Tar Spot’s Silent Strike: Why Waiting to Act Could Cost You 60 Bushels an Acre
July 23, 2025 First identified in 2015 in a field in northwestern Indiana, tar spot (Phyllachora maydis) can reduce corn yield by as much as 60 bushels per acre, according to Purdue University. In the decade the fungus has spread to Missouri, Iowa, Kansas, Nebraska and other Midwest states.
For a decade, growers around the country have been battling tar spot, a relatively new fungus. We interviewed Kim Tutor, Technical Marketing Manager for Row Crop Fungicides, BASF, to learn how growers and retailers can work together to battle this yield-reducing fungus.
CropLife: When did tar spot first start posing a problem for corn growers?
Kim Tutor: Tar spot was first identified in the US in 2015, and we didn’t really see it take hold or cause a major problem initially (until) 2018. 2021 was the last really significant tar spot year in terms of overall crop loss, and level of infection. It was identified in Kansas in May of 2024, but it didn’t necessarily take off in terms of disease severity, like everybody thought that it would. That was primarily driven by environmental conditions (i.e., the weather turned hotter and drier). So, the environment wasn’t necessarily the best for continual disease proliferation and high levels of disease severity.
CL: What should growers or retailers look for when they’re surveying their fields?
KT: Tar spot lesions look like tiny little black flecks, similar to tar, which is how the disease got its name. Tar spot can also sometimes be confused with insect frass, but the insect frass will scrape off with your fingernail. Tar spot will not.
Tar spot is an ascomycete, which is sometimes referred to as a “spore shooter.” As part of the reproductive cycle, tar spot’s reproductive structures burst releasing ascospores upwards and outwards. Like a lot of fungal pathogens, tar spot over winters in the corn stubble. So, we’ll start to see lesions from the bottom of the plant and working upwards. But tar spot is a little different in that when those spores get released upwards and outwards into the air, they can get caught by a wind current. That’s how they can come in and affect other fields, and that’s why this disease can also show signs of infection from the top of the plant moving downward.
It’s a two-way infection in terms of the plant. That’s part of the reason why it continues to be such a troublesome disease to manage.
CL: What are growers seeing in 2025?
KT: Similar to 2024, tar spot was identified early in Kansas (i.e., June). We continue to see the spread of the disease across the Midwest. It has been confirmed in in Kansas, Missouri, Nebraska, Iowa, Illinois, Indiana, Wisconsin, Michigan, Pennsylvania, and New York. It is continuing to spread outward in somewhat of adradial pattern and has also been confirmed in parts of Ontario.
In order for this disease to spread, it comes back to a very basic agronomic concept: The disease triangle. For a disease to take hold, you have to have three ingredients. You have to have the pathogen, which we know we have, because it overwinters in corn stubble. You have to have the host plant, which is corn. We have lots of corn planted across the Midwest this year, and then you have to have the right environmental conditions. So, it’s pretty safe to say that we have two out of three of those parameters that have been met. So, it’s just a matter of the environment and with nighttime temperatures in mid 60s-70s and ample chances of rainfall in the forecast, odds are high that we will continue to see an increase in both disease incidence and severity. And not just with tar spot, there have also been confirmations of other troublesome corn diseases across the Midwest – Northern corn leaf blight, grey leaf spot, and Southern rust. Folks are seeing more than one of these diseases in their fields, which indicates that the corn crop is at risk.
CL: What makes tar spot so challenging?
KT: Tar spot has a really long, latent period of 14 to 20 days. So that’s upwards of two to three weeks (in which the) disease has infected that plant. It’s inside the plant, robbing it of water, of nutrients, and reducing its overall photosynthetic ability before you can even detect a lesion with the naked eye.
Because of the really long latent period, by the time you see those lesions the disease has been present for quite some time, reducing your yield potential. And that’s why it really pays to be proactive instead of reactive.
CL: What is the outlook for tar spot right now?
KT: Based on what we are seeing and hearing across the Midwest, coupled with the weather forecast, the situation is ripe for significant damage from tar spot and other corn diseases. While in Iowa recently, we found several diseases present within a single field (i.e., tar spot, Northern corn leaf blight, and grey leaf spot). We know other folks are seeing various complexes of corn diseases as well. This is concerning because we know plants have a certain amount of resources. They can put those resources towards growth, development, and photosynthesis, which drives yield, or they can spend those resources, that energy, on fighting off stress, whether those stresses are diseases like tar spot, gray leaf spot, northern corn leaf blight, or environmental stress, things like heat and drought.
CL: Can you discuss treatment for tar spot?
KT: Veltyma Fungicide (can be used) to combat these troublesome diseases. It is broad spectrum in nature and has really long residual disease control. But it’s also important to note that if offers additional benefits as well. We never know what the stress is going to be, but our crops are always battling some kind of stress. Veltyma fungicide helps mitigate that stress, so that your corn crop can continue to focus on photosynthesis and drive yield. We are seeing incredible corn yield potential across the Midwest; the crop looks phenomenal, and if your yield potential is 200-250-plus bushels an acre, every bushel counts. In arming that plant with a product like Veltyma fungicide, you are hedging your bets as best you can in order to protect that yield potential.
CL: What are the expectations post application?
KT: It’s important to understand that even if you apply a fungicide to your corn crop, you may still see tar spot lesions. For example, let’s say a retailer and a grower are walking one of the grower’s corn fields. They find a couple of tar spot lesions. They pull the trigger and make a fungicide application the next day. Then they go back out in that same field a couple weeks later, and they’re still seeing some tar spot lesions. The grower is frustrated saying, “Well, you told me if I applied fungicide, I was going to stop the infection.” In actuality, you did stop all those new tar spot infections. But tar spot is polycyclic, meaning it goes through multiple cycles of infection and reproduction within a single season. It’s also important to know that fungicides with curative activity only have 72 hours to stop an infection. If there are tar spot infections in the field that are more than 72 hours old, you can’t stop those infections, but you can stop new ones from occurring. That’s why growers may still see tar spot lesions, even after a fungicide application.
CL: Any last advice?
KT: Having a fungicide as part of an agronomic plan every year is a sound decision. We know we only have a certain number of airplanes across the Midwest, so the sooner you book the fungicide, it’s much easier to get across all of your corn acres in a timely manner.
Any fungicide that’s worth its weight will control disease. But a performance-driven fungicide like Veltyma offers additional benefits. Not only does Veltyma fungicide combat disease with broad spectrum, longer-lasting disease control, but it also helps optimize the growth efficiency of the plant by supporting nitrogen uptake and root growth. It also helps mitigate environmental stressors like heat and drought and keeps stalks healthy enabling better harvestability. If you would like to experience Veltyma fungicide yourself, BASF has launched the 2025 Real Results Yield Challenge. Set up a side-by-side comparison of Veltyma fungicide versus untreated, or versus another fungicide available on the market today. By submitting some photos of the fungicide performance and sharing your yield results you can be entered into sweepstakes for a chance to win one of three F250 two-year truck leases. To learn more, visit yieldchallenge.basf.us, or reach out to your local retailer or BASF representative.
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