Tar spot (Phyllachora maydis) has become one of the most concerning diseases for corn growers in the U.S. Since its first detection in 2015, it has spread rapidly across the Midwest, causing significant economic damage. According to the Crop Protection Network (CPN), yield losses can reach 50 bushels per acre under severe infections, with the worst cases reported in Illinois, Indiana, and Wisconsin.
Symptoms and Spread
The disease is easily identified by its black, raised stromata embedded in leaves, which cannot be rubbed off. In advanced stages, lesions may appear on husks and tassels, further reducing photosynthesis and grain fill. Unlike other foliar diseases, tar spot thrives in cool (60-70°F), humid conditions, with infection requiring at least seven hours of leaf wetness.
Recent studies from Purdue University (2023) confirm that the fungus survives on crop residue, making continuous corn rotations particularly vulnerable. Additionally, irrigated fields face higher risk due to prolonged moisture.
Management Strategies
- Fungicide Applications – Research shows that two well-timed fungicide sprays (VT-R2) with multiple modes of action (e.g., QoI + DMI + SDHI) provide the best control. Products like Miravis Neo® and Veltyma® have shown strong residual activity.
- Hybrid Selection – Companies like Syngenta and Pioneer are developing tar spot-tolerant hybrids, which can reduce losses by 20-30% in high-pressure areas.
- Residue Management & Crop Rotation – Reducing infected residue through tillage or rotation with soybeans can lower spore loads.
- Scouting & Early Detection – Fields should be monitored from V8 through R4, as tar spot can explode within days under favorable conditions.
Tar spot is no longer an emerging threat—it’s here, and it’s costing growers millions. However, with proactive scouting, smart hybrid choices, and strategic fungicide use, farmers can mitigate losses. As research continues, staying updated on the latest recommendations from university extensions and agronomists will be key to long-term management.
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