Be Proactive When Fighting Tar Spot

Opinions Vary on Best Time to Spray for Tar Spot

Matt Wilde
By  Matthew Wilde , Progressive Farmer Crops Editor
Fungicide is applied to corn in August 2021 near Monona, Iowa. Tar spot, a foliar disease, is spreading across the Corn Belt. (DTN photo by Matthew Wilde)

ANKENY, Iowa (DTN) -- Farmers need to be proactive when planning and spraying corn to mitigate potential yield loss from tar spot, university plant pathologists and ag chemical retailers agree. But spraying too early could mean more than one application may be needed.

Damon Smith, a University of Wisconsin-Madison plant pathologist, said farmers are understandably nervous about tar spot this year since the fungal disease -- known to reduce corn yields by more than 50 bushels per acre (bpa) -- was confirmed in Wisconsin on July 6, which is earlier than ever. It also was identified in the V8 growth stage of corn, which is earlier than normal.

"There's a lot of anxiety out there. With diseases, the earlier they start, the more impact they can have on yield," Smith said. "We want to be proactive scouting for tar spot and lining up applicators (if needed). We also want to make sure we're protecting the important parts of the plant because it's the upper half or upper third of the plant (ear leaf and leaves above) that will do the most yield work.

"If farmers spray now, I'm OK with it," he continued. "But they're setting themselves up probably for a two-pass program. Farmers need to sit down and do the math to make sure that fits with their operation."

Smith said an aerial fungicide application typically costs $25 to $35 per acre. In corn tar spot research plots last year, he said a second treatment didn't generally provide an economic benefit compared to one timely application.

Smith and Alison Robertson, an Iowa State University Extension plant pathologist, suggest farmers wait to spray until tar spot is found. They also recommend not to spray until corn is the VT to R3 growth stages (earlier that time frame is better, if possible) to provide the best chance to combat the disease with only one pass.

"That's what seven years of research tells us," Robertson said. "Some people think you should spray as early as V8, but we want to protect the ear leaf and development later in the season. Since fungicides only last for three to five weeks, if you spray too early, the first application may not be as effective, and you may have to spray again."

Keep up on where tar spot has been identified at….


While tar spot lesions show the severity of the plant's infection, yield loss happens long before that visual damage, according to Kimberley Tutor, technical marketing manager - plant health, for BASF. The company, which produces a range of fungicides for use against diseases such as tar spot, conducted a webinar recently for selected ag reporters to discuss this latency issue.

BASF estimates more than half of U.S. farmers will make at least one fungicide application.

Tutor presented a collection of electron microscopy images, taken by BASF, that showed how disease begins affecting plant growth within hours of infection. It can take 14 to 20 days following infection before tar spot lesions begin to show up, she said. In the meantime, the disease is producing toxins and reducing the plant's photosynthetic capability. Both can potentially cost yield.

BASF promotes applying fungicides early within the label recommendation period, and using more than one application if needed, to protect plants. A wait-and-see approach could be costly, according to the company, even though the reality of rising input costs is already taking a bite out of high grain prices for farmers.

"There are more farmers that are starting to adopt fungicide applications," Tutor told DTN in an email after the webinar. "However, a lot of those farmers aren't traditional/historical users of fungicides. Therefore, it's critical to educate them as to why they need to be proactive with their applications and not reactive.

Smith said he has not seen peer-reviewed research to support the proactive approach.

"I think it was common in years past that growers, who were looking to save a few bucks toward the end of the season, would consider cutting their fungicide application as it's often the last trip across a field," Tutor stated. "However, I think folks are starting to understand the profitability of fungicide applications, the benefits of BASF Plant Health products in addition to disease control. Generally speaking, fungicide applications are becoming more of a mainstream practice."


There are several fungicides with 2ee labels that can be used to manage tar spot, according to the Crop Protection Network (CPN).

Check out CPN's fungicide efficacy chart at….

Smith said recent research at the University of Wisconsin-Madison and other land-grant institutions show spraying fungicides with two or three modes of action works best against tar spot, though one mode of action still provides some efficacy compared to not spraying at all.

Fungicides primarily delay the fungus from damaging corn plants to allow ears to fill and kernels to add weight. "Some products keep the disease in the lag phase longer, and that's where we (preserve) yield," Smith said.


Here's several tar spot articles DTN recently published:




Greg Horstmeier contributed to this article

Matthew Wilde can be reached at

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Matt Wilde