Tar spot is a fungal disease initiated by the fungus Phyllachora maydis. First identified in the United States in 2015, it has since spread to several midwestern states.
The disease is notorious for its tar-like spots, which can cause significant yield loss and negatively impact quality if not managed effectively.
The black spots caused by the fungus can cover up to 50-60% of the leaf surface, reducing the plant's ability to photosynthesize. Effects on different parts of the plant include premature death of leaves, which can lead to poor kernel fill and reduced grain quality. Tar spot can also cause stalks to become brittle and prone to lodging, making harvest more difficult.
In terms of physiological effects, tar spot can reduce the plant's ability to take up water and nutrients, further impacting yield. Additionally, the disease can make plants more susceptible to other diseases and pests.
(Image Credit: Ed Zaworski, Bugwood.org)
How to Identify Tar Spot
Tar spot appears on the leaves, husks, and tassels of corn plants. These black spots, or stromata, are roughly circular and can vary in size from a pinhead to a pencil eraser. They often feature a thin, tan or brown halo.
Other signs of tar spot include early leaf senescence and a general decline in plant health. Infected plants may also exhibit reduced ear size and poor kernel fill.
Geographic Impact of Tar Spot
Tar spot is most commonly found in Illinois, Indiana, Iowa, Michigan, and Wisconsin. However, it has also been identified in other corn-growing regions, including parts of Mexico and Central and South America.
(Image Credit: Kiersten Wise, Bugwood.org)
Timing of Tar Spot
Tar spot typically appears mid- to late in the growing season, often after silking. However, the disease can develop earlier, particularly in areas with high humidity and moderate temperatures.
How to Manage Tar Spot on Corn
Unlike other diseases that cause similar symptoms, these spots cannot be rubbed off or removed.
To manage tar spot on corn, our agronomists recommend applying Trivapro between R1 and R3. Research indicates that multiple modes of action help to improve disease control, and Trivapro has three active ingredients with three different modes of action.
Another product our agronomists recommend for managing tar spot on corn between R1 and R3 in certain states is GCS Azoxyprop.
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