What is Tar Spot?

Tar Spot Dean Malvick, Universtiy of Minnesota Extension plant pathologistTar Spot is a corn fungal disease that has been infiltrating the Midwest within the last few years. Tar spot has the capability to severely impact yields up to 30%. Concern about the intensity of this fungus grows as researchers learn more about it. We urge farmers to scout their fields through harvest for tar spot since this fungus can survive over winter year to year.

Tar Spot Identification

Tar spot will show up as small, raised blacks spots, on the upper and lower corn leaves. The raised black spots are also known as stromata. If severe enough, it can spread to leaf sheaths and husks. Typically, it will start lower in the canopy and move upward. Tar spots are usually accompanied with a yellow halo or fish-eye effect around the black spots. Note the stromata, cannot be rubbed off unlike a common or southern rust pustule.

The Importance of Scouting

This corn fungus has nearly made it across Iowa and is continuing to spread in Wisconsin, Illinois, Indiana and Missouri. Tar Spot likes cool temperatures and humid conditions, with prolonged leaf wetness or heavy dew and will concentrate towards the middle of the field. It can cause lodging and hasten maturity.

Final Thoughts

This is one of the variables in farming we continue to learn about. Scout now and start planning for next season. Product selection is key when fighting these variables year to year and important to integrate into your plan for the next growing season.

“Threats of Tar Spot Complex disease of maize in the United States of America and its global consequences” (Mottaleb et al. 2018).

“Threats of Tar Spot Complex disease of maize in the United States of America and its global consequences” (Mottaleb et al. 2018).

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