Tar Spot May Arrive Early in Missouri Cornfields This Season

By Blake Jackson

Persistent rainfall and cooler-than-normal temperatures throughout May have created ideal conditions for tar spot to emerge in Missouri cornfields earlier than usual this season.

According to University of Missouri Extension plant pathologist Mandy Bish, growers should prepare for the possibility of seeing the disease in June. “We’ve detected tar spot in June in each of the last three years, and I anticipate the same for 2026,” said Bish.

Tar spot is a fungal disease that survives in corn residue left in fields. It thrives when temperatures remain in the mid-60s to low 70s for extended periods.

Since Missouri recorded its first case in 2018, the disease has spread across multiple counties throughout the northern and central portions of the state and continues to expand.

The disease appears as small, raised black spots known as stromata on both sides of corn leaves. In severe infections, lesions may also develop on husks and leaf sheaths. Corn remains the only known host of the tar spot fungus.

Bish encourages producers to scout fields regularly but cautions against making immediate treatment decisions. “That said, we don’t want to panic if we observe tar spot in June,” said Bish.

“When we confirm its presence, we need to monitor the disease’s progress. We want to consider disease severity, frequency, the location of the symptoms and weather forecast before making a fungicide application. For example, are symptoms confined to the lower canopy, or are they moving toward the ear leaf?”

Bish notes that disease forecasting tools can help evaluate risk. “Temperatures tend to rise as we approach July, and the fungus that causes this disease tends to slow its growth in warmer conditions,” Bish adds.

Research indicates fungicide applications between the VT and R3 growth stages offer the greatest economic return when disease pressure is high.

“If we apply fungicide too early, before tar spot begins moving up the canopy, we risk losing residual activity just as southern rust may move in during July,” Bish warns.

While no corn hybrid offers complete resistance, selecting tolerant hybrids, rotating crops, and maintaining regular scouting programs can help manage tar spot and support informed fungicide decisions.

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