Overall disease pressure for the 2026 growing season has been relatively low, especially compared to the southern rust outbreak we experienced last growing season. Gray leaf spot, northern corn leaf blight, bacterial leaf streak, Curvularia leaf spot, southern rust, tar spot, Goss’s wilt and Physoderma brown spot have all been observed in corn fields across Iowa this growing season. However, no one disease has become particularly severe and threatened yields so far.
In fungicide trials across the state, disease severity on the ear leaf has averaged less than 2% so far, this growing season. When disease severity at R5 (dent) is greater than 5%, a positive return on investment for fungicide application is likely. Despite periods of high humidity and extended leaf wetness, the yo-yoing of temperatures has likely helped slow a specific disease from really taking off.
With the recent wet conditions there have been concerns raised about late-season disease development and potential management considerations. Since we don’t have a crystal ball on exactly what the weather will bring in the next few weeks before maturity, it is difficult to predict what disease may become more prevalent.
Since diseases develop exponentially, it wouldn’t be surprising to see disease pressure in fields increase. Regardless, it is likely too late for it to impact yield much. If fields were sprayed with a fungicide closer to the R2 (blister) or R3 (milk) growth stage, they are likely still being protected from that fungicide. However, for fungicides that were applied around VT (tassel)/R1 (silking), they have likely reached the end of their effective period, and those fields will be susceptible to infection.
Given the fact that a lot of corn fields across the state are now in the R4 (dough) and some are even at R5 (dent), there really isn’t much we can do at this point in the growing season to manage for disease. Once corn reaches R4 and beyond, fungicide applications are much less likely to provide an economic return. The exception is in years with unusually high disease pressure, such as the southern rust epidemic experienced in 2025. We haven’t seen that yet this year.
We can, however, be thinking of disease management for 2027. All diseases, except southern rust, overwinter in corn residue. Consequently, if a disease does escalate in these last few weeks, there will be more inoculum available next year. Visiting with your seed dealer about which hybrids have better tolerance to disease, starts you off on a firm foundation for disease management in 2027.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 17, 2026. The information contained within may not be the most current and accurate depending on when it is accessed.
- Field Agronomist in EC/SE Iowa
- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist