Tar spot in corn was so rampant in some areas that the black spots from this disease may appear in your nightmares. For others, perhaps it was a nuisance but didn’t hit yields hard. Either way, 2024 showed that one thing is certain: If you grow corn in the Corn Belt, your fields are not immune from this threat.

Tar spot was widespread in 2024, creating abundant questions. Answering them are Darcy Telenko, Purdue Extension plant pathologist; Dan Quinn, Purdue Extension corn specialist; and Dean Malvick, University of Minnesota Extension plant pathologist and program leader for crops plant pathology.

Telenko and Quinn answered questions posed by Farm Progress. Tar spot was confirmed in all 92 counties in Indiana in 2024, with significant yield reductions where fungicides were not applied.

Malvick answered questions from fellow University of Minnesota Extension agronomists Dave Nicolai and Seth Naeve during a Minnesota CropCast titled “Corn Tar Spot Disease: A 2024 Minnesota Fall Update.” Find the complete transcript or listen here.

Minnesota is representative of states where tar spot showed up later, after first appearing in the U.S. in Indiana and Illinois in 2015. “We’ve seen the most damage in southeastern Minnesota, but this year, there were indications that it moved to other sections of the state and surrounding states,” Malvick says. Tar spot expanded in South Dakota and Wisconsin this year and was confirmed in several counties in North Dakota.

Related:Early corn yield estimates fail in face of tar spot

What made this a tough year for tar spot?

Telenko: Moisture events in June and July kicked it off in Indiana. Once the disease got started, there was probably enough relative humidity in some areas to allow the disease to continue to develop in the canopy.

Quinn: Tar spot became most severe in fields where no fungicide was applied. One scouting trip around tasseling isn’t enough. In many cases, it came in after that time. Our research trials indicate around 30-bushel-per-acre losses where no fungicides were applied, and we’ve heard farmer reports of higher losses.

Why did tar spot attack some fields that were sprayed?

Telenko and Quinn: It is all about timing. But even if you sprayed a fungicide at R1 or brown silk, even the most effective fungicides don’t last forever. So, where tar spot was widespread, even those fields exhibited some level of tar spot in September. However, if corn was well dented or better when tar spot invaded, yield likely wasn’t affected. Plants were more susceptible to stalk rots due to stress and to lodging from storms.

Related:This field beat tar spot to finish line

Can you talk about timing of infection in Minnesota and areas where it wasn’t as intense?

Malvick: This year, for example, in Rosemount, Minn., we found it by the fourth week of July. We expected, given the amount of rain we were having, it might really take off, but it didn’t. It was very widespread there across research fields, but I don’t suspect it caused any yield loss, even though conditions seemed conducive, and we had early infection.

Early infection seems critical in those fields showing yield loss because it takes time for the disease to ramp up and become truly severe. Oftentimes, we see it much later in the season, in August and even into September. Usually, if it’s initiating that late, we don’t really see any yield loss.

Can tar spot overwinter in the Midwest?

Telenko: Yes, it overwinters here in residue. So, after a year like 2024, we have even more inoculum for future seasons.

Should farmers chisel or plow under residue?

Telenko: No. Since tar spot stroma are so small and there are millions of them, plowing under residue won’t help. Besides, the disease has secondary cycles with spore movement. By midseason, there are spores moving across the Midwest. This aerial aspect will negate any rotation or tillage practice. You can see this in how the disease continues to move across the U.S.

Related:Studying tar spot just got easier

We have learned about movement of spores via wind recently, and we had a lot of moisture early on here [in Minnesota], with high record rainfalls this spring. Was there a sweet spot for temperature, moisture on leaves, free water and such? Did it occur at the right time as southerly winds moved across the Upper Midwest? Was it the perfect storm?

Malvick: That’s probably true. I think we’re still trying to understand conditions that are truly needed to create high levels of tar spot. I expected it in some fields near Rochester, Minn., where there is a high level of inoculum around from previous crops. I thought the disease would really take off given the amount of rain we had. But it didn’t.

So, there was another factor that wasn’t there that was needed to really create high levels of the disease. We’re still trying to understand that a little bit better. At the same time, though, we saw tar spot this year in fields where we’ve never seen tar spot before.

Can it affect other crops? For example, is it OK to grow wheat after tar spot?

Telenko: Corn is the only host for the tar spot pathogen. There are other species that go to other grasses, but corn is the only host here. So, wheat would not be affected.

Is it OK to bale up cornstalks full of tar spot inoculum for bedding?

Telenko: Bedding can be used. However, realize you could be moving tar spot inoculum to new areas.

What about any impact on corn silage?

Malvick: Yes, this also affects silage corn because corn can dry down earlier than it would normally. That affects moisture content and could impact the product that comes out after ensiling.