Battle for the Belt: Season 3, Episode 16 – Tar Spot in Corn

Season 3, Episode 16 of Battle for the Belt is now available: https://www.youtube.com/watch?v=QIx3GTm5L44

In Episode 16, Dr. Stephanie Karhoff, Ohio State University Extension Agronomic Systems Field Specialist, discusses tar spot symptoms and scouting techniques.

Tar spot is a relatively new disease in Ohio, first appearing in 2018. Pathologists and agronomists are still researching tar spot to understand which environments favor this disease. This year, OSU Extension on farm research network, eFields, is conducting a scouting survey called the “Tar Spot Surveillance Survey” to further explore optimal environments for tar spot infection to improve management of this disease in Ohio. County Extension Educators across the state are scouting three corn fields in their county weekly, starting at the V7 growth stage to report tar spot infection. Field management information is collected to identify if there are any management practices that correlate with tar spot occurrence. Towards the end of the season, disease incidence (number of plants with disease) and severity (percent of leaf covered by disease) will be evaluated in the fields that have tar spot. Look for results and more information on this study in the 2025 eFields report!

If you are out in your corn fields in the next couple of weeks scouting for tar spot, look for small, raised bumps that look like flecks of tar on the leaf. The black spots are stromata that are embedded into the leaf surface. This means that tar spot stromata cannot be scratched off. Insect frass is often mistaken for tar spot but is easily scratched or wiped off (Figure 1). While scouting, if you think you have come across tar spot, wet your finger and try to rub off the black spot, and if it comes off, then it is not tar spot.

Though tar spot is prevalent throughout Ohio, if you detect tar spot in your field, please contact your local county educator to assist Ohio State Extension in tracking when and where tar spot is occurring and the impact of this important disease.

Crop Progress Updates from the Field

Corn at the Western Agricultural Research Station reached R1 (silking) in the first planting date (4/18). As of now, this is the only location that entered the reproductive stage of corn. Because we have four different hybrids, disease resistance varies. The 100-day hybrid tends to show disease symptomology first.

 Low levels of gray leaf spot were seen on the ear leaf of the 100-day hybrid, but the more resistant hybrids showed no disease (Figure 2). In disease management, the ear leaf is the most important to protect, so when scouting, it is imperative to note what leaf the disease is on. The first planting date of soybean reached R3 (beginning pod) and the only present disease in soybean has been Septoria brown spot, which is not normally a yield limiting disease in Ohio. Much of Western and Northwest Ohio are in need of the rain as corn was showing signs of stress with rolling leaves. The rest of the stages for this location are presented in Table 1.

Corn at the Northwest and Wooster locations are in vegetative stages. Soybean are in the R3 and R2 stage at Northwest and Wooster, respectively (Figure 3). These two locations are without disease, in the earliest planted soybeans. The dry weather is not conducive for frogeye leafspot, which is our main foliar disease concern.

Follow the Battle for the Belt campaign all season long to stay informed and up to date.
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