News Story

Agronomy Update - Corn Diseases

Brian Weller, Lead Agronomist

Our recent weather pattern is causing a few conversations about corn diseases and the use of fungicide. Early training taught me that in a dry year you worry about insects and if it’s a wet year you worry about diseases.


We may have a few diseases to be concerned about if this holds true. Some of the corn diseases we see are Northern Corn Leaf Blight (NCLB), Gray Leaf Spot (GLS), EyeSpot, Common Rust, Southern Rust and Physoderma. Things these diseases have in common is they require water on the leaf surface of 6-18 hours. Remember, even a heavy dew can cause a leaf to be wet for 6 hours or more. Northern Corn Leaf Blight, EyeSpot and Common Rust and are prone to happen in temperatures between 60-75°F. Gray Leaf Spot, Southern Rust, and Physoderma happen when temperatures are between 75-90°F.


Cooler Temperature Diseases

Warmer Temperature Diseases

Northern Corn Leaf Blight 64-81° F

Gray Leaf Spot 75-85° F

Eye Spot

Southern Rust 77-82° F

Common Rust 61-77° F

Physoderma 73-90° F


Another factor that most diseases have in common is that they overwinter on residue and infect the plant from the lower leaves first and move up into the canopy, removing the photosynthetic capabilities of the plant. Common and Southern Rust are blown in on mainly southernly breezes. By decreasing the photosynthetic area of the leaf, you take away yield. Critical to the leaves on the upper portion of the plant. Holding onto leaves in the lower canopy is also critically important. The lower leaves produce energy to maintain a functioning root system and stalk which increases standability/harvestability of the plant. 


In some instances, some of the diseases can look similar which may make identification more difficult.

Symptomology by disease is as follows:


•    Northern Corn Leaf Blight-long narrow “cigar-shaped” tan or greyish lesions that form parallel to leaf margins are 1 to 7 inches long



•    Eye Spot-small 1/16-inch water-soaked circular lesions with a tan center and darker brown margin. This spot is surrounded by a yellow “halo” which when held to the light looks like an eye


•    Common and Southern Rust- this spore creates a brown or brownish red spore/pustule that when rubbed, leaves a brown or orange stain that to me, looks just like rust on a vehicle.


•    Gray Leaf Spot- starts out as a small lesion with yellow halos like Eye Spot but as the lesions mature they elongate into narrow brown and gray rectangles. Agronomists who are color blind may not distinguish between NCLB and GLS in its early stages. These lesions expand out parallel to the vein and are 1.5-2 inches long


•    Physoderma-Brown spot lesions appear as small round type, yellow spots on the leaf blade, sheath and stalks. These spots may occur in bands across the leaf blade and with a creative eye/mind could be likened to the “W” on Charlie Browns shirt or the Batman symbol.


Fungicides work best when they cover a large portion of the plant or with applications used in a sequence. Examples would be V4 to V8 applications, followed by a tassel application. It is my recommendation that regardless of a single or multiple application the following tips allow fungicides to work better:


1.    Water and coverage are a must. WATER! WATER! WATER!


2.    Quality Nutrients, or Micronutrients, help the fungicide find its way into the plant and work better. They may make the mixture heavier, helping the water droplets go further into the canopy and provide better coverage.


3.   Quality Drift Control Agents, like Masterlock®, put more active ingredients on the plant, deliver more activity, reduce driftable fines, and provide a higher ROI.


4.    Newer, more active fungicides with multiple modes of action may cost a few dollars more but provide a quicker response in the plant and lessen the potential for fungicide-resistant diseases.


Not all Hybrids or Genetic Families react the same to a fungicide application. Some provide a better ROI than others. Contact your local Central Farm Service or Central Advantage Representative to see which hybrids show a better ROI.