Corn School: Will tar spot spell trouble in 2023?


Tar spot has been spreading across North American corn growing regions ever since the leaf disease was first detected in 2015.

Characterized by tar-like speckling on the upper surface of corn leaves, the fungal pathogen can deliver yield hits ranging from 20 to 60 bushels per acre (in highly infected fields). On this episode of the RealAgriculture Corn School, plant pathologists Darcy Telenko, Purdue University, and Martin Chilvers, Michigan State, share what researchers have learned about the disease and how growers can best manage the yield robber.

Telenko says moisture is the real driver of tar spot and will play a key role in determining what impact it has in 2023. High relative humidity and leaf wetness fuels the pathogen’s ability to infect corn plants. When that moisture is present early in the season, the disease can move through the canopy, infect leaves and cause significant yield loss. Corn yields are less impacted when the earlier part of the growing season is drier, because the disease establishes later in the season. She adds this contrast is very evident when comparing the wet conditions and higher yield losses of 2021 to much lower impact in 2022 when seasonal weather was much drier.

In the interview, recorded last month after the pathologists concluded their presentation at the Ontario Agriculture Conference, Chilvers notes the importance of hybrid selection in reducing tar spot.

“I think we’re in much better place now than we were a few years ago, just because some of those really susceptible hybrids have been removed from the company lineups.” He adds that seed companies are also “more aware now of what material they have with better resistance in it and that information is coming through to growers.”

Another consideration is tar spot’s ability to over-winter and whether crop residue can be a harbinger for the disease to infecting crops in following years.

“We’ve looked at the implications of residue,” says Telenko.  “We do know the pathogen is surviving over winter on the residue where it’s infected. But what we’re seeing is that if we bury this residue, it really isn’t going to influence the epidemic because we’ve got multiple stages of the pathogen. That primary infection may come from the residue, but once we’re in season, with the regional aspect of this, we’ve got spores moving throughout the whole region.

“And once those spores are moving, it really doesn’t matter what field specific agronomic things you put in, the spores are there, so we really have to think about how do we protect the plant and reduce that infection within the plant,” she adds. (Story continues after the video.)

Chilvers notes that fungicides continue to be one of the key tools in the tar spot toolbox to protect the corn crop.. When it comes to timing fungicide application, he says “somewhere between VT and R3 tends to win hands down.”

The pathologists note that some fungicides perform better than others, but an efficacious product can deliver a strong return on investment. “We know we can get some return on low disease pressure, it’s two to three bushels,” says Telenko. “But under a high disease pressure, where we know tar spot may take 20 to 60 bushels per acre, we do see that return on investment.  When taking out the cost of the fungicides, we saw that we could get a $30 return and nine bushels or more when we have tar spot epidemics that are severe.”

Also in the video, Chilvers and Telenko discuss in-season tar spot detection and monitoring tools, as well as information sources to help manage the disease.

Click here for more Corn School videos.

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