When Corn Diseases Pile Up, Yields Fall Fast — Here’s How to Fight Back

Every year, when talking with growers during the summer, one question always seems to be on their minds: “Are you seeing tar spot this year?” That particular pathogen dominates the corn disease conversations, but nevertheless tar spot appears to be the most damaging when it’s accompanied by another disease or diseases, which together can lead to substantial yield losses. In 2021, tar spot in combination with fusarium stalk rot decimated fields in the central part of the state, so much so that in many cases, fields were taken for harvest just shortly after Labor Day out of concerns over rapidly deteriorating stalks. In 2025, much of the state saw both tar spot and southern rust cause substantial yield losses where pressure was high. The underlying message is that although tar spot can be a very damaging disease, it’s also important not to overlook the many other diseases that have infected U.S. corn fields long before tar spot was ever a concern, and the best way for growers to manage the risk of late-season disease infections is with a fungicide. In a 2025 research study, Winfield United observed an average gain of 12.7 bushels per acre with a single late-season fungicide application¹. When considering what fungicide to use for a late-season corn application, look first at the number of active ingredients in the product. A single-mode-of-action product is not recommended for these types of applications because they are simply not effective enough at combating the dynamic disease environment often present in corn fields late in the growing season. A product with at least two modes of action is required, and three is better yet. Currently, many fungicide products are available for corn, and growers should work with their local agronomists to determine which product best meets their needs. Along with choosing the appropriate fungicide, growers should consider including an adjuvant with the application. Corn fields often exceed eight feet in height at the time of late-season fungicide applications, and the effectiveness of the application can be increased when an adjuvant is used to help increase the overall coverage of the plants and drive more fungicide across the leaf wax cuticle layer and into the plant. In a 2019 study, MasterLock® adjuvant increased the average response of a late-season fungicide application by 4.7 bushels². Also, the late application window for fungicides on corn can be a great opportunity for growers to address other problems that their crop may be struggling with, such as micronutrient deficiencies and other abiotic environmental stresses like drought and/or excessive heat. Alleviating these non-disease issues can help add more bushels and boost the overall return on investment of the application. Again, growers should work with their local United Cooperative agronomists to help identify these potential opportunities and determine the best course of action.
 
 
 
¹Research was conducted at several 2025 Winfield United Answer Plots. Yield responses from fungicide applications can be variable and do not always reflect the same results as an individual study.
²Results are from a study conducted in 2019 at Answer Plot locations in ND, SD, NE, KS, IA, MN, WI, OH, IN, CO, TN, and AR