Disease Guide
University-rated fungicide efficacy, pricing per acre, and spray timing for tar spot (Phyllachora maydis). Based on Crop Protection Network 2025 data.
Tar spot is a foliar disease of corn caused by the fungus Phyllachora maydis. First confirmed in the United States in 2015 in Indiana and Illinois, it has since spread across the Corn Belt and become one of the most economically significant corn diseases in the region. The pathogen produces raised, black stromata on leaf surfaces that resemble spots of tar — giving the disease its name.
Tar spot thrives in humid conditions with cool nighttime temperatures (60-70°F) and extended periods of leaf wetness. Fields near wooded areas, waterways, or low-lying terrain with poor air circulation are at higher risk. The fungus overwinters on infected corn residue, making continuous corn and reduced-tillage fields particularly vulnerable.
Yield losses from tar spot can be severe — 20 to 60+ bu/acre in epidemic years. The disease reduces photosynthetic area during grain fill, leading to premature leaf death, lightweight grain, and significant yield penalties. Since its arrival, tar spot has driven more fungicide applications in the upper Corn Belt than any other single disease.
Efficacy ratings from the Crop Protection Network 2025, based on multi-year university field trial data. Only products with tested ratings are shown — untested or not-listed products are excluded.
| Product | Mode of Action | Tar Spot Rating |
|---|
Source: Crop Protection Network 2025. Ratings reflect multi-year, multi-location field trial performance.
Efficacy ratings cross-referenced with real purchase prices. Products sorted by efficacy rating — best performers first.
| Product | Tar Spot Rating | Best $/Acre |
|---|
Prices as of early 2025 from AgChem, FBN, and Farmerceag. Actual pricing varies by geography, volume, and timing.
Preventive applications at VT-R1 are critical. Tar spot is difficult to control once it is established. Unlike some foliar diseases where you can wait to see symptoms before spraying, tar spot management relies on getting a fungicide on the plant before infection takes hold. In fields with a history of tar spot, a VT-R1 application is strongly recommended regardless of whether symptoms are visible at application time.
Scout starting at V10. Early scouting helps gauge the season's risk. Look for the small, raised black spots on lower leaves first — they move upward through the canopy. If you find tar spot before VT, you are likely in a high-pressure year.
Late applications can still help in severe epidemics. Research from Purdue University (Ross et al. 2024) demonstrated that even R2 fungicide applications provided meaningful yield protection when tar spot pressure was extreme. If you missed the VT-R1 window and tar spot is exploding, a rescue application at R2-R3 with a premium product may still be worthwhile.
Use premium, multi-MOA products. Single-mode strobilurins (FRAC 11 only) have limited tar spot efficacy. For this disease specifically, invest in products rated Very Good or better — the yield protection under high pressure more than justifies the cost difference.
Efficacy ratings for all 26 products across 7 diseases, pricing from 3 suppliers, decision frameworks, and an ROI calculator.
View full guide →Search and filter 9,196 yield records from 867 university field trials by year, state, and crop.
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View guide →Based on Crop Protection Network 2025 data, the top-rated fungicides for tar spot include Topguard EQ, Veltyma, Revytek, and Delaro Complete — all rated Very Good. Products like Miravis Neo, Trivapro, and Lucento rate Good to Very Good. Single-mode strobilurin products (FRAC 11 only) generally have limited tar spot efficacy.
Tar spot appears as small (1-5mm), raised, black stromata on corn leaves that look like spots of tar. Unlike other leaf diseases, the spots feel rough and raised when you run your finger across the leaf. In severe infections, spots coalesce and can cover most of the leaf surface, sometimes with tan halos called "fisheye" lesions that indicate more aggressive tissue damage.
The optimal spray timing for tar spot is VT (tasseling) to R1 (silking), applied preventively before symptoms appear. In fields with known tar spot history, a VT-R1 application of a premium fungicide is strongly recommended. Scout starting at V10 for early signs. In severe epidemic years, a second application at R2-R3 may be justified — Indiana research showed even late applications provided yield protection under extreme pressure.
Tar spot yield losses range from 20 to 60+ bu/acre in severe epidemics. Since its U.S. discovery in 2015, tar spot has caused some of the largest single-disease yield losses in Corn Belt history. Fields with early infection (before R2) and susceptible hybrids are at highest risk. Even moderate infections can reduce yield by 10-20 bu/acre through premature leaf death during grain fill.
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