Tar spot, caused by the fungal pathogen Phyllachora maydis, has once again appeared in the U.S. Corn Belt, this time confirmed in six counties across Missouri: Nodaway, Chariton, Saline, Cooper, Howard, and Audrain. The announcement, based on data as of June 24, 2025, signals an expanding disease footprint that first reached Missouri in 2019.

Since then, tar spot has been confirmed in 63 of Missouri's 114 counties, with the northern region showing the greatest concentration of outbreaks. Its rapid spread is becoming an increasing concern for corn growers and agronomists alike, particularly given the weather-sensitive nature of the pathogen.

This year, the fungus has also been detected in Kansas, Nebraska, Iowa, Illinois, and Indiana, suggesting that regional conditions are conducive to the disease's development. Tar spot thrives in humid, warm environments, especially following periods of rain and canopy closure in corn stands.

Critical Action Steps for Farmers

To counter the rising threat, experts recommend that producers:

Step up field scouting, particularly in areas with a known history of tar spot. Early detection is essential for informed fungicide decisions.

Target fungicide applications between the VT (tasseling) and R3 (milk) stages, when efficacy is highest. Applications made before VT or after R4 have not shown significant economic return.

Monitor weather patterns closely, especially temperatures above 60°F with 7+ hours of leaf wetness, which create ideal conditions for tar spot to flourish.

Mandy Bish, Missouri's extension plant pathologist, reported early-stage tar spot in Nodaway County corn at growth stage V10, likely triggered by the cool, moist conditions in May. These findings stress the urgency of scouting before tasseling begins, especially in fields where fungicides were applied preventively last year.

Yield Loss and Economic Impact

Tar spot is not merely a visual nuisance-it can be economically devastating. In severe infestations, it can reduce corn yields by 20 to 60 bushels per acre, depending on hybrid susceptibility, weather conditions, and fungicide timing.

There are no commercially available hybrids fully resistant to tar spot, though some offer partial tolerance. This reality underscores the need for integrated pest management, including:

Hybrid selection with tolerance traits

Crop rotation and residue management

Accurate growth-stage-based fungicide applications

Research continues into better forecasting models and AI-based diagnostic tools that could support farmers in recognizing risk windows and optimizing treatment schedules.

Missouri Corn Crop Conditions

As of late June, Missouri's corn emergence was 98% complete, matching the five-year average. The silking stage was observed in about 5% of fields, slightly ahead of historical trends but behind the pace of 2024. Overall crop condition ratings remain strong: 62% good and 11% excellent, outperforming the national average. However, with 21% rated as fair, the potential for yield drag from tar spot is real, particularly in fields lacking fungicide protection.