ByLacy Christopher
Today at 4:00 PM
Tar spot is a relatively new corn disease in North Dakota, but it is one that Richland County producers should keep on their radar this growing season. The disease was first confirmed in Richland County in fall 2024, marking the first detection of tar spot in North Dakota. Although disease levels remained very low and no measurable yield losses occurred, its presence serves as a reminder that continued scouting is important
Tar spot is caused by the fungus Phyllachora maydis. It appears as small, raised, black spots scattered across corn leaves that often resemble tiny flecks of black tar. The spots cannot be rubbed off with a finger, helping distinguish them from insect frass or dirt
The disease thrives under cool temperatures between 60 and 70 degrees, frequent rainfall, high humidity and extended periods of leaf wetness. Fields with heavy dew, frequent rainfall or irrigation are generally at greater risk for disease <a href="https://healthylife7.com/odds-increase-for-development-of-invest-91l-in-the-gulf-with-heavy-downpours-expected-in-southeast/” title=”Odds increase for development of Invest 91L in the Gulf, with heavy downpours expected in Southeast”>development
Fortunately, tar spot was detected late in the growing season in North Dakota, resulting in little to no impact on grain yield. However, NDSU Extension specialists observed higher disease incidence in some southeastern North Dakota fields during the 2025 growing season, including Richland County. Those observations suggest the disease may become established and could appear earlier in future years if weather conditions are favorable
Scouting remains the most important management tool. Begin monitoring corn fields during the late vegetative stages and continue through grain fill. Pay particular attention to fields with a history of tar spot, continuous corn production or abundant corn residue. If black, raised lesions are found, producers are encouraged to contact their local NDSU Extension office to confirm the diagnosis
Hybrid selection is another important management strategy. Although no corn hybrid is completely resistant, differences in tolerance exist. Selecting hybrids with improved tar spot ratings can help reduce disease severity. Crop rotation and residue management also may help reduce disease risk
Fungicide applications can be effective when disease develops before or around tasseling through the early grain-fill stages and environmental conditions remain favorable. However, fungicides should not be applied simply because tar spot has been reported in the area. Management decisions should be based on field scouting, crop growth stage, weather conditions and disease pressure. In Richland County, tar spot has not been found before or at tasseling, but continued scouting remains essential
Although tar spot has become a significant concern in parts of the Midwest, North Dakota producers have time to prepare. Continued scouting, selecting well-adapted hybrids and making informed management decisions will help minimize potential impacts if the disease becomes more common in our region
If you suspect tar spot in your corn fields, contact the Richland County Extension Office for assistance with identification and management recommendations
- NDSU Crop & Pest Report No. 18 (Oct. 10, 2024)
- NDSU Crop & Pest Report No. 19 (Sept. 25, 2025)
Lacy Christopher Contributed
Lacy Christopheris the Agriculture and Natural Reon-Richland County
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Lacy Christopher
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