Reports of tar spot from Madison, Jaspar and Calhoun counties have been received over the weekend and today (June 13-15, 2026). The disease is present at very low incidence (one to a few plants in a field) and low severity (one spot on a leaf). In addition, the disease is being observed on the lowest leaves of the canopy.
The Crop Risk Model shows Iowa has been at high risk for tar spot development since soon after planting (Fig. 1). Note however, the model should only be used when corn reaches growth stage V10. I expect that as summer temperatures warm up, tar spot risk will decrease as it did in 2025 (Fig. 2) since temperatures above 73°F significantly slow or stop disease development.
An early application of fungicide is not recommended in this situation. More than 10 years of data from across the Midwest show an application of a fungicide at tassel or prior to R3 (milk)) is best for managing tar spot.
Fig 1. Tar spot risk model for Madison County, IA from 15 May 2026 through 30 June 2026.
Fig. 2. Tar spot risk model for Madison County, IA from 15 May 2026 through 15 September 2025
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist