How late is too late for corn foliar fungicides?
Early fungicide applications are intended to serve multiple purposes. Some of those purposes are to protect the photosynthetic leaf area in the upper canopy, to prevent disease development that could predispose the plant to stalk rots and to reduce plant stress during the critical pollination period (VT-R1).
By the R2 growth stage (also known as the blister stage), the kernel number is set and disease management objectives narrow to protect the photosynthetic leaf area required to add dry matter weight to the kernels.
Deciding when to spray
The likelihood of driving total yield response at later growth stages is diminished relative to the opportunities that exist in early reproductive stages.
The answer for fungicides is rarely clear, but our best guide is to consider the risk factors in the disease triangle and make an educated guess on what is likely to happen.
Risk factors could include being a susceptible host, a hybrid with low-to-moderate resistance ratings, a favorable environment for disease incubation, and the presence of infectious pathogens — such as lesions progressing above the ear leaf, increasing severity of a disease.
Rainy and/or humid weather is most favorable to foliar diseases. In growing seasons when these conditions prevail, the risk for disease development increases.
With that said, we will soon be experiencing the R3 growth phase in many corn fields in Iowa. Data from university studies suggest that a fungicide application after R2 will slow disease development, but may not always result in an increase in yield.
There are more fields that did not get sprayed for diseases than fields that did. Now is the time to go back and look at disease like northern corn leaf blight and evaluate the recommendations that were given.
Barker is a Growmark western region senior field sales agronomist. His email address is email@example.com.
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