When it comes to hunting down crop diseases, nothing can replace going out to a field and scouting — even if it is hot and muggy in July.

That’s especially true for managing tar spot.

“Early July is the most critical time to be out there scouting fields, searching for it,” Paul Esker, professor of epidemiology and field crop pathology at Penn State, told a group of crop advisers and farmers at a recent crops and soils conference in Grantville, Pa.

Tar spot was found earlier than ever this year, with positive confirmation of the disease July 9 in a field in southeast Pennsylvania, 10 days earlier than the previous first find.

Branch County in Michigan saw a reported infection June 24. Jackson, Shiawassee, Saginaw and Monroe counties, also in Michigan, saw early infections from July 8-11. Below is a map of positive tar spot identifications this past season.

Finding tar spot early is critical to staying ahead of it, Esker said. Once the disease covers 25% of a leaf’s surface, the game’s over. It will shut the plant down.

“The good thing is if you spray a fungicide and it was warranted, more than likely it will clean it up. But again, this depends on the given situation,” he said.

Using a fungicide timing and model validation study done in Indiana in 2020 — where the first positive confirmation was July 28 — Esker said it took about a month or so for the disease to develop enough spores to rapidly spread. This is important to know, he said, because every season presents a window for effective control.

Related:Tar spot identified in Pennsylvania

The key is to know when conditions are right for development — when temperatures are between 59 and 70 degrees F, relative humidity of at least 85%, and more than seven hours of leaf wetness — and to attack it with a fungicide. But beware of applying fungicide too early after you first see it. According to the Crop Protection Network, most fungicides only effectively manage disease for about 14 to 21 days after application.

When is the best time to spray? The Crop Protection Network recommends between VT/R1 and R3, depending on the fungicide being used. Multiple sprays could also be needed. Between R3 and R4 is a prime time for spore growth and spread, depending on if conditions are right.

“Once you get past 5% leaf severity, that fungicide effectiveness goes down as the inoculum has built up,” Esker said.

In that same 2020 study from Indiana, calculated net returns from foliar fungicide and proper application timing averaged between $29.20 and $48.50 an acre under high tar spot disease pressure, in comparison to no fungicide being sprayed. On the other hand, spraying under low disease pressure averaged out to a loss of minus $25.80 to just over a $1 net return.

Related:Prepare now for crop diseases next year

Even as feared as tar spot is, Esker still thinks gray leaf spot is more concerning. The good news, he said, is that you can treat both at the same time. “If you hit the right window for gray leaf spot, you will clean up the most important period for tar spot,” he said.

Tips for 2025

Here are some tips for dealing with tar spot in 2025:

Select the right hybrid. If you haven’t selected a corn hybrid yet, this is your first line of defense. Some hybrids are better at tolerating tar spot than others.

Esker recommends shorter-season corn hybrids, as they will most likely mature faster than the disease can do most of its damage.

“So again, if you can plant early, maybe dial back a little that RM, you’ll probably avoid some of the risk," he said.

Rotate crops. Crop rotation can also be beneficial, as it will allow residue to decompose and reduce the primary inoculum, but it’s unclear how beneficial it is.

Same thing with tillage. According to the Crop Protection Network, tilling will bury infected residue and increase the rate of decomposition, which can help reduce overwintering inoculum, but it won’t reduce the risk of infection from locally dispersed inoculum.

Related:Sweet cherry growers receive USDA disaster declaration

Report the disease. If you see tar spot, report it. Esker said local reports are valuable because they help researchers understand when the disease is occurring relative to crop growth stage.

"That changes our recommendations very quickly," he said. "One of the big things we have seen with tar spot, it doesn't matter what else is on there, it will do very well and thrive. So gray leaf spot, others, tar spot can do very well with it. You can have tar spot lesions right on top of gray leaf spot.

"Just assume we're starting the season with inoculum out there. It's really how that environmental condition comes together.”