Minnesota: Tar Spot of Corn Continues Spread

    Tar spot on corn leaf. Photo: University of Minnesota

    Tar spot of corn has continued to spread in Minnesota. This disease was found in Minnesota for the first time in four counties in 2019. In 2020, it has been confirmed in 11 counties in southeastern Minnesota, including one county west of I35 ( https://corn.ipmpipe.org/tarspot-2/).

    The last sample of this year with suspected tar spot symptoms was received last week, but it turned out to be the black lesions that are common with mature rust infection (https://corn.ipmpipe.org/tarspot-2/). Most of the counties with tar spot had many different fields confirmed with this disease. Fortunately, all or most of the confirmed tar spot developed only to low levels or late enough such that yield loss was none or minimal.

    These results confirm what we suspected, that tar spot would continue to spread in Minnesota. Dry weather in parts of southern Minnesota in July and August likely suppressed tar spot development and spread this year. These confirmed counties only represent where tar spot was found, and do not reflect how much further north or west it may have spread undetected.

    Symptoms and management

    Figure 2. Tar spot in corn.
    Corn tar spot is caused by the fungus Phyllachora maydis. This fungus infects leaves and husks and produces small raised black structures on leaf surfaces. The black structures are firm, smooth, and do not rub off or break open as do rust pustules (Figures 1 & 2).

    Tar spot can result in significant corn yield loss, depending on weather, severity, and timing of disease development. Yield loss due to tar spot has not been confirmed in Minnesota. If tar spot develops to high levels before maturity, fungicides can be used to manage it, although optimal timing of applications need to be determined. Proactive scouting can guide potential need and timing for fungicide applications. Risk of this disease is highest where the disease has occurred previously.

    Tar spot appears to respond strongly to weather conditions, in some ways similar to white mold on soybean, and likely is not a major threat when hot and dry weather prevails in late July and August. However, we still have much to learn about tar spot and the risks (high or low?) that it presents to corn production across Minnesota.



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