Kentucky Corn: Tar Spot Confirmed in Lincoln County

    Tar spot on corn was confirmed by the University of Kentucky Plant Disease Diagnostic Laboratory (PDDL) from samples collected in Lincoln County. This is our first confirmation of tar spot for 2022, after finding it in Ohio and Todd counties in 2021.

    So far, this is the only location in Kentucky in 2022 where tar spot has been confirmed, despite extensive scouting by university employees and others in the crop scouting industry.

    Tar Spot Symptoms & Signs

    Tar spot on corn, caused by Phyllachora maydis, is usually first observed when the causal fungus produces small black structures called stromata on leaf tissue (Figure 1). 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.

    When Southern Rust Can Resemble Tar Spot

    There have been several “suspicious” samples submitted to the PDDL with suspected tar spot that were diagnosed as southern rust. As the season progresses, the southern rust fungus’s pustules darken and can resemble the stromata of tar spot (Figure 2).

    These signs are difficult to distinguish in the field, but microscopic examination of spores within the fungal structures can easily determine if southern rust or tar spot is the causal disease. If you suspect you have tar spot, please contact your County Extension Agent to submit a sample to the PDDL for diagnosis.

    Management, Range & Yield Loss

    At this point in the season, no management is needed if tar spot is confirmed in a field; however, documenting confirmed cases to monitor for future disease spread and impact is still needed.

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    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 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 how management recommendations for the disease in other areas will apply in Kentucky.

    It is possible that current management practices for diseases that are common in our area (like southern rust) will also effectively manage potential tar spot outbreaks, but more research in our specific climate is needed.

    Figure 1. Signs of tar spot observed on a corn sample from Todd County, KY, 2021.  (Photo: Kiersten Wise, UK)

    Figure 2. Southern rust pustules can resemble tar spot (Photo:  Kiersten Wise, UK)

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