Kentucky Corn: Tar Spot Confirmed in State

    Tar spot on corn leaf. Photo: University of Minnesota

    Tar spot on corn was confirmed by the University of Kentucky Plant Disease Diagnostic Laboratory (PDDL) in Princeton, Kentucky last week. Corn samples submitted by Curt Judy, Todd County Extension Agent, had signs of the tar spot causal fungus Phyllachora maydis present on leaf tissue.

    The sample received was NOT from a commercial corn field but was on inbred corn in a private corn breeding trial. So far, this is the only location in Kentucky where tar spot has been confirmed.

    Identifying Tar Spot

    Tar spot is usually first observed when the causal fungus produces small black structures called stromata on leaf tissue. These structures protrude from the leaf surface, and affected areas of the leaf feel rough or bumpy. The stromata can also be present on leaf sheaths and husks.

    At this point in the season, no management is needed if tar spot is confirmed in a field, but it is important to alert your county Extension agent if you observe the disease so we can document distribution in the state. If you suspect you have tar spot, please contact your county Extension agent to submit a sample to the PDDL for diagnosis.

    Tar Spot in Other States & Impact

    Tar spot was first confirmed on corn in the United States in 2015. Since 2015, it has been reported in multiple midwestern states and Ontario, Canada, and as far south as Georgia (confirmed 2021) and Florida. A map of the current tar spot distribution in the United States can be found on the corn ipmPIPE website.

    Yield loss due to tar spot varies, and depends on hybrid susceptibility, infection timing, and environmental conditions. Research on tar spot is ongoing and has primarily occurred in northern states. It is unknown what impact tar spot will have in Kentucky, and how management recommendations for the disease in other areas will apply in Kentucky.

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