Pest Control · September 2025 · 5 min read
Tar spot has rapidly become one of the most significant corn diseases in the U.S. Midwest and Great Lakes region. Caused by the fungus Phyllachora maydis, it produces distinctive raised black spots on corn leaves that reduce photosynthesis and can cause severe yield losses. Understanding how to identify and manage tar spot is now essential for corn growers.
Tar spot produces small, raised black stromata on leaf surfaces that cannot be rubbed off—a key distinction from insect frass or dirt. The spots feel rough to the touch and are embedded in the leaf tissue. In severe cases, a tan or brown halo called a fish-eye lesion surrounds the stromata, and these lesions are associated with the greatest yield loss.
Tar spot thrives in cool, wet conditions with temperatures between 60–70°F and prolonged leaf wetness from dew, fog, or frequent rain. High humidity and dense canopies create ideal infection environments. Fields near wooded areas or low-lying ground with poor air circulation are often affected first.
The pathogen overwinters on corn residue, so continuous corn fields and reduced-tillage systems face higher risk.
Fungicide applications at VT to R2 (tasseling through blister stage) provide the best protection against tar spot yield loss. Products containing strobilurin and triazole active ingredients have shown good efficacy. A second application 14–21 days later may be warranted in high-pressure years.
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