Tar spot is a topic of angst and anxiety. A severely infected cornfield can see yield losses upward of 60 bushels per acre. Yield losses are often a result of reduced photosynthetic capacity during grain fill, resulting in poor grain fill, kernel abortion and reduced kernel weight. Severe infection can reduce stalk integrity and cause significant lodging.

Tar spot is caused by the fungus Phyllachora maydis. It produces small, raised, black circular spots on leaves, stalks and husks. These spots are fungal fruiting structures called stromata, each of which can produce thousands of spores. Overall, tar spot infection and severity can vary based on environmental conditions, total amount of pathogen present and hybrid choice.

Here’s more key information about tar spot and how to control it:

Favorable conditions. Tar spot pressure in corn is fueled by 60 to 70 degrees F, relative humidity above 75% and prolonged leaf wetness for longer than seven hours. Pressure is typically higher in northwest Indiana, river bottoms and irrigated acres. Previous research found that the pathogen that causes tar spot can overwinter on infested corn residue on the soil surface.

Scouting and diagnosis. To properly manage problems in your fields, identify which specific problems are present in your fields. Proper diagnosis and early detection of tar spot is critical for successful management of the disease.

Tools such as the Tar Spot Distribution map and the Tar Spotter app are useful for tracking presence of the disease in your specific county and recognizing conditions conducive for tar spot development.

Fungicide use. Multiple fungicides from various companies help control tar spot. The fungicide efficacy chart available through the Crop Protection Network is useful.

However, fungicide efficacy is often largely controlled by fungicide timing. In most instances, a fungicide application between VT and R2 growth stages is most effective for controlling tar spot. A mixed mode-of-action fungicide product is often more effective than a single mode-of-action product.

In severe tar spot situations, as was observed in certain areas in 2021, two fungicide applications may be required. However, this is often dictated by history of severe disease and conducive environmental conditions in a field. It’s why tracking the disease and field scouting are important.

Hybrid selection. There are no corn hybrids with complete resistance to tar spot. However, different hybrids show different levels of partial resistance, which can be effective in reducing tar spot severity. Work with your seed company and examine local hybrid trials for proper hybrid selection.

If you have tar spot, scout and pay attention to different hybrids planted across your farm later in the season. Look for noticeable differences in tar spot severity and yield.

Cultural practices. Practices that help reduce infested corn residue and reduce survival of tar spot fungal structures can help reduce tar spot presence in cornfields. Two common practices for reducing disease inoculum are crop rotation and tillage. Rotation away from corn to soybeans allows for further breakdown of infested corn residue. In addition, tillage can help bury infested corn residue and reduce fungal spore movement.

However, these practices can produce mixed results and are not the sole solution for preventing tar spot.

Quinn is a Purdue Extension corn specialist. Telenko is a Purdue plant pathologist.