Get Ready to Tackle Tar Spot

Tar spot showed up in early June in corn fields in several Midwest states in 2024. Does that mean the disease will be problematic this year?

Not necessarily. But it does increase the need to look for the presence of the disease, said Darcy Telenko, Purdue University plant pathologist.

"Each year will be different depending on environmental conditions from the end of May through June. I would encourage growers to start scouting at the V8 growth stage by looking for signs of the disease in the lower canopy," she recommended.

The key environmental factors favoring tar spot are moderate temperatures, high humidity and prolonged leaf wetness, particularly in areas with a history of the disease and a presence of corn residue.

Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces. Give it the fingernail test -- tar spot will not scratch off the leaf.

Last year, the disease was found in every county in Indiana and was well-established in the upper Midwest. "Again, environmental conditions will dictate when and where it is first found this year and how fast it moves in the canopy," Telenko said.

Prioritize scouting by knowing what fields contain the most susceptible hybrids. Currently, no hybrids are available that are completely resistant to tar spot. However, if you selected hybrids with some moderate resistance, you'll have a better chance of reducing disease in the canopy.

Fungicides do help control the disease, but you need to catch the disease before severity ramps up. The optimum application window based on Purdue research is between VT and R3 growth stages (tassel to milk), but timing is critical.

"The fungicide target is to protect the ear leaf and above for yield protection," she said. "Therefore, we need those upper leaves exposed to be protected by fungicide."

Farmers are always looking to reduce input costs, but this year current market scenarios have many sharpening the pencil further. Even more reason to scout carefully, Telenko said.

"If you trigger an early application, most likely you'll have to come back to make sure coverage makes it to black layer," she added.

In a University of Missouri news release, plant pathologist Mandy Bish was also urging farmers in her state to scout carefully. According to the Crop Protection Network, most fungicides only effectively manage disease for about 14 to 21 days after application. Applications earlier than VT and after R4 have not demonstrated a good return on investment, she said.

There are many good resources to help make decisions. Find a map of where tar spot is being detected and a handy table to determine when to spray from the Crop Protection Network: https://cropprotectionnetwork.org/….

Find a new tool to help sort the efficacy of fungicides for the disease here: https://cropprotectionnetwork.org/….

The Crop Protection Network also has a new forecasting tool called the "Crop Risk Tool," which replaces the TarSpotter app. It's available here:

https://cropprotectionnetwork.org/….

Telenko's lab has in-season updates on tar spot: https://indianafieldcroppathology.com/….

In Indiana, tar spot samples submitted to the Purdue Plant Diagnostic Lab are covered with support from checkoff funding from the Indiana Corn Marketing Council. Find more information here: https://ag.purdue.edu/….

Find the University of Missouri release about tar spot here: https://extension.missouri.edu/….

Pamela Smith can be reached at pamela.smith@dtn.com

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