A fungal disease called tar spot has been found in North Dakota for the first time. The disease was first identified in Richland County on September 20, and scouting efforts by the NDSU Extension Cereal Crop Pathology team have now confirmed it in nine counties across the state.
Low Impact This Year
The good news is that both the number of infected plants and severity of the disease have been very low. In most field scouting efforts, only one tar spot lesion was found on every 50 to 100 corn plants.
Because tar spot occurred late in the growing season, no yield loss is expected from this disease in North Dakota in 2024.
About Tar Spot Disease
Tar spot is considered a new disease of corn in the United States. The disease has historically been very problematic in:
- Southern Mexico
- Central America
- Northern South America
The first U.S. report of tar spot came in 2015 in Indiana and Illinois. Since then, the disease has been expanding across the country.
What Causes Tar Spot to Spread
The disease prefers:
- Cool temperatures (64-73°F)
- Humid environments
- Extended periods of leaf moisture (like dew)
The combination of August rain events from the south and cool temperatures likely helped the pathogen’s spores arrive in North Dakota.
What to Look For
When scouting for tar spot, look for:
- Slightly raised black spots
- Spots may appear circular, irregular, or diamond-shaped
- Lesions appear on both upper and underside of corn leaves
- Spots cannot be rubbed off
- With a magnifying glass, you may see an orange spore mass coming from the center of the lesion
Looking Ahead to 2025
The big question now is what to do for 2025. The first step will be determining if the tar spot pathogen can survive North Dakota winters.
One of the best ways to check this is to periodically scout corn fields for tar spot presence. If you think you’ve found tar spot in your fields, work with your local County Extension Agricultural Agent for confirmation.
While tar spot’s arrival in North Dakota is something to monitor, the late timing and low severity this year means farmers have time to prepare and learn more about managing this new disease.
