As the crop season moves into early August, the high variability in weather conditions and crop growth stages has affected the risk for tar spot in corn. While there haven’t been any significant incidents of tar spot reported to date, this is a disease that can develop and escalate quickly. Dean Malvick, University of Minnesota Extension plant pathologist, encourages growers to scout for this new corn disease.

Tar spot was discovered for the first time in the U.S. in northern Illinois and northern Indiana. Since then, it has spread across much of the corn belt, from Wisconsin and Michigan to southern Illinois and as far east as Pennsylvania. In some states, tar spot has caused yield losses of 40 to 50 bushels, although that level of loss has not been seen in Minnesota.

In Minnesota, it was first detected in Fillmore County in the southeastern part of the state in 2019. Since then, it’s been confirmed as far north as Stearns County, so it can spread rapidly.

Tar spot gets its name from the small, black, raised fungal structures that are produced on the leaves. At this time of the year, they’re hard to find, because the initial spots can be mistaken for insect frass or dirt. Unlike frass or dirt, tar spot lesions cannot be easily wiped off the leaf. It can also be difficult to find if it develops in patches in the middle of the field. Scouting requires eagle eyes at this early stage.

Catching this disease early is important, though, because it can progress from scattered black spots on the leaves to leaves being completely covered and killed in two to three weeks under the right weather conditions.

In most cases, the inoculum is probably coming from infected residue within the fields, especially in early-season infections. However, fields can be infected from locations outside the field. “We know that this disease can spread by wind, but we don’t have a good understanding how effective that is,” explains Malvick.

Under the right conditions, spores infect the leaves, and symptoms appear in 14-21 days. If moderate temperatures and ample moisture continues, these tar spot lesions can release more spores, and the disease keeps cycling in the field.

There is still a lot to learn about risk factors, but several conditions favor tar spot, including a previous infection, late planting, moderate temperatures, and ample moisture.

Malvick offered several tar spot management tips. Scouting is an important management tool. Start looking for tar spot in early July to document where the disease is developing and to identify where and if potential fungicide applications are needed. The earlier tar spot develops, the higher the risk for damaging levels, depending on favorable weather conditions.

Avoid the most susceptible hybrids. More seed companies are identifying hybrids that are lower in susceptibility and that can make a significant difference in disease outcomes.

Fungicides can be effective, but application timing is extremely important. While multistate research has identified the best timings to be between VT (tasseling) to R2 (blister), the applications need to be made near the onset of symptoms to be effective.

Applications aren’t recommended unless disease is developing in the field and weather conditions look favorable. Insurance applications at VT may lose their protective power before the disease even ramps up. Unless there’s reason to think that an application is needed at VT, it might be wise to wait until R1 or R2.

Finally, manage irrigation to keep leaf wetness at a minimum to reduce the risk of developing disease.

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We wish to thank our generous sponsors, the Minnesota Corn Research and Promotion Council and the Minnesota Soybean Research and Promotion Council for their support of this program.