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Tar spot targets Midwest corn

THERE IS A NEW FUNGUS AMONG US, and it’s been spotted in northern Missouri cornfields.

This emerging fungal disease is tar spot, which attacks leaf tissue in corn and causes the plant to rapidly deteriorate. Tar spot limits water and nutrient movement, resulting in reduced photosynthesis that can affect yield and grain quality.

“Tar spot has the potential to be a very aggressive disease,” said Kevin Moore, MFA senior staff agronomist. “However, there are several factors that affect the severity of the disease—environmental conditions, stage of the crop, hybrid tolerance and crop health to name a few.”

Although tar spot has been considered a major foliar disease in several Latin American countries for more than a century, according to the American Phytopathological Society, it was not a concern in the United States until 2015 when it was discovered in northern parts of Indiana and Illinois. For the first couple of years, the pathogen was scattered and resulted in minor yield loss. However, by 2018, tar spot had spread quickly around the Midwest.

The disease is now found throughout Indiana, Illinois, Iowa, Michigan and Wisconsin, and it has moved further into MFA territory. Tar spot has been reported in cornfields in 22 Missouri counties. According to the Crop Protection Network, it’s estimated that tar spot caused U.S. farmers to lose around $3 billion from 2018 to 2021 and has the poten­tial to be more destructive in states that are just now seeing cases.

Because this disease is relatively new to the U.S. corn industry, it is a hot topic for dis­cussion and research. There is very little information on the biology of the pathogen that causes tar spot as well as the epidemiology and management of this disease. It is now top of mind with agronomists such as Darcy Telenko, assistant professor, field crop patholo­gist and extension specialist at Purdue University, who is working with researchers across the country to unlock the mystery of tar spot.

“The problem with this pathogen is that it requires living host tissue to reproduce, so I can’t grow it in a lab in a culture and try to learn a little bit more about the biology,” Telenko said during a presentation on tar spot at the 2023 Commodity Classic in Orlan­do, Fla. “That’s why there’s very limited literature on it. We’ve shown that it does over­winter in residue, so we can isolate those spores and get them to germinate, but we’re still working on those tools.”

Telenko’s work is based in the region considered a “hot pocket” for tar spot, which has been steadily pushing outward from the area around Lake Michi­gan. She said she came on board at Purdue in 2018 when the disease reached epidemic proportions. The speed at which tar spot can overtake a field is one of reasons for concern, she said.

“The thing that shocked a lot of our northern Indiana growers is that they can drive by the field, not see tar spot, come back two weeks later, and it’s covered,” she said. “If the inoculum is on your farm, and you have the right environmental conditions, it can lead to significant yield losses.”

Telenko said that Purdue has documented losses of 20 to 60 bushels per acre, but the severe outbreak in 2018 cut yields of some infected fields in half. So far, MFA agronomists said the impact of tar spot in this region has not been so drastic.

“In Missouri, we have observed late-season infection that has had little measurable yield loss,” said Scott Wilburn, MFA se­nior staff agronomist. “Many of the infected fields have had oth­er diseases, such as gray leaf spot, northern corn leaf blight and southern rust, that were more concerning than the tar spot.”

Moore agrees. “The late tar spot has not caused yield losses because the corn has been far enough along in its development to where yields are nearly determined. However, earlier infec­tions are possible, and we are on the lookout for that in 2023.”

IDENTIFY

Tar spot symptoms include irregularly shaped black spots called stromata that are found on healthy or dead tissue of leaf sheaths, stalks and husks. A narrow tan halo, known as a fish-eye lesion, often surrounds the stromata. Raised and bumpy tar spots vary in shape from small pinhead structures to more elongated structures.

Tar spots on leaves and corn husks will appear on the upper and lower surfaces and might look like dirt. When scouting corn, if black spots can’t be scraped off or broken open, the pathogen is present. Lookalike spots will generally scratch off easily, Moore explained.

Unlike the pustules of southern corn rust or common rust, the stromata of tar spot do not break through the leaf’s surface, Telenko said.

