Skip to main content

Strategic Farming: Let's talk crops focused on tar spot of corn

Phyllis Bongard, Extension content development and communications specialist

Tar spot of corn was problematic in portions of Minnesota last year. It can spread rapidly, develop quickly, and cause significant yield loss. Since tar spot is a relatively new disease, it’s one to watch for.

Drs. Dean Malvick, Extension plant pathologist, and Darcy Telenko, Extension field crop pathologist from Purdue University, tackled these questions from tar spot basics to management in the March 8th Strategic Farming: Let’s talk crops session.

Tar spot basics


Tar spot of corn was first discovered in 2015 in northern Indiana and northern Illinois. For the first couple of years, it didn’t cause much concern. By 2018, however, it started to spread quickly around the Midwest.

In Indiana where Telenko is based, tar spot can be found in 86 of 92 counties. Between 2018 and 2021, parts of the state saw yield losses between 20 and 60 bushels per acre, with some fields experiencing a 50% loss in 2021. The disease has been found in every Iowa county and a large part of Wisconsin.

In 2019, it was confirmed in southeastern Minnesota, where it has caused significant yield losses in the past couple of years. Tar spot continues to spread in the state, though primarily at very low levels outside of the southeastern region.

To see a distribution map where tar spot has been confirmed and real-time disease activity during the growing season in the US, visit

Disease development


Figure 1. Tar spot of corn. Photo: Dean Malvick, UMN
Tar spot of corn is caused by the fungus Phyllachora maydis. It produces small, raised black spots - called stromata - on leaves and corn husks that can’t be rubbed off, unlike dirt or insect frass (Figure 1). These spots generally go through the leaf, showing on both upper and lower surfaces.

The fungus that causes tar spot requires a living host to grow and reproduce. The black spots overwinter in infected residue, then when temperatures warm and it’s wet, they release ascospores that can infect new corn plants. Once infected, a corn plant will show symptoms in 14 to 21 days. After the initial infections, the new tar spots can produce more spores, resulting in secondary infections. Leaves with secondary infection may have a mix of larger spots and many smaller ones.

Favorable conditions

Tar spot can develop rapidly under wet conditions and moderate temperatures. Several risk factors are reported to favor disease development, although we have more to learn about the conditions that favor tar spot:
  • Moderate temperatures (50-70 degrees F)
  • Average relative humidity greater than 75%
  • Average of 7 hours/night of leaf wetness
  • Monthly rainfall total greater than 6 inches
Under these favorable conditions, tar spot can progress from a few scattered lesions to a fully involved leaf in just 2 to 3 weeks (Figure 2). Each season is unique, however, and even with what we think are the right temperature and moisture conditions, sometimes the disease doesn’t develop as expected.
Figure 2. Tar spot can progress from scattered lesions to killed leaves in 2 to 3 weeks. Photos: Dean Malvick

Sustainable disease management

Purdue research is focusing on an integrated approach for managing tar spot. Identifying tolerant hybrids, conducting fungicide trials and looking at residue management are components of this effort.

Tolerant hybrids

Could tolerant hybrids reduce or eliminate the need for fungicides? To address this, Telenko identified two tolerant lines and compared them to a susceptible line with and without fungicide at sites where there is significant tar spot pressure. In this three-year study, researchers saw significantly reduced disease severity by growing the tolerant lines. When fungicides were applied at VT/R1, disease was reduced in the susceptible line, but not significantly in the tolerant lines.

How did this translate into corn yields? In this study, yields were quite variable. Until the 2018 epidemic in Indiana, there was little effort to breed for tar spot resistance. As a result, tolerant hybrids may not have as much yield potential as top performers. Even when disease is present, a susceptible top hybrid may outperform a tolerant one if protected by a fungicide, although that may change as new resistant hybrids are developed.

Fungicide trials

Until resistant hybrids are widely available, applying fungicides to protect the crop is a management option. However, fungicides have some limitations when it comes to treating tar spot. Most fungicides are active for about two to three or maybe four weeks. If tar spot starts in early July, like it did in 2021, the corn would need a much longer period of protection than if the disease started in September. Once fungicide activity diminishes, tar spot can rapidly redevelop if conditions are favorable and green tissue is available.

