Managing Tar Spot Late in the Season

As the season has progressed, so has the spread of tar spot in corn. Since it arrived in the United States in 2015, this fast-acting pathogen has been identified across the Midwest and into the Northeast and Southeast. Although tar spot has jumped out to a commanding lead, ongoing research is helping producers catch up with management programs designed to mitigate risk.

“I’ve not seen a corn disease that could take 50 to 60 bushels off a yield in my career,” says Brad Hammes, Product Specialist for Helena Agri-Enterprises based in Iowa. “This is a new one”

According to Hammes, tar spot is “unmistakable” in a field. Growers can recognize it by the shiny black, raised lesions it causes on the corn leaf that resemble specks of tar. As those speckles kill tissue around them, they create a halo of yellowing and dying tissue. Inside the plant, a tar spot infection attacks the vascular system, disrupting the movement of nutrients and carbohydrates into the ear during grain fill and resulting in early, rapid death in severe cases. In addition to these tell-tale signs of tar spot, there are certain environmental conditions conducive to its development.

“What we’ve found now is that if you have seven to ten hours of high humidity, that can be enough moisture in the air to allow for infection,” says Hammes. “If you think about the conditions we often have with our overnight temperatures in the Midwest, that's a pretty big footprint that could be impacted.”

Current forecasts are calling for a cooler and wetter than average August thanks to the El Niño, and Hammes says that shapes up for an increased level of risk for infection from tar spot. He recommends downloading the University of Wisconsin’s Tarspotter app, which uses the location, current environmental conditions, and weather forecasts to determine your risk for tar spot. For active infections this season, the window is closing on effective application timings.

“Being out into that mid-July timeframe, most of the crop is tasseled at this point,” says Hammes.  “The general guidance is that R2 timeframe, or visually, if you start to see brown silks, the ear’s probably in that R2 timing, and that’s going to be your number one spot to manage it.”

Hammes has seen fantastic results in tar spot environments with Helena’s fungicide Odyssey®, which offers four modes of action from triazole, strobilurin and potassium phosphite fungicides. While fields with less severe infections can get by with only one pass, harder hit areas need a second application no later than exactly 3 weeks after the first treatment. A great follow-up to Odyssey is Avaris® 2XS that contains two modes of action and a fungicide activation system. Hammes says, timing these late-season applications correctly can help ensure more bushels at harvest.

“We have 10 percent of our yield yet to be determined even after dent,” says Hammes. “With how quickly tar spot can kill a plant, if you were to have a late R4 infection and have that plant die very rapidly while it was still trying to fill grain, now you're out potentially 10 plus bushels on that acre.”

Looking forward, Hammes also recommends talking to your seed representative this fall about how different varieties are handling tar spot. It’s one more step you can take to adapt to this new threat. For more advice on managing tar spot from Brad Hammes, tune in to Episode 39 of Helena’s FieldLink Podcast, available on YouTube or wherever you listen.