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Strategic Farming: Let's talk crops! February 16 session covers tar spot, a new disease in Minnesota corn

By Angie Peltier, UMN Extension crops educator, and Phyllis Bongard, UMN Extension educational content development and communications specialist

Figure 1. Tar spot of corn. Photo: Dean Malvick
Over the last couple of years, a new fungal disease has been observed in corn fields in southeast Minnesota. Called tar spot of corn, this disease was observed for the first time in the United States in 2015 in northern Illinois and central Indiana. As with any new disease, there is a steep learning curve to characterize risk factors and management strategies and field crop plant pathologists throughout the north central US have been hard at work in this endeavor. 

On February 16, 2022, Drs. Dean Malvick, University of Minnesota Extension corn and soybean plant pathology specialist, and Nathan Kleczewski, Growmark technical agronomist, joined UMN Extension educator Ryan Miller for a wide-ranging discussion of how best to identify tar spot, what environmental factors favor disease and what research suggests is best way to manage this new disease. This was the seventh episode of the 2022 Strategic Farming: Let’s talk crops! webinars in this series.

How best to scout for tar spot

Common names for plant diseases can be quite descriptive. It is the dark black, raised spots that resemble small droplets of tar that gives tar spot its common name (Figure 1 ). Although it appears that all corn growth stages are susceptible to infection by Phyllachora maydis, the fungus that causes tar spot, in the few years that it has been observed in Minnesota, its range has been increasing steadily. Symptoms of tar spot in Minnesota haven’t been observed until the middle of the growing season, in July or later. This disease is also very likely in many more corn fields in Minnesota than are currently known.

Environmental conditions that favor disease

Three factors must come together for every corn disease to occur: a susceptible hybrid, the presence of the pathogen and the environmental conditions that allow the pathogen and plant to come together to result in disease. There are differences among corn hybrids in susceptibility to tar spot, although most seed companies in Minnesota maturities have not yet added tar spot ratings to their seed catalogues. The fungus survives the cold Minnesota winter in debris from previously infected crops and is thought to be wind disseminated and capable of traveling by this means as far as at least 0.75 miles.

Tar spot is favored by cool, wet weather. Periods of leaf wetness and high humidity appear to be important for both the first infection of the season and repeating cycles of infection throughout the season. The pathogen survives in residue from previously infected corn, and so the more tar spot in previous and neighborhood corn crops and the more residue on the soil surface, the more inoculum is present to cause primary infections. Delayed planting will mean that plant growth and development will also be delayed and primary infections will occur on plants that have more time than a timely planted crop to be exposed to repeated cycles of tar spot infection before they reach maturity. Yield loss from tar spot does not occur every year, but can be more than 30 bushels per acre when severe. Severe tar spot can also predispose the crop to stalk rotting pathogens that affect standability and can also impact how quickly one will need to chop a crop for silage.

What is currently known about tar spot management

Each observation about a new disease can provide clues about how best to manage it. Researchers have found that hybrid susceptibility varies, meaning that -if they are available in hybrids adapted to your region- one should consider planting corn hybrids with high levels of disease resistance to tar spot. Long periods of leaf wetness increase disease severity, meaning that if one grows corn under irrigation, carefully planning irrigation to minimize the duration of leaf wetness can decrease disease pressure. One disease management strategy is to rotate away from susceptible crops. 

There are several protectant fungicides labeled for managing tar spot of corn that have proved effective against additional infection when tar spot has reached approximately 3% severity on the leaf below the ear leaf or higher in the plant during the reproductive growth stages but before plants have reached the milk (R3) growth stage. While tending to be more pricy than fungicides with a single active ingredient, those premixes containing active ingredients with more than one mode of action tend to provide better protection and so are recommended for the first application, with cheaper, single mode of action fungicides reserved as a last, second resort.

Fielding audience questions

Drs. Malvick and Kleczewski answered many audience questions that were either asked live during the webinar or posed when attendees registered for the series, including: Are any of the fungicides labeled for tar spot systemic?, Are any of the fungicides labeled for tar spot curative?, How does a pathogen that needs a host to survive (like the fungus that causes tar spot) survive the Minnesota winter?, What about these products that are marketed as being able to degrade residue faster; would they have an effect on tar spot?, What about in-furrow fungicides for tar spot management?, What about the tar spotter app for predicting risk?, How does tar spot in the Midwestern US differ from the disease in the highlands of Mexico and Central America, its center of origin?, Was tar spot to blame for all of that moldy corn at harvest time last year?, Are toxins produced by the fungus that causes tar spot that could negatively affect livestock or humans?, How can one distinguish tar spot from northern leaf blight, for example?

Watch a recording of this webinar.  

Join the webinar series

University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2022.

This Wednesday, join Drs. Jochum Wiersma, UMN Extension small grains specialist, and Jared Goplen, UMN Extension educator, as they answer audience questions and discuss small grains production, challenges encountered in 2021 and how best to manage these crops in 2022 and beyond. For more information and to register, visit

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!

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