Will tar spot be back in force in 2025? Can you prepare by making sure corn hybrids in your lineup have resistance to the disease? Are hybrids with significant resistance available?

These are key questions as you prepare to turn the page on the 2024 season. It will be remembered for tar spot confirmations in all 92 counties in Indiana, plus the discovery of the disease in counties across the Midwest where it had not appeared before, indicating it is spreading.

Providing answers to these questions and others are Darcy Telenko, Purdue Extension plant pathologist; Dan Quinn, Purdue Extension corn specialist; and Dean Malvick, plant pathologist and program leader for crops plant pathology at the University of Minnesota. 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 represents a state that has dealt with tar spot before, but not to the extent seen in Indiana, Illinois and parts of other Midwestern states. It was most intense in southeastern Minnesota in ’24, but also appeared in other counties.

Because there is lots of inoculum from 2024, will tar spot be back next year?

Related:Tar spot update: Experts answer questions after tough year

Telenko: Yes, possibly, but the weather conditions will play a big role in how bad it develops in each season.

Quinn: You need a susceptible host, disease inoculum and weather conditions which favor the disease. You can’t control weather conditions. We thought we knew conditions that favored tar spot, including extended periods of wetness, and warm but not hot weather. What we saw in 2024 was that it didn’t always act like we expected. It showed up late, even though it turned dry. So, we are still learning.

What can a farmer do to prevent tar spot next year?

Telenko: It is here to stay. You need to make plans on how to manage it on your farm. The severity and risk of yield loss may change year to year, but it will be here.

Quinn: You need to keep scouting for it throughout the season. Don’t stop scouting until corn reaches maturity.

If we think about 2025, with farmers either having it in their fields or in the area, how should they think about preparing for next year? Where should we start?

Malvick: So, at this point, still the best way to manage this disease is through fungicides. Now to back up, are there hybrids that are less susceptible? Yes.

All seed companies are working hard on this and trying to identify more and more resistant hybrids. It’s an important part now, and it will become more important in the future. But right now, even from some studies I’ve seen in Indiana where they’ve been working on it for a long time, there can be a difference in how much tar spot develops on a moderately resistant hybrid versus one that’s very susceptible.

What have we learned in Indiana so far? Are there hybrids that do better than others with resistance?

Telenko: Yes, there are hybrids that reduce disease development. I would ask for hybrids with resistance. We’ve seen in our trials that a moderately resistant hybrid may provide enough resistance to disease that an additional fungicide may not provide a return on investment.

That is the biggest question. In areas hard hit by tar spot, a single application of fungicide generally will provide yield protection and a return on ROI. However, you may not see a better ROI by adding a second application, even if the corn looks a bit cleaner.

If you apply a fungicide, don’t expect corn to be clean at harvest. Tar spot will move back into the canopy once the fungicide residual activity is gone. The goal is to protect as much yield as possible. Just like with gray leaf spot, there is still going to be disease in the canopy at the end of the season.

Rely on resistance or fungicides?

If you agree that fungicides and genetic resistance are tools, do you rely on one, the other or both?

In the CropCast episode, Naeve posed the following scenario: “So, let’s game this out with an example. Suppose a farmer is approached by a seed company rep. They’re riding in the combine together, with the salesman giving the farmer a rundown of hybrids to plant next year. The salesman says, ‘This is a really great hybrid.’

“Yes, it’s got top yields, but it seems to be more susceptible to tar spot. So, does the farmer just plan on a prophylactic application of fungicide? Or should the farmer just stay away from that hybrid completely? Does that depend on whether the farmer had tar spot in the past?”

Malvick offered this response: “There are a lot of parts to that scenario, and there is no absolute answer to any of it. Outside of southeastern Minnesota, I still think it’s significant to remember we haven’t seen significant levels of disease in very many places [in Minnesota]. Now, is that because we don’t have conducive conditions generally? Or is it because the disease did not have enough time to develop a buildup yet? There is a lot we have to figure out about tar spot.”