Illinois Researchers find Exotic Sources of Resistance to Tar Spot in Corn

By Lauren Quinn

When tar spot—a fungal disease of corn capable of causing significant yield loss—popped out of nowhere in 2015, Midwestern corn growers were left scrambling to manage the outbreak with few effective tools. The industry has since made some progress toward management with fungicides, but many researchers agree resistance is the path forward for living with tar spot.

"There are people looking at using mixed-modes-of-action fungicides, but more research is needed to optimize the timing for that. Ultimately, I think having resistant hybrids is the key to managing this disease long term," says Tiffany Jamann, assistant professor in the Department of Crop Sciences at the University of Illinois and co-author of a new study suggesting certain tropical corn germplasm may provide good sources of tar spot resistance.

Jamann and her collaborators grew 25 corn lines from the USDA's Germplasm Enhancement of Maize (GEM) project in nine locations across four states. Two accessions derived from germplasm from Cuba and Brazil—GEMS-0066 and GEMS-0226—showed promising levels of tar spot resistance, regardless of where they were grown.

"They consistently performed among the top of all the lines in each environment. Showing similar levels of resistance across locations is a really good indicator they're reliable," says Sarah Lipps, doctoral student and lead author on the study. "And because these lines are available to the public, anybody can use them in a breeding program to develop resistant hybrids."

Jamann notes that "these two accessions weren't perfect. 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."

In addition to identifying two promising sources of resistance, the study 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% scale," Lipps says. "But with tar spot, because it makes these small dots on the leaf, it's really difficult to accurately estimate 5% versus 20%, etc. It's also hard to estimate disease on a percentage scale across thousands of plants for this disease."

"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."