Tar spot is a fungal disease of corn that produces small, raised black spots on leaves, giving them a splattered-tar appearance. It can cause yield losses up to 50 bushels per acre in susceptible hybrids, making it one of the more economically significant foliar diseases in the Corn Belt. First confirmed in the U.S. in 2015, it has since spread rapidly across Midwest corn-growing states and continues expanding its range.
What Tar Spot Looks Like
The signature symptom is small, raised black spots with dark borders scattered across the leaf surface. These spots are fungal structures called stromata, which contain the spores responsible for spreading the disease. They have a distinctly “tarry” look, almost like drops of black paint dried onto the leaf. As the spots age, some develop a tan or brown halo around them, creating what’s called a “fisheye” appearance.
Tar spot typically shows up first in the lower canopy, then works its way upward as the disease intensifies. Spots can appear on leaves, leaf sheaths, and husks. In severe cases, the leaf tissue between spots dies off, leading to premature leaf death across large portions of the plant.
Telling It Apart From Other Problems
At low levels, tar spot can be tricky to find while scouting, and the lesions are easy to confuse with insect frass (droppings) or rust pustules. Two quick field tests help confirm it. First, try rubbing the spot with your fingers. Tar spot lesions will not rub off, unlike insect frass or some rust spores. Second, flip the leaf over. Tar spot stromata grow through the leaf tissue, so the raised spots are visible on both sides.
How the Disease Develops
The fungus overwinters in corn residue left on the soil surface. When conditions turn warm and wet the following season, spores release from that old residue and land on new corn leaves. Prolonged leaf wetness is the key driver. Extended periods of high humidity, heavy dew, fog, or frequent rainfall create ideal conditions for infection and rapid disease buildup. Fields in low-lying areas or near waterways, where humidity tends to linger, face higher risk.
Once infection starts, the disease can progress remarkably fast. Under favorable wet conditions, tar spot can kill plants in less than two weeks, causing them to dry down well ahead of schedule. This rapid timeline is part of what makes the disease so damaging: by the time symptoms become visually obvious across a field, significant leaf area may already be compromised.
Yield and Silage Impact
The economic toll varies widely depending on hybrid susceptibility, weather patterns, and how early infection takes hold. In severe outbreaks, losses can reach 50 bushels per acre in susceptible hybrids. Illinois estimated its 2024 statewide yield loss from tar spot at roughly 3%, which translated to about $29.75 per acre. That statewide average masks much larger losses in individual fields where conditions were especially favorable for the disease.
For silage growers, the risks go beyond tonnage. Severe tar spot reduces silage moisture, digestibility, and energy content. Plants that die prematurely lose the sugars needed for good fermentation, so even if you harvest on time by the calendar, the quality of what you’re putting in the bunker may be significantly lower. If tar spot is advancing quickly in a silage field, having harvesters ready to go on short notice can at least preserve correct moisture levels, even if digestibility has already taken a hit.
Choosing the Right Hybrids
Hybrid selection is the single most important management decision for tar spot. No commercial hybrid is immune, but there is significant variation in susceptibility. A hybrid with partial resistance slows disease development considerably and may eliminate the need for a fungicide application altogether. Seed companies assign tar spot tolerance or susceptibility ratings to their hybrids, and these ratings are worth reviewing before you buy, especially if your fields have a history of the disease.
When evaluating hybrids, look at the full disease package. A hybrid’s ratings for tar spot, northern leaf blight, and gray leaf spot all matter, particularly if you’re in continuous corn or reduced-tillage systems where multiple foliar pathogens persist in residue. Your seed dealer should be able to walk through a hybrid’s specific risk profile for your area.
Fungicide Timing and Effectiveness
Fungicides with multiple active ingredients provide the most consistent tar spot control. Multi-state trials have shown that products combining two or three different modes of action outperform single-ingredient options. The timing of application matters as much as the product choice.
Research across several states points to applications between tasseling (VT) and the milk stage (R3) as the most effective window for reducing disease severity and protecting yield. Applications at silking through the blister stage (R1 to R2) tend to give the best results overall. In high-risk situations, such as fields with a tar spot history or stretches of persistently wet weather, spraying closer to tasseling may offer the greatest benefit by getting ahead of the disease curve.
Later applications around R3 can still help if the disease arrives late, the hybrid is highly susceptible, and wet weather continues. Keep in mind that most of the more effective multi-ingredient fungicides carry preharvest intervals longer than 20 days, so late-season applications need to be timed with harvest planning in mind.
Cultural Practices That Reduce Risk
Because the fungus survives in corn residue, anything that accelerates residue breakdown reduces the amount of inoculum available the following season. Tillage that buries residue helps, though it comes with its own tradeoffs for soil health and erosion. Crop rotation away from corn for at least one year lowers the local spore load, though spores can also travel from neighboring fields, so rotation alone won’t eliminate the disease in areas where tar spot is well established.
Scouting is essential for making timely fungicide decisions. Start checking the lower canopy before tasseling, particularly in fields that had tar spot the previous year or are planted to susceptible hybrids. Early detection gives you the widest window to respond. By the time symptoms are obvious from the road, you’ve likely already lost some yield potential.