“It is easy to confuse stromata with structures associated with other fungal diseases, such as the black pustules that corn rust pathogens produce,” she said. “A laboratory diagnosis is required to distinguish tar spot from rust pustules or other pathogens. Tar spot can also be easily confused with the black saprophytic organisms that grow on dead leaf tissue.”

The fungus can overwinter in infected residue and reproduce as temperatures begin to rise. Wind and heavy rain disperse the spores that can infect new corn plants. Once infected, a corn plant will show symptoms in 14 to 21 days.

“Weather is going to dictate this pathogen,” Telenko said. “Having 85% relative humidity with more than seven hours of leaf wetness appear to promote infection and disease develop­ment. The severity of the disease seems to hinge on moisture and the amount of pathogen in the field.”

Compounding the problem, the pathogen can infect plants multiple times in a single season. After the initial infections, the stromata produce more spores, resulting in secondary infections that appear as a mix of larger spots and many smaller ones.

“The problem is, once we’re into those secondary cycles, the spores are everywhere,” Telenko said. “That limits our man­agement tools because those spores are moving. That’s another reason why it’s important to be out there monitoring early on.”

MANAGE

Keeping the plants healthy, being aware of weather conditions that promote the spread of the fungus and scouting fields are all ways to help manage tar spot, Moore said. As with any plant disease, the existence and significance of tar spot are impacted by three factors—the host, the pathogen and the environ­ment—known as the disease triangle.

“Know what’s on your farm and assess your risk potential,” said Telenko. “Scout fields when conditions are right for the disease to blow up. If you find tar spot, the inoculum is present. Scout the lower canopy so you can determine when the disease is starting. If conditions are favorable for development, then you can decide if you need to protect the upper canopy.”

Moore said that adequate fertilizer will help plant health. “Tar spot tends to pick on weak leaf tissue, such as plants showing nutrient deficiencies,” he explained. “Selecting hybrids with good disease tolerance will also help, specifically a good tar spot rating, if that is available.”

Research shows that fungicide application before the disease becomes severe in the canopy may provide significant yield protection. Currently, there are several fungicides from different crop protection companies that work to help control tar spot in corn. To be effective, timing is critical. Wilburn recommends making a foliar fungicide application at the VT-R1 stage.

“Applying fungicide at VT or R1 is already a common practice on many corn fields, so we are fortunate that this also appears to be the best timing, in most cases, for tar spot,” he said.

Telenko encourages the use of fungicides with mixed modes of action to protect against resistance development. She said cultural practices, such as residue management and crop rota­tion, can also help to reduce the pathogen inoculum available to infect corn.

“Rotation away from corn to soybeans allows for further breakdown of infested corn residue. In addition, tillage can help bury infested corn residue and reduce fungal spore movement,” she said. “However, these practices can produce mixed results and are not the sole solution for preventing tar spot.”

TRACK

Staying proactive and being aware of what is going on in your cornfields are keys to preventing tar spot from negatively im­pacting yields, Wilburn said.

“We are tracking the disease through MFA’s Crop-Trak pro­gram and then either submitting samples or notifying the plant diagnostic lab to verify new counties where tar spot is found,” Wilburn said.

MFA agronomists are also keeping up to date with laboratory confirmations through the University of Missouri Extension Service. Producers can report sightings and submit samples to the MU Plant Diagnostic Clinic at This email address is being protected from spambots. You need JavaScript enabled to view it. or to Integrated Pest Management at corn.ipmpipe.org/tarspot/.

Wilburn said he and other MFA agronomists are also investi­gating the utility of the Tarspotter app, a free disease forecasting tool developed by the University of Wisconsin to assist farmers in making management decisions for tar spot in corn.

“We are cautioning growers that it may not be a practical tool yet as Missouri has limited inoculum in the soil, therefore the app may currently over-predict the possibility of tar spot,” Wilburn added. “This may change in the future.”

Moore said that many producers are asking if and how tar spot will impact their crops and are naturally concerned. He encourages them to work with their MFA agronomists to learn more about the disease and its control.

“We are giving the best advice that we can which is to be pro­active and use appropriate management practices throughout the season,” Moore said. “MFA is here to help advise growers about best practices and proper products for tar spot as well as other diseases.”

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