Late-Season Tar Spot Scouting Pays

Late-Season Tar Spot Scouting Can Pay Dividends Now and in 2023

Matt Wilde
By  Matthew Wilde , Progressive Farmer Crops Editor
Precision Planting used a Rantizo drone in 2021 to apply fungicides to battle tar spot with good results at the company's Precision Technology Institute research farm in Pontiac, Illinois. (Photo courtesy of Precision Planting)

ANKENY, Iowa (DTN) -- Farmers are urged to scout unharvested cornfields for tar spot, even if combines and silage choppers are about to roll, crop specialists say. It's too late to spray for the foliar disease, but confirming the disease's presence can help prioritize fields for harvest and get a start on 2023 management decisions.

Tar spot is known to reduce yields by as much as 50 bushels per acre (bpa). But the disease can also hurt stalk quality and cause lodging, according to University of Wisconsin-Madison Plant Pathologist Damon Smith.

With grain and silage harvest ongoing in some areas or beginning soon, he said late-season scouting is beneficial to prevent future losses.

"It (tar spot) is all over Wisconsin. I think the biggest thing farmers need to do right now is get in fields and look for it," Smith said. "See what looks decent and what doesn't. See what hybrids are more susceptible to the disease.

"In Wisconsin, we're knee deep in silage harvest. Tar spot really sucks the life out of silage hybrids, so time is of the essence to get those fields with higher severity chopped sooner than later," he continued. "We've also learned fields with a high severity of tar spot like to lodge first. Prioritize your (grain) harvest by the level of severity so you're not dealing with a bunch of corn that's on the ground."


Tar spot was first discovered in the U.S. in 2015. It has quickly spread across the Corn Belt. Keep up where tar spot has been identified at….

The pathogen is by far the nation's biggest yield robber of all leaf and above-ground diseases for corn in 2021, according to figures from the Crop Protection Network. Tar spot reduced production by an estimated 231.3 million bushels (mb). Gray leaf spot came in second in losses at 116.5 mb, followed closely by Southern rust at 114.4 mb. Find the complete list at….

2022 tar spot yield reduction estimates aren't available. But farmers can take steps to curb losses next year.

The pathogen that causes tar spot, Phyllachora maydis, can survive over the winter in corn residue. Extreme cold doesn't kill it, which makes its return in subsequent years not a surprise.

That's why late-season tar spot scouting and testing is so important, said Mandy Bish, a University of Missouri Extension specialist in plant science and technology. She reported the university's Plant Diagnostic Lab recently received corn leaf samples from farmers in Harrison, Carroll and Marion counties in Missouri. The samples were all positive.

Tar spot has been diagnosed in 12 counties since its arrival in Missouri in 2019, though its likely present in much of the northern part of the state, Bish wrote in an article on the University of Missouri's Integrated Pest Management website. Read the latest University of Missouri report on tar spot at….

"If you see (tar sport) this year, then you can anticipate that the pathogen will be present in 2023," Bish said in an email to DTN.


If farmers find the pathogen in 2022 or if it's confirmed in their region, Bish and University of Missouri Extension Specialist Peng Tian advise growers to consider the following in 2023:

-- Check with your seed dealer for appropriate tar-spot-resistant hybrids.

-- Scout fields. A well-timed VT or R1 application when tar spot lesions are present seems to be as effective as an early fungicide application to corn that has low to no incidences of tar spot. An early application (V8 to V10) is more likely to require a second pass if the pathogen is present.

-- Know the conditions that favor tar spot. Leaf wetness is an important factor. Even in dry years, such as the majority of 2022, dew forms on leaf surfaces most mornings due to a weather condition known as a temperature inversion.

-- Consider residue management and crop rotation to reduce the amount of pathogen inoculum available to infect corn.

"If there is a positive to this pathogen, it is that corn seems to be the only host," Bish told DTN. "So, most crop rotation will be helpful as far as keeping the pathogen inoculum low."

Smith said tillage and crop rotation can help fight tar spot, but knowledge, hybrid selection and timely fungicide applications are the best ways to curb potential yield losses. He said a National Predictive Modeling Tool Initiative among land grant universities studying tar spot, among other things, indicates tillage provides a 1% to 2% drop in severity of the disease, while tolerant hybrids reduce severity by as much as 50%. Learn more about the research at….

"We're telling folks not to abandon conservation tillage practices. Spend more energy on the hybrid side," Smith said.

Missouri farmers that suspect tar spot in corn are encouraged to send leaf samples to the University of Missouri Plant Diagnostic Lab. Find out more at…. Other state land grant universities and private labs have testing programs as well.


Precision Planting conducted tar spot fungicide trials in 2021 at its Precision Technology Institute (PTI), the company's research farm in Pontiac, Illinois.

Jason Webster, Precision Planting commercial agronomist and PTI manager, said tar spot hit corn at the research farm hard, making for good fungicide testing conditions. Fungicide application timing and return-on-investment were examined.

In one study, BASF's Veltyma fungicide was sprayed at 7 ounces per acre at two different growth stages. The untreated check yielded 243.5 bpa; VT application only, 260.1 bpa and VT and R3 applications, 269.2 bpa.

It cost $27 per acre for the fungicide and application. The corn price was set at $5 per bushel for corn. The VT application netted an extra $56 per acre compared to the untreated check, and two applications earned an extra $74.50 per acre.

In another study, FMC's Topguard EQ fungicide was sprayed at 5 ounces per acre at three different growth stages. The untreated check yielded 238.2 bpa; V10 application only, 250.1 bpa; V10 and VT application, 257.3 bpa and V10, VT and R3 applications, 270.5 bpa.

It cost $25 per acre for Topguard EQ and application. The corn price was set at $5 per bushel. The V10-only application netted an extra $34.50 per acre compared to the untreated check, the V10 and VT applications netted an extra $70.50 per acre and the three applications (V10, VT and R3) made an extra $111.50 per acre.

"If you scout and find tar spot during the growing season, fungicides are the only good way to control it," Webster said. "Scout again 12-14 days after initial fungicide application, as a sequential application may be needed to continue to control tar spot. We're showing sequential applications of fungicides paid in 2021, making our top 10 list of return-on-investment items in that year."

Find 2021 PTI research results at….

Webster also said research indicates drone application of fungicides to battle tar spot is just as, or more, effective than ground rig applications.

When it was time to spray in 2021, Webster said, it was too wet for a sprayer and airplanes weren't available. "We brought in a drone and that really saved us. We were able to get the fungicide on to knock tar spot down," he said.

DTN has extensively reported on tar spot this year. Here are some previous articles and a video:





Matthew Wilde can be reached at

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Matt Wilde