We have seen positive results in terms of disease control and yield for foliar fungicide applications made before tassel, at tassel, and even as late as R3.
However, from year to year, the most consistent results and the greatest benefits are seen when applications are made at tassel or silking (VT/R1).
The diseases we are most concerned about in Ohio such as gray leaf spot and eye spot usually develop from the lower to the upper leaves of the plant. Fungicides are applied to prevent these diseases from moving up and reaching the ear leaf and the leaves above the ear leaf before grain fill is complete.
Some diseases like rust and northern corn leaf blight may show up first on the upper leaves, but in most years this usually occurs too late (after R3) to significantly affect grain yield. By the time we get to brown silk (R2), the number of rows of kernel per ear and the number of kernels per row are already made.
Therefore, applying a fungicide at or after R2 will not change these numbers, even if disease lesions develop on the upper leaves. However, severe blighting of the upper leaves after R2 may affect the size of the kernels, and as such, we may see a benefit form applying a fungicide between R2 and R3.
Based on reports coming in from across the state, most of the corn in Ohio is at the R2-R3 growth stage, with very little or no disease on the upper leaves.
However, gray leaf spot, eye spot and southern corn leaf spot have been reported on the leaves below the ear in some fields planted with susceptible hybrids.
Before making a decision to apply a fungicide, walk the field and look for disease lesions and determine the growth stage of your crop. If your crop is at R1 or early R2, and you have a susceptible hybrid, and there are disease lesions on the leaves below the ear, you may see a benefit from a fungicide application.
However, if your hybrid is resistant or moderately resistant, your crop is past brown silk (R2), and lesions are restricted to the leaves below the ear, you are less likely to see a benefit from applying a fungicide. In addition to not always being as effective as VT/R1 applications, late applications may be off label. Some fungicides cannot be applied after R3 or R4.
Before making an application, please read product labels carefully.
By Pierce Paul, Ohio State University