Corn Tar Spot – A New Disease

by Kolton Ingles, Sustainable Plant Systems – Agronomy major

Last year, a disease that has never been seen in the United States before made its debut on corn plants in the Midwest.  Farmers began to notice brown lesions and small black spots appearing on the leaves of their corn.  The effects of this mysterious disease were unknown, and there was no application known to stop its spread.

corn tar spot symptoms

Initial symptoms of tar spot are brownish lesions on the leaves. Black, spore-producing spots appear later, making the leaf feel rough or bumpy. (Purdue Botany and Plant Pathology photo/Kiersten Wise)

After analyzing samples of plants infected by the disease, researchers have discovered that it is Tar Spot, a disease that mainly infects the leaves of the corn plant. The fungus has existed in Mexico but has never before been seen in the United States. Phyllachora maydis is the pathogen that causes this disease, and is suspected to have arrived here through the high speed winds of a hurricane or storm.

Plants infected with tar spot typically will have necrotic areas and brownish lesions on the leaves. On these lesions, are the small black spots that resemble tar and give this disease its name. These spots can also appear on the corn husks.
Fortunately, the worst that Corn Tar Spot will do is slightly reduce yields and lower the aesthetic appeal of a corn field. Another reason to not worry too much about this disease is that the fungus doesn’t overwinter well on dead plant matter because it requires warm weather to survive, and winter temperatures in the Midwest should be cold enough to kill it.

Although Tar Spot is a relatively weak disease, the quick response of farmers and researchers in discovering and identifying it is what is necessary to protect the harvest from total devastation. The next new disease discovery could be powerful enough to wipe out an entire crop, so speedy identification and control is imperative to both farmers and scientists.

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This blog post was an assignment for Societal Issues: Pesticides, Alternatives and the Environment (PLNTPTH 4597). The views expressed are those of the author and do not necessarily reflect the views of the class, Department of Plant Pathology or the instructor.

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