Halfway through 2025, volatility is still the name of the game. The Midwest got a cooler, wet spring and early summer — a perfect playground for the fungal pathogen that causes tar spot, Phyllachora maydis.

Corn farmers and consultants are no strangers to fungal threats, but tar spot causes special angst and anxiety, says Darcy Telenko, associate professor of botany and plant pathology at Purdue University. 

Tar spot can cause a farmer to lose between 50 to 100 bushels per acre, she emphasizes. Plus, the disease is far more widespread than most think. In Indiana, it’s been confirmed in all 92 counties.

“With this disease, I would say the environment is really key on when it gets started in the season and how bad it will be by the end of the season. A cool, wet spring could lead to more disease starting earlier. Once it’s going, it doesn’t matter how hot it gets — we’ll still see tar spot ramp up.”

The fungal pathogen reduces photosynthetic capacity, which lessens grain fill and kernel weight. In severe cases, tar spot can deplete stalk tissue, causing stress during grain fill and increased risk for lodging — and increased headaches come harvest time. 

The disease affects silage feed quality by drying plants abnormally fast, so packing a bunker and starting fermentation becomes an issue. The produced silage also has less feed energy and digestible components.

First discovered in the United States only 10 years ago, tar spot has been reported in the majority of the Corn Belt and beyond. Its relative newness means resistant corn hybrids are not available, and while it’s impossible to fully prevent the pathogen from causing damage once it takes hold, she says fungicide application is the best route for yield protection. 

Tar spot is generally present 14-20 days¹ before signs are visible (see sidebar for tips on recognizing the disease), so prevention is paramount. According to the Crop Protection Network, two major factors affect fungicide efficacy: fungicide active ingredients and application timing.

Telenko explains products with multiple fungicide classes are preferred because they tend to exhibit higher efficacy. They attack the fungal pathogens differently and delay fungicide resistance development. Recommended fungicide classes include QoI, DMI and SDHI, and more than one mode of action is recommended. A product like Veltyma^® fungicide from BASF mixes two of these recommended classes — Qol and DMI.

Fungicide efficacy is also largely affected by fungicide timing.

“Prioritizing early fungicide applications is extremely important because it can mitigate the risk of harvesting early to get ahead of the shut-down process and protect your yield,” says Telenko. 

Timing is also critical because even a good product can only do so much if applied at the wrong time.

For yield protection and disease mitigation, she recommends applying fungicide between the VT and R1 growth stages (tassel emergence and silking) to R3 (milk). Effectiveness drastically reduces if applied at or after the R4 growth stage (dough). In bad years, she admits a second application can be necessary to protect against the disease before the crop reaches black layer. This is especially important if a late vegetative fungicide is applied, verses waiting until VT or later.

“I've been in fields where they sprayed too late. There's a point where if you have 5% or more tar spot on the ear leaf and above, you’ve missed the window. You're probably just wasting products at that point because you're not going to slow it down. I've seen decent control spraying when there’s 1% of the ear leaf,” Telenko notes. “But if I'm finding 1% severity of tar spot on the ear leaf, it's on every plant in that field.”

She recommends using the many resources developed by land-grant universities at the Crop Protection Network to help make timing decisions. Among these resources is the fungicide efficacy tool, which objectively ranks available products. For instance, Veltyma fungicide is one of only two fungicides with a “Very Good” ranking. There are also maps that include historical disease data, in-season maps for scouting and forecasting tools. Another new tool is a return on investment calculator.

Proactive fungicide application is key to protecting both yield potential and disease management. This year’s neutral weather patterns are opening the door for tar spot, but farmers can put the ball back in their court.

Veltyma fungicide offers proven advantages that optimize yield potential. Regardless if you’re battling yield-limiting diseases like tar spot or other environmental stresses, Veltyma fungicide has been shown to provide a 7.0 bushel-per-acre advantage over other corn fungicides on average.²  

For help with your fungicide application strategy, contact your BASF representative. See how Veltyma fungicide can protect your fields at Veltyma.com