URBANA, Ill. — As tar spot continues to spread in the Midwest and eastern Corn Belt, researchers are coming to a consensus that resistant hybrids hold the key to managing the corn fungal disease. 

Though progress has been made with fungicide management, a recent study by the University of Illinois (U of I) appears to support the theory that resistant hybrids are the key to long-term management of the yield-robbing disease, which was first discovered in the Corn Belt in 2015. This is according to Tiffany Jamann, assistant professor for the U of I Department of Crop Sciences, who led a team that grew 25 corn lines from the USDA’s Germplasm Enhancement of Maize (EM) project in nine locations across four states in order to study tar spot. The team’s research identified two varieties derived from germplasm ( a collection of multiple, related germs) from Cuba and Brazil (GEMS-0066 and GEMS-0226) which showed “promising” levels of tar spot resistance, regardless of location.

“These two accessions weren’t perfect,” Jamann said. “There was still a little bit of disease on them, but I think it’s a good starting point. There are definitely improvements to be made, but they were consistent across a lot of environments for us.”

Study author and doctoral student Sarah Lipps said the accessions consistently performed among the top of all the lines in each environment, and showed similar levels of resistance across locations. “(This) is a really good indicator they’re reliable. And because these lines are available to the public, anybody can use them in a breeding program to develop resistant hybrids,” she said, adding that the study also offers a new method of scoring tar spot incidence and severity in the field. 

“Generally speaking, when we rate foliar diseases, we use a 0-100 percent scale. But with tar spot, because it makes these small dots on the leaf, it’s really difficult to accurately estimate 5 percent versus 20 percent, etc. It’s also hard to estimate disease on a percentage scale across thousands of plants for this disease,” Lipps said. “Looking in the literature, we found a rating scale used for anthracnose — another fungal disease — that considers incidence as well as coverage in the plot. So we developed a one-to-nine rating scale for tar spot. It is somewhat similar to what is being used in Latin America to score tar spot and works well for our purposes.”

Researchers looking into tar spot have faced a major stumbling block in that the disease can not be cultured in a laboratory. To introduce tar spot into their test plots, the U of I team scattered infected plant residue into plots, or, in some cases, planted corn into fields that had been infected during the prior year. 

According to AgriGold agronomist Joe Stephan, it is important for corn growers to understand that no hybrid is resistant to tar spot. Though highly tolerant hybrids usually stay healthier for longer periods of time, they may still be affected by the disease. In an April 26 report, Stephan advised growers who are concerned about tar spot to plan their fungicide strategy immediately.

“Farmers should scout for the disease as the canopy starts to close and humidity rises. Keep in mind if it’s too wet to scout, conditions are ideal for tar spot,” said Stephan, who recommends growers make two fungicide applications to treat tar spot.

“Some farmers apply fungicides at planting and then plan for aerial application at the R3 stage. Others choose to spray at V8 to V10 stages and then follow up with another spray later,” Stephan said. “For those looking to make one application, the ideal time is just after tasseling, or before tasseling if humidity is high.”

Speaking at the Illinois Fertilizer and Chemical Association convention in Peoria in January, University of Wisconsin-Madison (UWM) associate professor and Extension plant pathologist Dr. Damon Smith said that untreated tar spot becomes apparent around the R3 corn growth stage, making the V8 to R4 growth stage the best time to manage tar spot. “By the time we get to R5 we can see a pretty high disease level, but a resistant hybrid can give you a nice reduction,” said Smith, who recommended farmers consult with their seed dealer when selecting resistant corn hybrids. 

Stephan strongly encourages farmers to avoid a “wait and see” attitude when it comes to fungicide applications and tar spot, due to potential limitations in fungicide supply and infrastructure to get it applied. “Farmers should be prepared and have their fungicide purchased and ready to go,” he said. “With current corn prices, spraying for tar spot is very cost effective when you factor in a potential loss of 20 or more bushels per acre.”

Though some researchers are supportive of mixed-mode fungicides to battle tar spot, Jamann said that more needs to be learned. “There are people looking at using mixed-modes-of-action fungicides, but more research is needed to optimize the timing for that,” she said. “Ultimately, I think having resistant hybrids is the key to managing this disease long-term.” 

Smith recommends the free TarSpotter mobile app, available for iPhone and Android phones, for farmers who want to diagnose tar spot probability in their corn fields. Developed by the UWM, TarSpotter uses GPS coordinates to determine if weather has been favorable for the development of tar spot fungus during corn flowering in a specific field. Models in the app use local weather to predict favorable conditions for most corn growing regions to generate site-specific risk predictions.