Photo Credit – Ed Zaworski, Bugwood.org
Tar spot, Phyllachora maydis, manifests as raised stromata or circular specks resembling tar spots. If you have found this on your farm before, then you know the inoculum for this disease is lurking there. The first key is not to confuse tar spot with bug poop, which will appear shiny on a leaf surface with sometimes evident leaf feeding, and can be scratched off (if you dare) with a fingernail. A tar spot lesion or “black dot” can be found both on both the front as well beneath the leaf surface. Dr. Alison Robertson, Iowa State Extension Field Crop Pathologist, has some great videos on Twitter to help you distinguish tar spot from bug poop and they can be found here:
Tar spot has not officially been found in Illinois yet but could be here as this disease has been reported in neighboring states. To track tar spot in corn, go to: Tar Spot – Corn ipmPIPE.
Tarspotter and Field Prophet are both smartphone applications that can help you determine if the weather in your neck of the woods has been conducive for tar spot to make its appearance. If it has been dry, don’t assume that you will not have tar spot. Yes, rain is helpful for disease development, but another key is “intermittent wet/dry cycles to give us intermittent leaf wetness” or more specifically, leaf wetness at night. This along with temperature are variables are considered to help predict the appearance of Tar Spot. For more details on these apps, check out this research publication. Please be aware that Tarspotter has been updated and improved since it made its debut in 2020.
Currently, this disease has been found at very low severity and most of time, is lower in the corn canopy. The last key is to make sure you know the tar spot disease rating of your corn hybrid and target fields that are the most susceptible to this disease. If tar spot severity is at 10% or more on the ear leaf or above, this will warrant a fungicide application. Research has shown thus far that you can generally control tar spot and preserve yield potential with one fungicide spray between VT (tasseling) and R3 (milk) growth stages. There are several fungicide options to choose from and you can check out the efficacy of various fungicides based on University research by going to the Fungicide Efficacy for Control of Corn Diseases via the Crop Protection Network.
Resources – Crop Protection Network
Tar Spot – Corn ipmPIPE
What Should I do About Tar Spot of Corn in 2023? – Badger Crop Doc
Photo – IPM Images on Bugwood.Org