Managing tar spot in corn

Martin Chilvers<chilvers@msu.edu>, Michigan State University Department of Plant, Soil and Microbial Sciences - August 07, 2025

Cool weather conditions and hybrid selection have a dramatic effect on tar spot development. Read on to understand your risk and what you can do to combat this disease.

A map showing tar spot confirmations in corn across the north-central and northeastern United States and parts of Canada. Counties are shaded by detection status: yellow for positive detections, light yellow for probable detections, blue for scouted but not found, gray for found in previous years, and white for no data. Most positive detections are clustered in Iowa, Illinois, Missouri, Indiana, southern Wisconsin, southern Michigan, and southwestern Ontario. — Map of tar spot confirmations as of Aug. 6, 2025, across the region. Map can be accessed at: Crop Protection Network - Tar Spot of Corn Map.

Take-aways

Understand your hybrids, what are their disease ratings for tar spot, northern leaf blight and gray leaf spot, are any hybrids more susceptible than others
Scout fields regularly, especially as we come into reproductive stages. The tar spot map is helpful as a guide to scouting, not for timing fungicides.
Most years when needed, a single fungicide application from tassel (VT/R1) through milk (R3) provides adequate suppression of tar spot, occasionally an application at dough (R4) will be valuable, very rarely at the beginning of dent (R5).
Leave a check strip if you are making a fungicide application so you can understand your return on investment (ROI). ROI calculators have been developed to aid in fungicide decision making.
Use the Crop Disease Forecasting tool (which replaces Tarspotter) to assess risk conditions and to aid in fungicide application timing.

Tar spot this season?

Tar spot is developing yet again this season, with some fields likely requiring a fungicide application. Weather conditions throughout the remainer of the season and hybrid susceptibility will be the primary factor for disease development. Farmers and crop consultants should make efforts to understand their corn hybrid susceptibility and be scouting for disease.

What causes tar spot?

Tar spot is caused by the fungal pathogen Phyllachora maydis. P. maydis can only infect green corn tissue and to date has not been found on any other crops of cover crops. The fungus survives on infected corn residue through the winter and releases spores in the spring that can infect new plants. It is not seed transmitted, though can be moved on infected leaf tissue.

Should I use tillage and crop rotation?

In short, no. Although the fungus that causes tar spot does survive on residue, the fungal spores can travel many miles on the wind. In severe tar spot epidemic years of 2018 and 2021 there were numerous fields that hadn’t been in corn for years that were “smoked” with tar spot. Any value in tilling fields will be short lived as inoculum blows in from outside fields. Crop rotation is an important tenant for disease and insect management, however variety susceptibility and weather conditions play a much larger role in disease development.

What about other agronomic factors?

Research has demonstrated that nitrogen rates did not influence tar spot development. Tar spot is caused by a fungal pathogen, not by a lack of nutrients. Tar spot can develop just as happily on healthy plants as it can on those that are mismanaged. Although we have seen an inverse relationship between plant population and tar spot development, i.e. less disease in higher plant populations. Increasing plant density to lower disease did not improve the economics of production. However, in the studies hybrid susceptibility had a strong effect on tar spot severity and yield and should be prioritized for disease management.

Scout, scout, scout!

You can’t manage what you don’t know. It’s critical to be aware of disease development, especially as those final leaves have emerged and we move into reproductive stages.

How do I identify tar spot?

Tar spot lesions are relatively distinct, consisting of a matte black raised spot (1/16 – 3/4 inch diameter) that is embedded within the leaf and will not rub off the leaf surface. Tar spot lesions form on the top side of the leaf but will protrude through the bottom side of the leaf. Lesions are typically drawn to some extent with veins in the leaf, and often have a slight brown margin around them.

Be sure of your diagnosis. Fields have been sprayed when no tar spot was present. If in doubt, send a sample to Michigan State University Plant and Pest Diagnostics. Or send good quality pictures via email chilvers@msu.edu or via X (Twitter) @MartinChilvers1.

Tar spot lesions on a corn leaf. — Tar spot lesions. Note the slight brown margin around lesions and how the lesions tend to be drawn with the veins. Photo by Martin Chilvers, MSU.

Insect frass (bug poop) is often confused with tar spot; however, these are easy to distinguish as frass will dissolve and wipe off the leaf with some water while tar spot will not. Initial gray leaf spot or anthracnose lesions may look similar to tar spot, but typically they will have a dark margin but light-colored center of dead leaf tissue. Physoderma brown spot can also look similar but lesions are not raised. Late season rust pustules can also take on a dark color and look similar to tar spot, but upon close inspection rust pustules erupt through the leaf leaving a torn margin, and rust spores will wipe off the leaf onto your finger.

How should I manage tar spot?

Using hybrids with tolerance/resistance to disease is the cornerstone of disease management. A hybrid with partial resistance to tar spot will greatly assist in slowing disease development and has the potential to negate the need for a fungicide. Unfortunately, no hybrid is immune to tar spot, but there is significant variation in hybrid susceptibility. Talk to your seed dealer about your hybrid’s risk potential. Michigan State University Extension continues to work with colleagues and industry to screen for resistance to tar spot.

If I apply a fungicide, what should I use?

In general, Michigan State University Extension recommends using a fungicide product with mixed modes of action. This typically provides better disease suppression than a single mode of action product. We are also concerned with the development of fungicide resistance in the tar spot fungus (Phyllachora maydis). Using a mixed mode of action fungicide should slow the development of fungicide resistance. Each year we conduct fungicide efficacy trials to assess current and new products and combinations. Those trials are summarized in fungicide efficacy charts that are updated annually and can be found at Fungicide Efficacy for Control of Corn Foliar Diseases.

When should I apply a fungicide, and should I make two applications?

If a fungicide is needed and depending on the timing of disease onset and weather, a fungicide application somewhere from VT/R1 through R3 will likely be the best timing. Early vegetative applications tend to have little effect on suppressing tar spot epidemics. Although we have seen greater disease suppression with two fungicide applications, this is not always profitable, especially with today’s lower corn prices.

What about disease forecasting tools?

Initial disease forecasting models used apps such as Tarspotter. However, our new platform, Crop Disease Forecasting, is now housed on the Crop Protection Network website. Users can assess tar spot disease risk based on weather variables for multiple pinned locations within the website. When using the crop risk tool, keep in mind that it is still important to scout fields to see if disease is present and be mindful of crop growth stage. We typically see the best fungicide timing for tar spot suppression from silking (VT/R1) through milk (R3) or dough stage (R4).

What is my ROI? Leave a check strip!

If you do decide to make a fungicide application(s), be sure to leave check strips and pencil out the costs and benefits, otherwise you will never know what your ROI was. It’s always important to pay attention to what is working for you and what is not. At the Crop Protection Network, we’ve assembled multistate fungicide trial data into a fungicide ROI calculator. The calculator will help examine the potential economic benefits by examining input costs, market price and production outcomes. These ROI calculators are intended to support the decision-making process. It’s important to note that these are not predictive, but they provide a framework for exploring trial data.

If you have questions about corn disease management, don’t hesitate to contact me at chilvers@msu.edu.

This work was supported in part by the Corn Marketing Program of Michigan, National Predictive Modeling Tool Initiative, Project GREEEN and MSU AgBioResearch.

Additional resources

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Note that the Crop Disease Forecasting tool has now moved to the Crop Protection Network and Tarspotter is no longer supported.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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