2021 – High pressure season

Trials conducted in several states during the severe epidemic in 2021 showed that multiple different fungicides (nine) significantly reduced tar spot severity. Any differences between the products may have been related to how long they were active. While there were no significant yield differences between the different fungicides, there was a 7 to 18 bushel advantage to applying fungicides over the control.

2022 – Low pressure season

Because tar spot started in Indiana so early in 2021, the researchers received questions about whether a two-application program was needed and how it should be structured. They compared a single VT/R1 application of five fungicides to two double applications: one that started with the top products followed 3 weeks later with a generic, and one that flipped these products.

While disease pressure was low during the 2022 season, the trial can still illustrate the importance of timing fungicide applications. In contrast to 2021, tar spot started later this season. By the time the disease ramped up, the VT/R1 application was no longer active. As a result, there were no significant differences in disease severity between the fungicide application and the control.

Since disease pressure was so low (2% severity), there were no yield differences for any of the treatments in this trial. Unlike 2021, there was no return on investment for the fungicides. Conditions were hot and dry at these sites, not the moderate temperatures and wet conditions that favor tar spot.

Since each year is different, the management practices that worked one year may not work the next. These studies illustrate the importance of detecting tar spot in its early stages and assessing whether conditions will favor disease development.

Application timing

Another three-year study went more in-depth on optimum application timing. Trials included applications at V8, VT, R3, and one triggered at detection. At +3 weeks, a second application was made for each of these stages (ex. 1st detection + 3 weeks), so that the trial included both single and double applications.

As noted earlier, each year had different weather conditions, which affected both the course of the disease and the efficacy of the fungicide programs. Although 2020 was moderately wet, tar spot didn’t get started until late July. The most effective applications were at first detection and VT. For the most part, the 2nd application didn’t add anything to control. There were no significant yield differences for any of the fungicide applications this year.

In 2021, tar spot was first detected at the beginning of July. There had been a lot of rain early in the season that kicked up the disease early. The best application timing was at V8 followed by a second application 3 weeks later, effective programs that reduced disease resulted in an 18 to 31 percent yield increase.

Finally, disease pressure was very low in 2022. A single fungicide application at first detection and two of the double treatments had the best control. The second applications did not add anything to either disease control or yield. There were no significant yield differences for any of the fungicide treatments this year, either.

Severe disease pressure vs. low pressure

What is the net return for using these fungicides for tar spot? Researchers combined data from all three years and locations and sorted it by disease pressure to answer this question. In a high disease environment (>5% severity), all fungicide treatments had a positive response with an average 9 bushel advantage. In contrast, there were fluctuations in the yield response for fungicide applications in the low disease fields with an average increase of 3 bushels.

What about timing? In a high disease situation, the best timing was at VT/R1 and also R2 and R3 with an average 14 bushel increase. In contrast, the average return in a low pressure situation was a loss equaling 3 bushels per acre.

In these high disease pressure fields, fungicides protected yield and provided a positive return on investment. For more information about fungicides from the Crop Protection Network, visit Fungicide efficacy for control of corn diseases.

Impact of residue on management

Tar spot stromata survive and overwinter on infected corn residue. Research suggests that they survive at least one year, but can it last longer? Work is continuing to evaluate that.

High and low residue systems with tolerant and susceptible hybrids were compared to evaluate their impact on the disease. While tar spot severity was significantly reduced in a low residue system, the biggest disease response came from using a tolerant hybrid.

Putting it all together

Tar spot can spread rapidly, develop very quickly, and cause significant yield loss, making this disease one to watch for.
  1. Assess your risk – Do you have tar spot in your fields? If so, the inoculum is present. Is it in the area? Scout the lower canopy so you can decide whether or not to protect the upper canopy. The distribution map and Tarspotter are tools that will help determine where the disease is active and if conditions are favorable for disease development.
  2. Hybrid resistance – Watch for and use tar spot resistant hybrids as they become available.
  3. Consider fungicide – Fungicides can protect a crop but they will not kill an infection and need to be applied before disease is well established in a field. Use the apps and scouting to figure out when the disease is starting and if conditions are favorable for development. To keep fungicides effective, be a good steward of the products.
  4. Irrigation management – Research is ongoing in Indiana to try and limit the wetting events that promote disease.
  5. Residue management – Residue management is less effective because of the secondary cycling and spread of the disease once it gets started.

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 2023. For more information and to register, visit

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

Print Friendly and PDF