Tar Spot Identified on South Dakota Corn

Farmers reported sightings of the fungal disease, though no yield losses occurred in the state during 2026.

Updated on Oct. 8, 2026 in Botany

Tar Spot Identified on South Dakota Corn

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Black spots discovered on South Dakota corn stalks in 2026 were identified as tar spot, a fungal disease that first appeared in the state in 2022. Despite these reports, the disease did not cause significant issues like early crop death or lodging.

Why it matters

Understanding the impact of fungal diseases is critical for farmers, as certain pathogens can produce mycotoxins that lead to grain rejection at elevators. While tar spot is a concern in some regions, South Dakota's dry 2026 conditions limited the spread of most corn diseases.

Tar spot is a saprophytic fungus that feeds on dead organic matter, with researchers noting it first appeared in Clay and Yankton counties in 2022. Fungicide application is only recommended if the disease is detected at the R2 growth stage or earlier.

The players

Fusarium

This is a genus of filamentous fungi that can produce harmful mycotoxins in agricultural crops.

Aspergillus

This is a group of fungi known for producing toxins that can lead to grain rejection if found in harvest samples.

The details

Although the fungal disease arrived in South Dakota recently, it remains distinct from other problematic fungi such as Fusarium and Aspergillus, which actively produce dangerous mycotoxins. The state has successfully avoided the severe crop damage and lodging issues that have been observed in other regions like Missouri.

2021: Tar spot may have been present in the state.
2022: The fungus was first officially identified in South Dakota.
2026: Extremely dry conditions resulted in low levels of corn disease across the state.

The Big Picture

The identification of tar spot in South Dakota follows the monitoring patterns established by the USDA corn disease surveillance protocols. This tracking is essential for distinguishing between purely saprophytic fungal growth and pathogens that threaten agricultural yields.

Farmers can differentiate between harmless saprophytic fungi and dangerous mycotoxin-producing pathogens to avoid unnecessary fungicide costs. Proper identification ensures that resources are only directed toward management when actual yield loss is likely at the R2 growth stage.

The takeaway

Producers should monitor for disease early in the season to determine if intervention is economically necessary. Identifying the specific fungus is essential to preventing potential grain rejection at the elevator.