Be on alert for tar spot in corn

Low levels of tar spot have been reported in Indiana, Iowa, Kansas, Missouri, and Nebraska. According to Dr. Darcy Telenko, Purdue University Extension plant pathologist, this is the earliest she’s found a natural infection of tar spot on young corn. In Indiana, it was found on corn planted in April on the lower leaves. “In the past, the earliest we found it was July 3. It’s here, we know it’s active,” she says. 

Environment is key for risk of spread

The jury is still out on whether the early detection of tar spot indicates a severe outbreak later in the season. Katie Strathman, a technical service representative with BASF in Kansas, explains that tar spot prefers cool, moist conditions. However, even in warm temperatures, if enough humidity is present, tar spot can still sporulate and spread.

Strathman suggests that if you can walk through your yard in the morning and you have wet feet, that’s probably enough time in hours of leaf wetness for tar spot to spread.

Ultimately, despite finding tar spot early, the weather in July and August will determine the yield impact of the disease in 2024.  

Management

Fungicide applications are an important tool for managing tar spot. Farmers should start scouting early for the disease to identify which fields may be infected and to properly time fungicides.

“Late vegetative stages is when we should start scouting for these diseases,” Telenko advises. “Can you find things in the lower canopy? If so, then be ready to spray at VT/R1.” 

While waiting can be hard, Strathman doesn’t recommend managing tar spot with fungicide applications until closer to the VT/R1 growth stage. For those who decide to treat earlier, a second application is needed.

“The rule of thumb for tar spot is that you really see about 21 days of protection from a fungicide application. There’s no product on the market that has long enough residual from V3 to V5 all the way to harvest,” she warns.

“With lower corn prices, let’s hold on to our fungicides and put a well-timed fungicide out where we need to protect that ear leaf and above,” Telenko further advises. “Those are the leaves we want to protect. Right now if you’re in V5, those leaves aren’t even out there to protect, so it will be a waste of product for disease management this early.” 

Strathman reminds farmers that protecting corn from other diseases in the reproductive stages is important as well. 

“While tar spot is top of mind because it is the new, flashy disease, we still have to have a mindset to manage the diseases that continuously come and knock on our door every season, like southern rust, that we still need to protect our crop from,” Strathman says. 

While it’s too early to know how severe disease outbreaks may be during the 2024 growing season, farmers should be on the lookout for symptoms so they can manage accordingly.