The progression of tar spot seems to have slowed in recent years due to overall drier conditions, but the corn disease remains a problem for farmers, and good rains this spring or summer could continue its spread.
Tar spot has been found across the Midwest, with Iowa and Illinois reporting it in nearly every county and Missouri starting to find it in its northernmost counties in 2022.
“I’ve seen it from western Iowa clear to the Missouri River,” said Tyler Steinkamp, crop protection product manager with WinField United. “It’s here, it’s just how bad is it?”
Daren Mueller, plant pathologist with Iowa State University Extension, said a quick turn to wet conditions would be tough for farmers as it would promote the spread of tar spot, but that could be a boon for those researching the disease.
“I keep waiting for a repeat of that 2018 year where it was inordinately wet most of the season,” Monin said. “And when we get that, with the increase in inoculum, it will be a good year for plant pathologists.”
With the spread of tar spot and the threat it poses to corn yields, many are looking for the optimal way to handle the disease. Agronomists say more research is needed for more in-depth protection, but current products have shown some effectiveness.
“Some of these newer three-way, longer-lasting fungicides will stay on them,” Steinkamp said. “The biggest thing we’ve learned from the past is that you have to really plan on making an application before it shows up.”
Steinkamp said tar spot often only shows on the plant leaf a week or two after the plant has been affected. When scouting, if there is a dark spot on the leaf and it doesn’t come off easily, it is likely a sign of tar spot.
“It takes a while to develop,” he said. “You are looking for any kind of black lesion. It does seem like heavier, wetter ground tends to get it quicker. There is speculation that it could be tied in with crown rot, but that’s more observational than anything. If it is a part of that plant, that means you should have sprayed a little while ago.”
The trouble with tar spot has led some farmers to make a second fungicide pass. Steinkamp said that advances in fungicides should limit the need for additional passes, but those may persist while agronomists figure out the best course of action.
“Right now it might be the norm in high-pressure areas, like the northeast corner of Iowa and down along the Mississippi or Missouri River,” he said. “That is likely to be for the next couple of years, but once we have breeders work with it more, we’ll see more resistance and shouldn’t need multiple applications.”
Mandy Bish, plant scientist with University of Missouri Extension, wrote the best time to get the single pass of fungicide out can be at VT/R1 stage, which should reduce the severity of any early pathogens. She echoed the idea of two-pass systems showing positive results if the need arises, but late-arriving disease presents a different scenario.
“When disease shows up later than the R3 growth stage, such as in 2022 (in Missouri), fungicide applications didn’t influence yield,” Bish said.
As it continues to emerge in Missouri, Bish said reporting any signs of tar spot will be helpful for agronomists and pathologists as they continue to learn about the disease.