Tar spot is a fungal disease of corn caused by the pathogen Phyllachora maydis. It produces small, raised black spots on leaves that look like flecks of tar, and in severe cases it can kill large sections of leaf tissue, causing yield losses of up to 50 bushels per acre in susceptible hybrids. First confirmed in the United States in 2015, it has since spread across the Corn Belt and become one of the most closely watched corn diseases in North America.
What Tar Spot Looks Like
The earliest symptoms are small yellow to tan flecks on the leaves, husks, or stalks. Within about 17 days of infection, the signature black spots appear. These spots, called stromata, are the fungal structures themselves, not just discoloration. They’re raised, round to slightly elongated, often shiny, and embedded directly in the leaf tissue. They extend through both the top and bottom surfaces of the leaf and are scattered across infected areas.
One quick field test: tar spot stromata cannot be rubbed off with your finger. This distinguishes them from insect frass, soil splatter, or rust pustules, all of which can be wiped or scraped away. Under high humidity, the stromata may release spores in small white or orange masses visible to the naked eye.
When infection is severe, individual spots merge together, killing large patches of leaf. This premature tissue death forces the plant to dry down early, cutting short the grain-filling period and reducing yield.
How It Differs From Similar-Looking Diseases
Several other corn problems can be confused with tar spot at first glance.
- Common and southern rust: Rust lesions also appear as raised bumps on leaves, but they erupt through the surface, causing small tears in the leaf skin. You can rub rust spores off with a finger. Tar spot stromata stay firmly in place.
- Physoderma brown spot: These spots are flat and embedded in the tissue rather than raised. They also tend to cluster near the base of the leaf, while tar spot stromata typically show up from the middle of the leaf blade toward the tip.
- Insect frass and soil: Both rub off easily. If the black spot stays put when you run your thumb across it, it’s likely tar spot.
What Causes It and How It Spreads
Phyllachora maydis is an obligate pathogen, meaning it can only grow and reproduce on living corn tissue. It cannot be cultured in a lab on artificial media, which has made it unusually difficult for researchers to study. Genomic analysis has revealed that the fungus lacks the ability to use certain basic nitrogen sources on its own, reinforcing its complete dependence on its host plant for survival.
The fungus overwinters inside the tough stromata embedded in leftover corn residue on the soil surface. When spring arrives, these structures release spores that land on new corn leaves and start the cycle again. Wind and rain splash carry spores within and between fields, so infection can occur even in fields with no corn residue present.
Weather Conditions That Favor Tar Spot
Tar spot thrives in cool, humid conditions. The optimal range is monthly average temperatures between 17 and 22°C (roughly 63 to 72°F), relative humidity above 75%, at least 7 hours of leaf wetness per night, and frequent foggy periods. Research has found that extended windows of moderate temperatures, averaged over about 30 days in the 18 to 23°C range, are the strongest predictors of tar spot development.
Shorter bursts of moisture over 14- to 21-day windows also play a role, though the relationship is complex. In practice, a stretch of cool, dewy nights through mid-summer into early fall creates the conditions most likely to trigger significant outbreaks. Hot, dry summers tend to keep tar spot in check.
Yield Loss and Economic Impact
The financial damage from tar spot varies widely depending on hybrid susceptibility, weather, and timing of infection. In the worst cases, yield losses reach 50 bushels per acre. Across Illinois in 2024, the statewide average loss was estimated at about 3%, translating to roughly $29.75 per acre. That may sound modest, but spread across millions of corn acres, the cumulative cost is substantial.
University trials have documented tar spot severity ranging from 2.5% to 44% across different hybrids planted in the same location, illustrating how much variety choice matters. A highly susceptible hybrid in a bad tar spot year can suffer dramatically more damage than a tolerant one growing in the same field conditions.
Where Tar Spot Has Been Confirmed
Tar spot has been confirmed across a growing number of states throughout the Corn Belt. It is well established in Illinois, Indiana, Iowa, Michigan, Wisconsin, Ohio, and surrounding states. It continues to push into new territory. In South Dakota, for example, confirmed counties for 2025 include Brookings, Deuel, Grant, Minnehaha, Lincoln, Union, Clay, Yankton, and Bon Homme, with suspected cases in additional counties. The disease’s range expands each year as spores move into new areas.
Managing Tar Spot
Hybrid Selection
Choosing a corn hybrid with better tar spot tolerance is the single most impactful decision you can make. In Latin America, where the disease has been present for decades, hybrid resistance is a cornerstone of management. In the U.S., seed companies now rate their hybrids for tar spot response, and those ratings are worth paying attention to. The difference between a tolerant and a susceptible hybrid can be enormous, potentially tens of bushels per acre in a bad year.
Tillage and Crop Rotation
Because the fungus overwinters in corn residue, burying or breaking down that residue reduces the amount of inoculum available the following spring. Tillage speeds decomposition and lowers disease risk, particularly when combined with rotation to a non-corn crop like soybeans. No-till, continuous-corn fields carry the greatest risk.
That said, tillage alone will not prevent tar spot. Spores travel on wind, so even clean fields planted after soybeans can become infected from neighboring areas. Think of residue management as lowering the starting inoculum load rather than eliminating the threat entirely. In a no-till field with a susceptible hybrid, it takes fewer hours of favorable weather for the disease to reach damaging levels compared to a tilled, rotated field.
Fungicide Timing
When tar spot pressure is high, fungicide applications can protect yield, but timing is critical. Research consistently shows that applications during early reproductive growth stages (around R1, or silking) are the most effective at reducing tar spot severity. Spraying too early during vegetative growth, such as at V12, often fails to provide lasting protection because the fungus hasn’t arrived yet or re-infection occurs after the product wears off.
Applications at R1 through R3 have shown the greatest yield benefits in trials, with gains of 9 to 10 bushels per acre over unsprayed controls when disease pressure exceeded 5% severity on the ear leaf. The decision to spray depends on your hybrid’s susceptibility rating, local tar spot history, and whether weather conditions are trending toward the cool, humid pattern the fungus prefers. In low-pressure years or with tolerant hybrids, the spray may not pay for itself.