It may be time to brace yourself for a summer heavy with tar spot outbreaks. According to ipmPIPE, an online pest mapping system, nearly 20 counties across four states have reported fields with tar spot by mid-June.
Timing fungicide applications will be critical to keep the disease from getting out of control. Darcy Telenko, Purdue Extension field crop pathologist, says Purdue’s research shows the optimal time to apply is between the VT/R1 and R3 growth stages.
“You have time to get out, look and scout for the disease to make an informed decision that tar spot is active in the canopy, but also understand you have to be there before you reach R4 stage,” Telenko shared in a video from Purdue Field Crop Pathology. “Some of the first lesions are hard to find, but give it time, go back next week and look around. See if your neighbors are finding it. Then understand what stage the corn is in.”
Telenko doesn’t recommend treating tar spot before the corn reaches the tassel stage, despite it showing up earlier in fields this year, to ensure the leaves of concern for yield are being treated.
“In 2001 when the disease started earlier like this year, two applications could provide a good return on investment, but that’s only happened in a couple of our research trials where we had gone out at V8 or V10,” she says. “If we go that early, we may need a 3-week-later treatment so that’s something to think about. If we just held on a little bit, maybe one application would’ve gotten us there.”
In addition to properly timing application, the Crop Protection Network recommends choosing a product with multiple fungicide classes because they provide better efficacy and delay the development of fungicide resistance.
For help selecting the right fungicide, check out The Scoop’s Tar Spot Product Guide.
To keep an eye on optimal tar spot conditions this growing season, Farm Journal field agronomist Missy Bauer advises charting your area’s daily relative humidity.
“Universities have found when average daily humidity is above 75%, crops are at risk,” Bauer says.
