Tar Spot on Corn: How to Prepare for It This Season

A new corn disease called tar spot has emerged in cornfields across the U.S. over the last few years. In some cases, the levels are so high that they affect stalk stability or yield production. Tar spot originated in Latin America and has migrated its way into the U.S. via Indiana, Illinois, Iowa, Wisconsin, Michigan, Minnesota, and several other Midwest states.

U.S. scientists are still working to understand the key elements of the disease, how it spreads, how much damage it can do, and what chemical solutions mitigate it.

What is Tar Spot?

Tar spot complex in corn is caused by the fungus Phyllachora Maydis and Monographella Maydis and has been historically found at high elevations in cool, humid areas in Latin America. Originally observed only in high valleys in Mexico, it has propagated and spread to parts of North America.

These two types of fungi can cause different corn yield effects, with Monographella Maydis having a significant economic impact within South America. Phyllachora Maydis is the fungal strain present in both South America and North America and has not been associated with yield loss as of yet. Both fungi are commonly found on the surface of corn leaves.

How to Identify Tar Spot

Tar spot has the following characteristics and display the following symptoms on maize leaves:

●There are brownish lesions on leaves. The actual tar spot is the physical sign that the fungus fruiting body, the ascomata, is developing on the leaf.

●A single ascomata contains black oval or circular lesions on the corn leaf.

●The texture of the leaf is bumpy and uneven when the fruiting bodies are present. Black structures cover the leaf.

●Larger, necrotic spots can develop around the ascomata, originating from water-soaked brown lesions with brown rings.

●The larger lesions can combine after 7-14 days. Areas between spots can become water-soaked and dry out.

●Tar spot spreads from the lowest leaves to the upper leaves, leaf sheaths, and eventually the husks.

●Phyllachora Maydis produces small, round, dark lesions. Monographella Maydis causes the brown ring. Together, they produce what looks like a fisheye.

Are There Currently Any Preventive or Treatment Measures for Tar Spot?

That is a question that both scientists and agricultural experts are trying to answer to date.

As it stands, we still have much to learn about tar spot on corn in the U.S. It will take time to generate quality data and information for the agricultural community. Plant pathologists in the region are working together to generate this information and share it with all agricultural entities associated with the problem.

There has been some discussion on the effects of tillage, rotation, and fungicide on tar spot. Currently, there is no official consensus about what may prevent tar spot or treat the corn. There is still much research that needs to be conducted to find out about both the harmful effects of tar spot and mitigation of the fungus.

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