot and Return on Investment in Corn” on Crop Protection Network. There is also a brief video talking about fungicide timing for tar spot based on our research.

Some good news: southern rust is yet to be detected in Indiana, although the common rust is present as it usually is in humid ole Indiana.

Telenko points out that it may not be an automatic decision to apply a fungicide to corn fields.

First, if the field has had tar spot in the past, it is more likely to have it in the future.

Second, you should be looking at the lower canopy and seeing the disease, which increases the risk the upper leaves will get infected and seriously hurt your crop yield.

Third, weather conditions that favor tar spot include warm temperatures, humidity and rainfall. One phone app called Tarspotter can give you insight into the current conditions.

The fourth consideration is the return on your investment. Will the cost of the fungicide plus the cost of the application bump your crop yield enough to cover the costs? Telenko and her counterparts around the Midwest have created a publication on the effectiveness of the current fungicides on the market.