As reported in August, tar spot was problematic throughout the Midwest this year and continues to spread. The disease is caused by the fungus Phyllachora maydis and can be detrimental to yields as it causes leaf deterioration, poor grain fill, compromised stalks and even plant death. In fact, experts from Purdue University Extension note that in cases of severe infection, yield losses can be upwards of 60 bushels per acre. Typically, the fungal disease affects corn later in the season during the critical grain fill stage, and unfortunately, there’s not much that can be done after it’s detected. That said, there are some effective strategies we can plan for now as we gear up for the 2023 season.
“Tar spot moved west in my regions and was detected late in the growing season,” says Tony Lenz, Stine® corn technical agronomist. “But what was really alarming in 2022 is that it moved quickly, even in a drought year. It’s bound to cause some angst for growers next year, so we must get ahead of it as early as possible as it’s likely to overwinter in infected fields this year to come back even stronger next growing season.”
Tony adds that the most important factors leading to this increase in tar spot are leaf moisture (when the leaves are wet for seven or more hours) and relative humidity 75% or greater.
While tar spot will likely be a problem next year, our experts provide a few tips to tackle the disease head-on next season.
Crop rotation and tillage
“If you experienced heavy tar spot pressure this last season, consider rotating to soybeans,” says Lenz. “It may be only one piece to a larger puzzle, but it’s a good start.”
Crop rotation gives the infested corn residue left over from harvest additional time to break down in the soil, reducing the presence of the fungal pathogens. Tillage is another strategy to help further break down the infected residue.
“Tillage and then crop rotation are a good one-two punch to get your fields prepped for planting season,” says Lenz. “Again, it’s not an all-in-one solution, but it’s shown to help slow down the disease before it rears its head in the growing season.”
Hybrid selection and timing
“Since tar spot is a relatively new disease, we’re still learning which hybrids are handling it well and which aren’t,” says Lenz. “Technically, there’s no good resistant hybrid available yet, but there are some with better resistance than others. Talk to your seed dealer or sales rep to walk you through which options might make the most sense in your environment.”
Even at Stine, we’re still researching tar spot and its impact on our corn products, including when the disease hits and how severely it affects the plants.
“It seems like late planting and planting a late-maturing hybrid are a bad recipe for having tar spot as the disease seeks and thrives off of green plant material,” says Lenz.
A resistant hybrid may come to market in the future, but in the meantime, it’s always best practice to discuss what options are available before purchasing seed.
Fungicides
“While seed treatments aren’t really known to control tar spot, there are some good fungicides on the market to help mitigate the disease,” says Lenz. “That said, the timing of application is the key to catching it before it explodes.”
Lenz recommends applying a fungicide at around VT to R2, and applying only one application has been the most effective strategy. That said, if the disease shows up later in the reproductive stage, two applications or a delayed application may be necessary.
For more tips on planning ahead to tackle tar spot, contact your local Stine rep or extension field agronomist.