Multiple corn diseases confirmed in Kentucky
Author: Kiersten Wise, Extension Plant Pathologist
Figure 1. Tar spot on corn (photo by Kiersten Wise)
There are multiple foliar diseases confirmed in Kentucky corn fields this year. Some are familiar and annually important diseases like gray leaf spot and southern rust, but newer diseases are also present in several areas of the state.
Tar spot on corn, caused by Phyllachora maydis, was confirmed by the University of Kentucky Plant Disease Diagnostic Laboratory (PDDL) from samples collected in Henderson County (Figure 1). As of July 30, there are several Kentucky counties with suspected tar spot, although samples will need to be confirmed by the PDDL. Crop scouts will likely continue to find tar spot as the season progresses, but the impact of the disease in each field is still to be determined.
In areas where the disease is confirmed, infections likely occurred a month or so ago. The fungus that causes tar spot has a very long latent period (the time between infection and symptom expression) under optimal conditions, but in our Kentucky environment, that exact time period is unknown. Recent research has suggested that it could be 19-41 days between infection and when symptoms are observed. This could explain why we are finding it now, even though conditions are currently hot and humid.
The Crop Protection Network has recently put together a fungicide decision table (Table 1). that shows the benefit of spraying fungicide for tar spot based on when symptoms are first observed. This is similar to our fungicide decision table for southern rust and will aid in making decisions of if/when to spray for tar spot based on disease detection. Remember, fields that do not have tar spot do not need a fungicide application to manage tar spot.
Research has shown that a single fungicide application at VT/R1 is effective at preventing yield loss from tar spot and is also the best chance of seeing a positive ROI. If fields have already been sprayed for southern rust or other diseases, the decision to make a second application should be made on a case-by-case basis.
Table 1. Possible benefits (by growth stage) from applying fungicides to protect against tar spot in corn.
In addition to tar spot, we are also seeing another new disease on corn in Kentucky (Figure 2). This Crop Protection Network article describes the symptoms of the new disease. This disease is not yet named but has been present in the state since 2020. It is easily confused with other foliar diseases like Curvularia leaf spot. There has not been confirmed yield loss associated with this disease yet, but like tar spot, it is important to scout and identify the disease through the PDDL so we can learn about its spread and impact in Kentucky.
Figure 2. New disease on corn in Kentucky (photo by Kiersten Wise)
