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Tar Spot in Allamakee County
by LuAnn Rolling, District Conservationist

Crop year 2021 has been subject to numerous stressors starting with an early drought and ending with tar spot on corn and army worms on forage. Tar spot has been slowly moving into Iowa and this year seems to have infected most corn fields in Allamakee County.  Depending on when the fungus started and the degree of infection yield loss seems to range from negligible to significant.

According to an article in Crop Production Network written and reviewed by multiple universities, tar spot is caused by a fungus that produces small (1/16 to 3/4 inch) round to irregular diamond-shaped, raised black structures called stromata.  These structures form on both the upper and lower surfaces of corn leaves.  Leaves of infected plants prematurely die when severity is approximately 30 percent or more.

The Crop Production Network article says that cool temperatures (60-70 degrees F) and high relative humidity (greater than 75%) favor tar spot development.  “In addition, disease increases when there is at least seven hours of free moisture on the leaves due to rain, fog or high relative humidity.”

Because of the recent development of this disease in the Midwest understanding of management is limited.  There has simply not been enough time to adequately research it.  According to the Crop Production Network article several management practices may help reduce tar spot development and severity.

1. Avoid highly susceptible hybrids. Speak to your seed dealer or crop adviser and check university corn performance trial data. Due to the recent establishment of tar spot there have been limited opportunities to screen hybrids and breeding material for resistance to tar spot. To date, all hybrids have some level of susceptibility.

2. Consider fungicides.  While fungicides have shown efficacy in managing tar spot, timing of fungicide applications is important in successfully managing this disease. There is little consistent data regarding the optimal time to apply fungicides for tar spot management.

3. Rotate to other crops.  It is not known how many years of rotation away from corn are needed to reduce inoculum.

4. Manage residue. Tillage may help reduce the amount of inoculum in a field but will not reduce the risk of infection from locally dispersed inoculum.

5. Scout for tar spot and be prepared to harvest heavily diseased fields early if push tests indicate that stalk integrity is impacted to avoid lodging.

According to Michigan State University plant pathologist Martin Chilvers it’s becoming clear that residue management strategies, such as crop rotation and tillage, only have limited effectiveness. “It is moving on the wind, airborne.” Chilvers explained. “So even if you did everything right to manage infected residue in your field, you can still get it because it’s a regional problem.”

According to Dr. Daren Mueller, an associate professor and extension plant pathologist at Iowa State University and also the coordinator of the Integrated Pest Management (IPM) program, there are several tar spot resources available for producers. One is a website called EDDMapS. It is a web-based alert system that emails users when new sightings for species or areas of interest are entered into the system. As new sightings are reported, a network of professionals will be available to verify new sightings and natural resource managers will be notified to take appropriate management actions. Another resource is the Crop Protection Network which is a multi-state and international partnership of university Extension specialists, and public and private professionals that provides unbiased, research-based information.

Mueller says that treatment for tar spot should not be attempted until the farmer is sure that the disease is present or the risk of the disease warrants an application. Also, the timing of a fungicide application will affect how well a fungicide works. While still in the early stages an app called Tarspotter can help determine the risk of tar spot and timing of fungicide application.

“I really hope that in five years’ time, tar spot will be more like northern corn leaf blight or gray leaf spot, in that they are always present, but we have a better handle on them,” Chilvers concluded. “But it will take time for more tar spot resistant hybrids to come through.”