All of the corn fields that I have seen this week look very similar to this one. I am seeing very little (if any) disease pressure.
SO … Do I spray or not? This is literally the same decision that we had to make with our wheat crop this year.
A fungicide application will cost will cost in the neighborhood of $20 – $25 per acre. At the current corn growth stage will this application pay?
Most corn studies have found the greatest yield benefit from a fungicide application to be at the VT (Tasseling) to R1 (Silking) growth stages. Intuitively, this makes sense since this is typically a time of high disease pressure, and the tissues we want to protect most are present and active. The ear leaf and those leaves that are younger are the solar panels that generate the sugars that then fill the grain. These later applications take us further into the season with the protection of this foliage.
Continued scouting is needed, especially in fields with susceptible hybrids and a history of foliar diseases. These 3 diseases are the most prevalent this time of year.
Gray leaf spot: symptoms of GLS include tan, rectangular lesions up to 2-4 inches long that first appear on lower leaves. Lesions are bordered by leaf veins and can turn gray later in season. Infection requires dew or foggy conditions and high relative humidity in the canopy.
Tar spot: this relatively new foliar disease prefers wet weather and moderate temperatures and can progress rapidly under the right conditions. The main signs of tar spot are raised, black spots called stroma that cannot be rubbed off and appear on both upper and lower leaf surfaces. In general, fungicides with multiple active ingredients (AI) are more effective against tar spot and should be applied between VT and R3 in a high disease environment (>5% severity).
Northern corn leaf blight: keep an eye out for tan, “cigar-shaped” lesions (Figure 3) that will expand up to six inches in length across leaf veins. NCLB is favored by high relative humidity and wet, but slightly cooler conditions than those that favor the development of GLS.