Tar Spot (Phyllachora maydis) was one of the hottest new corn diseases to hit the US in 2015 and has continued to rear its very distinct, speckled head in subsequent years. We had major outbreaks in 2018 and again in 2021 in various parts of our marketing area. Now that it is here, and we won’t be getting rid of it any time soon, how can we best scout for it and combat this disease?

For those of us who have experienced “the joys” that come from dealing with Tar Spot before, we’ve learned the campaign against this disease begins with proper hybrid selection. Beck’s offers several hybrids that show a much stronger tolerance to Tar Spot than others in our lineup. Infection can happen at any developmental stage, but Tar Spot has historically shown up as we approach the tassel timeframe. However, favorable conditions and significant inoculant load may cause it to show up even earlier.

Tar Spot needs prolonged leaf wetness (around seven hours), cooler temperatures (60 - 70° F), and high humidity before infection will occur. A two to three week latent period will follow before the stromata (black, glossy fungal structures scattered or clustered on leaf) appears, which will produce additional spores that can continue the spread of infection through the season in favorable conditions. This is a reason for the rapid infection and spread of this disease. Previously infected fields can see infections that start from the bottom of the plant and work up the canopy. All fields have the potential to see a top-down infection that is attributed to wind and rain dispersion of the fungus, though.

When scouting for Tar Spot, start your search at the upper canopy unless you know that Tar Spot has previously been found in that field. In that case, check both upper and lower portions of the plant. Tar Spot will appear like small, random, black paint smatterings that cannot be rubbed off, and have a raised, bump-like feel (see below.)

Rare Tar Spot infections can show a fisheye effect, which appears as a small necrotic area around the black stromata. This is known as the Tar Spot complex in other parts of the world due to the presence of a second fungus (Monographella maydis). This second fungus has not yet been found in the US, so the fisheye presence has scientists puzzled for the moment.

Tar Spot reduces a plant’s ability to translocate nutrients through its system, and scouting ratings are based on a percentage of severity. The Tarspotter app from the University of Wisconsin Nutrient and Pest Management Program is a good way to stay on top of the moving target that this disease represents.

If local fields reach 5 to 10% on the severity scale, it’s time to consider applying a fungicide with multiple modes of action and a 2(ee) label for Tar Spot to help slow the spread, regardless of the growth stage. The overall goal is to get to at least the R5 growth stage with as few foliar symptoms present as possible. Increased pressure from this disease can cause significant yield losses and reduced late-season standability.

Because Tar Spot is a relatively new disease in the U.S., we learn more about it each year. How to manage and scout for this disease is a process that will continue to change in years to come. For now, all we can do is manage it the best we can, with the tools we have, and continue to learn.