Forecasting Corn Disease Risk For Kentucky

DR. KIERSTEN WISE

LEXINGTON, KENTUCKY

Corn growth stages in Kentucky vary widely, with some corn in western Kentucky likely to tassel this week, while in other parts of Kentucky, corn is still in the early vegetative stages. The challenges to the 2025 planting season will make foliar disease monitoring and management decisions more important going through the summer.

Frequent rainfall across the state has led to some detections of anthracnose leaf blight (caused by Colletotrichum graminicola), and some low levels of gray leaf spot (caused by Cercospora zeae-maydis) in the lower canopy of corn that is close to tasseling. Other diseases observed recently include Diplodia leaf streak (caused by Stenocarpella sp.) and Holcus leaf spot (caused by the bacteria Pseudomonas syringae pv. syringae). To date, no concerning levels of disease have been observed, but it is important to scout frequently, monitor University resources, and be prepared to act if conditions warrant foliar disease management. In most cases, applying a foliar fungicide once at tasseling/silking (VT/R1) is the most effective way to prevent yield loss from foliar diseases and offers the greatest potential for a positive return on investment (ROI).

Tar Spot Risk

Recent detections of tar spot in Indiana and Kansas have prompted concern about disease development in Kentucky, particularly in areas where corn has at least ten leaf collars (V10). Tar spot is a newer disease in Kentucky and was detected widely in the state in 2024, but was not present at levels that caused yield loss. The new Crop Risk Tool can forecast disease risk for tar spot and gray leaf spot for corn that is between growth stages V10 and R3. Model predictions for multiple areas in western Kentucky for June 9 through June 21 indicate that corn at V10 or later is at high risk for gray leaf spot development, which is not surprising with the frequent rainfall and high humidity. Tar spot model predictions for several areas across western KY indicate that corn at V10 or later is at moderate risk for tar spot development, with risk decreasing, and low risk predicted by the end of the week (June 21; Figure 1). These predictions indicate that risk for tar spot development is decreasing as temperatures are increasing over the week. It is important to continue to monitor the Crop Risk Tool as corn approaches VT/R1.

 Southern Rust Risk

One of the most important corn diseases to monitor in Kentucky is southern rust. The fungus that causes southern rust does not overwinter in Kentucky, but spores of the fungus move north on wind currents and weather each summer. Movement of southern rust can be tracked by watching the map on the Crop Protection Network (Figure 2). On the map, red counties/parishes indicate that southern rust has been confirmed by university/Extension personnel. To date, southern rust has been confirmed in Alabama, Florida, Georgia, Mississippi, and Texas. Southern rust typically arrives in Kentucky in mid-July, and whether a fungicide will be needed to manage southern rust at that time will depend on the crop growth stage at the time it is detected in an area. Fungicide applications may be needed to manage southern rust through the milk (R3) growth stage, although if corn receives a fungicide application at VT/R1, it is less likely to need additional applications if southern rust arrives after VT/R1.

Importance of Scouting

Before applying fungicides, remember to scout fields first and check hybrid resistance ratings prior to fungicide application. Hybrids that are moderately resistant or resistant to foliar diseases like gray leaf spot are less likely to demonstrate an economic response to fungicide application.

Because of the variation in corn growth stages and detections of important diseases in other states, farmers may be tempted to alter their planned fungicide application timings. Multiple foliar fungicide application timings are promoted in corn, but research at the University of Kentucky and in other states has shown that a fungicide application at tasseling/silking (VT/R1) is most effective at preventing yield loss due to foliar diseases and also provides the greatest chance of seeing a positive return on investment.  Research has demonstrated that early foliar fungicide applications that occur at V4-V6 are less likely to provide an economic gain. Another common fungicide application timing in Kentucky is the pre-tassel stage, typically between V10 and V14. Research from the University of Kentucky shows that this timing can provide disease control for gray leaf spot and southern rust comparable to a VT/R1 application in some years. However, if disease onset occurs later in the season, a second application may be required at R2 or R3. This can double fungicide input costs and reduce the chances of achieving a positive ROI.

Scouting over the next few weeks and just prior to tasseling can help determine if fungicide applications are needed.  Although disease levels will continue to build over the course of the season, University research indicates that foliar fungicides applied at tasseling or early silking (VT-R1) provide optimal foliar disease control for diseases like gray leaf spot and tar spot compared to applications that occur earlier or later in the season. For southern rust, a fungicide application may be needed through milk (R3). Always check with your county agent for updates on the diseases present in your specific county and help determining if management is warranted.   ∆

DR. KIERSTEN WISE

UNIVERSITY OF KENTUCKY