Is this Curvularia leaf spot or something else?

The corn crop is advancing in growth stages, and farmers, agronomists, and crop scouts are all keeping a close eye on fields to monitor for the development of foliar disease. While some diseases are present in fields, pressure is significantly lower than in 2025. Many images have come in of other corn issues – some disease, some not. Questions about what is and isn’t Curvularia leaf spot, a new disease to Iowa, are by far the most common.

Curvularia leaf spot (Curvularia lunata) was first reported in Iowa in 2025. It has also been reported in other states since 2017. As reported in a prior article, this is a major disease of corn in China, and the effect on U.S. corn yields is currently unknown. Crop Protection Network reports that this has been “more of an aesthetic issue than a yield-limiting disease in the United States.”

Curvularia typically shows up in the mid- to upper canopy of corn, and looks very similar to eyespot – identification of spores under a microscope is usually required to distinguish between the two diseases. Eyespot typically shows up earlier in the growing season and, thus, is usually lower in the canopy. Curvularia lesions are generally circular or oval-shaped, tan- or white-colored with a brown margin and will often have a yellow halo (Figure 1). Younger lesions may be tan- or white-colored and have a greasy or water-soaked appearance around the center (figure 2). Many online reference images show numerous lesions on leaves, but we’ve noted only one to a few lesions on most leaves in corn fields this summer.

Many reports have come in of suspicious small spots on leaves in the upper corn canopy wondering whether these are Curvularia leaf spot. In some cases, they’re widespread across a single hybrid in corn fields. Often, these spots are difficult to spot from the top of the leaf but easy to see when backlit by the sun (Figure 3). These appear to be abiotic and unrelated to foliar disease. Our Plant and Insect Diagnostic Clinic investigated several samples with these suspicious spots under a microscope and found no evidence of bacterial or fungal disease aside from saprophytes on the corn leaf surface where pollen and dust had accumulated. 

These spots might be a hypersensitive reaction to the saprophytic fungi and bacteria colonizing the pollen and dust.  A hypersensitive reaction is death of few cells in response to infection by non-host pathogens. Since these spots seem to occur on certain hybrids, this symptom may also be the result of a genetic x environment interaction. 

As we continue through reproductive stages, the potential ROI of foliar fungicide decreases, but the importance of correct identification of possible diseases before making decisions does not. 

1. Consider sending leaf samples to a lab like our Plant and Insect Diagnostic Clinic.
2. Monitor fields for change – do suspicious lesions in the canopy appear to get worse over several days or do you notice no change?
3. We currently have no efficacy data for foliar fungicides on Curvularia leaf spot and most corn is past the growth stage where a fungicide application would be profitable.

Keep an eye out for Curvularia since it is a relatively new disease to Iowa and be wary of potential look-a-likes in corn fields this time of year, especially if the infestation seems remarkably severe or evenly distributed through a field or hybrid.