Tar spot continues surfacing across many fields in the Midwest. Originating from Latin America, tar spot was first confirmed in 2015 in Indiana and now has a presence in Illinois, Iowa, Wisconsin, Michigan and Ohio. While knowledge of how this disease impacts corn production is still limited, the
University of Illinois Extension notes that since infections are typically sparse and often occur late in the season, yield loss has been minimal. However, heavy disease pressure during grain fill may compromise grain and stalk quality. We encourage you to know how to identify this late-season disease, since tar spot can overwinter in the soil and residue, potentially becoming a larger problem in coming years.
IdentificationCaused by the fungus
Phyllachora maydis, tar spot seems to show after a foliar disease as a secondary infection. As you walk your cornfields,
Purdue University recommends scouting for black, circular bumps that appear like specks of tar. These spots are often, but not always, surrounded by a small, tan halo. While they may cover the entire corn leaf, tar spot might also surface in a few spots. Note these spots don't rub off easily.
Tar spot fungal structures can be easily confused with other pressures, such as rust pustules and saprophytic organisms, or organisms that feed on dead organic matter. To confirm proper identification, consider submitting samples to a
National Plant Diagnostic Network lab.
Development
Transported by wind or dry plant residue, tar spot symptoms generally appear at corn flowering. The fungal growth typically begins on the lower leaves and moves rapidly up the plant. Tar spot thrives in cool, humid conditions with prolonged periods of wet leaves. Specific hybrid tolerance or resistance remains a largely unknown factor.
If you think your field may have been hit with tar spot, contact your
Golden Harvest Seed Advisor.
References:
1. Purdue University Extension, Diseases of Corn – Tar Spot, BP-90-W
2. University of Illinois Extension, IPM Bulletin – Tar Spot in corn - requesting your help
3. Iowa State University, Integrated Crop Management – Tar Spot Confirmed in Corn in Eastern Iowa
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