A new fungal threat is emerging in U.S. cornfields. Curvularia leaf spot, a disease historically associated with tropical climates, has now been confirmed in central Iowa, marking its first appearance in the Corn Belt and the farthest north the disease has been detected in the United States.

Alison Robertson, field crops pathologist with Iowa State University Extension, verified the presence of the disease on August 14, after samples from a field trial and demonstration site in central Iowa showed signs of infection. The arrival of the disease in a major corn-producing state is prompting heightened concern among agronomists, pathologists, and corn producers alike.

"We don't know a ton about this disease," Robertson stated, "but there are definitely differences in hybrid susceptibility. We still don't really understand what kind of impact on yield it will have, if any."

Curvularia lunata, the fungal pathogen responsible, was first reported in the U.S. in 2017 in Louisiana and has since spread to states including Delaware, Kentucky, and Mississippi. Its discovery in Iowa underscores how climate patterns and cropping systems may be expanding the disease's range.

Symptoms and Identification
Curvularia leaf spot presents as small, tan-colored lesions with brown margins, often surrounded by a yellowish halo. Lesions typically appear on leaves in the mid to upper canopy, but can form at any growth stage. They may be scattered or cluster to create larger necrotic areas. The disease can be mistaken for eyespot, though it favors hot, humid conditions, especially after rainfall during reproductive stages.

According to the Crop Protection Network, the disease tends to develop when corn follows corn and thrives during frequent precipitation and extended warm periods-exactly the conditions much of Iowa experienced this summer. "The warm and wet season definitely favored the disease," Robertson noted. "Whether we'll see it again depends on whether we continue to have such summers."

Impact and Global Context
Though its effect on U.S. yields remains unknown, the disease has caused significant damage abroad. In China, Curvularia leaf spot is considered the third most destructive corn leaf disease, following northern and southern leaf blight. Yield losses in China have ranged from 1.7% to 36.9%, with an average of 15.4%, depending on the estimation model used.

The concern now is whether the disease will become established in the Midwest. "The big question here in Iowa is, will it survive our winters?" said Robertson. Cold temperatures could limit the pathogen's persistence, but that remains to be seen.

Management Options and Future Outlook
Currently, no foliar fungicides are labeled for managing Curvularia leaf spot in the U.S. However, crop rotation and tillage are effective in reducing the fungus's survival. Ongoing surveillance and research will be key to understanding the threat it poses.

Efforts are underway to model the disease's spread. Curvularia leaf spot is now among the pathogens included in the National Predictive Modeling Tool Initiative, and it will eventually be part of the Crop Disease Forecasting Tool developed by the Crop Protection Network.

"The next step for Iowa," Robertson concluded, "is to determine how widespread the disease is this season. Then, we'll monitor its progression in the coming years to evaluate whether it becomes a persistent threat."

As Iowa farmers prepare for harvest and begin planning for 2026, experts urge them to remain vigilant for symptoms and consult with Extension specialists on the latest diagnostics and recommendations.