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Tar Spot Disease in Corn Takes to the Midwest

If you noticed what looks like spots of tar on your corn leaves last year, you weren’t alone. Known as tar spot, the fungus has continued to infiltrate the Midwest since it was first identified in the U.S. in 2015. As wet, moderate weather persists this spring, farmers should be on the lookout for this relatively new disease.

Timeline and Progression

Agronomists currently believe that tar spot blew up from Mexico on a storm system and was first identified in a limited capacity in fields of Illinois and Indiana in 2015. Throughout 2016 and 2017, it slowly moved out from that epicenter to the Great Lakes region, into parts of Michigan, southern Wisconsin, Ohio, eastern Iowa, and even Florida.

Agronomists at first couldn’t predict if tar spot would overwinter in the Midwest. But after witnessing the spread over the last few years, it’s clear the disease remains in the soil and residue. Like many fungal diseases that produce spores, tar spot is carried by wind, and can easily spread 100-plus meters from field to field.

In 2018, we saw the most fields infected plus the biggest yield losses to date – around 30 to 40 bushels per acre in some fields.

Tar Spot Characteristics

Tar spot symptoms manifest as small, raised, irregular-shape black lesions scattered across the leaf surface. It spreads from the lowest leaves to the upper leaves, leaf sheaths, and eventually the husks of developing ears.

When you are out in the fields scouting, be aware that these visual symptoms are similar to other pathogens like common and southern rust. An important differentiator is that tar spot lesions may be surrounded by a tan halo and cannot be rubbed off.

Should you find tar spot in your fields, contact your seed provider or agronomist to determine next steps.

Ideal Environments

Nathan Kleczewski, research assistant professor and Extension specialist in field crops plant pathology at the University of Illinois, gives background on tar spot’s ideal conditions: “It really likes the 60°F. to 70°F. temperatures and lots of humidity; at least 85%. What that relative humidity is doing is allowing the spores of the pathogen to generate and colonize the tissues.”

The disease remains a concern even during consistently warm spring and summer days because cool evenings with high relative humidity foster germination. The fungus only needs about seven to eight hours of its ideal conditions to favor infection.

According to Karl Butenhoff, Channel technical agronomist in Iowa, “What we saw this past year and the year before is that it wasn’t just infecting corn on corn acres – those would be the highest risk with having more infected residue if you saw tar spot the previous year – but we saw first-year corn being infected with tar spot as well. There are some situations with corn coming out of alfalfa into soybeans and then corn – where it wasn’t a corn soybean rotation – and still seeing tar spot.”

Because tar spot spreads readily from field to field, it’s wise to talk with your neighbors to learn if they’ve had issues. “If you didn’t have tar spot in your fields, and your neighbors’ fields are in close proximity to yours, your fields could be infected as well if the weather cooperates this year,” he says.

First Line of Defense

We continue to have above-average moisture with saturated, wet field conditions, which are conducive to early-season disease.

Butenhoff advises, “We are still in the infancy of making a really sound fungicide recommendation to farmers but anecdotally, what we’ve seen is that an earlier fungicide application has provided the greatest protection. VTR1 is still going to be the No. 1 recommendation for the best ROI potential for that fungicide protection.” However, timing of fungicide application is difficult.

“We did see some cases of tar spot moving in earlier. What is different from other pathogens we deal with is that its latency period, from what we’ve gathered, is 15 to 30 days. So it might take 15 to 30 days to see visual symptoms. But there could be a pretty severe infection already if you have the moderate, cooler temps, leaf wetness, and high relative humidity,” says Butenhoff.

In theory, practices that bury the residue should help, but they certainly aren’t an option for no-till or minimal-till fields. Even if you manage residue and rotate crops, you are always going to have to deal with your neighbor’s farm, from which tar spot can spread.

And while rotation might be a good option, as Kleczewski admits, “We don’t know how long this organism persists in residue, so we don’t know how many years you’d have to go out of corn; is it one season, two, three, before you lose the potential?”

Managing the stress level on your plants, planting at the right population, and using good, balanced nutrient and fertilizer programs will be key. And if you overhead irrigate, don’t do so at night. Irrigate during the day so that the leaves dry out before you enter the evening hours.

Work to be Done

Many questions remain unanswered, including identifying the specific genetic tolerance, determining how fast tar spot will spread, and variable weather conditions moving forward.

For Kleczewski, two of the biggest factors are when the rains will come and how saturated soil will affect the spread of the disease. “We’ve seen tar spot every year since 2015. Typically, we don’t see tar spot until later in the growing season – after R5 when it doesn’t really impact yield.”

However, 2018 was a different story. “We had a lot of rain early, and the temperatures were conducive. The disease started to take off and build up in June.”

More wet weather later in the season only added fuel to the fire and caused the disease to “just blow up,” Kleczewski says.

Outlook for Farmers

From a grower’s standpoint, it’s not all gloom and doom. While tar spot has been present in the U.S. since 2015, it was only in 2018 that we saw yield impact from the disease. That timing indicates a slow progression.

And there are plenty of practices farmers can implement. Like any fungal disease, the most impactful action is to begin scouting for tar spot in the fields by checking every week, especially if the weather has been conducive to disease development and if you or your neighbors have a history of tar spot in previous seasons.

Kleczewski also recommends understanding your risk level based on the severity of tar spot in your fields the previous year. He says, “Figuring out where you’re at in terms of the level of susceptibility could give you an idea of potential risk. If it’s in your field, you could get that local infection earlier and then the disease could potentially take off earlier and have more impact on yield.”

Tar Spot Isn’t the Only Concern

Kleczewski says, “This is just one disease we’re interested in that could cause yield loss. But if you’re in a severe high-risk area and you see it coming on early before tassel, and you were maybe on the fence about a fungicide application, that might be an indication that maybe you should consider something. In that case, we recommend a premix fungicide with two modes of action. But this is a disease you want to get before it takes off on you and you want to make sure you protect that ear leaf and leaves above as long as possible.”

In the end, don’t be too narrowly focused on tar spot. As Kleczewski warns, “There are going to be other diseases that we encounter more frequently like gray leaf spot and northern corn leaf blight and they shouldn’t be forgetting about those. Those are still at the top of the list for management, and tar spot can really add to it.”

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