Gray Leaf Spot – A Growing Concern in Central Iowa

By: Craig Allaman, Lead Agronomist at Cornelius Seed

I recently got word from our DSM in central Iowa, and the news isn’t great: Gray Leaf Spot (GLS) is showing up in significant amounts. With the warm, humid weather we’re seeing, GLS is finding a perfect home to cycle and spread across corn fields. As your agronomist, I want to make sure you’re armed with the knowledge and tools to keep this disease from eating into your yields.

What Makes Gray Leaf Spot a Problem?

Caused by Cercospora zeae-maydis, GLS shows up as rectangular, grayish-green lesions on leaves, often starting in the lower canopy. These lesions block sunlight, reduce photosynthesis, and can lead to significant yield losses, especially in high-yield environments. The current humid conditions are like rocket fuel for GLS, and it’s already making its presence felt in central Iowa. If Southern Rust or Tar Spot joins the party, the damage can escalate fast.

Your Defense Strategy

· Fungicide Application: Applying a long-residual fungicide at the R1 stage is critical for GLS control. These products have shown strong efficacy against GLS, so if you’ve applied one, keep scouting to ensure it’s holding. If you haven’t, now’s the time to act.

· Hybrid Selection: Our Product Guide lists ratings for hybrids with strong GLS resistance. Top picks include C5704, C6203, C6204, C6377, C6523, C7048, C7235, and C7399 (all rated Excellent for Anthracnose Stalk Rot, which often correlates with GLS resistance). Hybrids like C5527, C5806, C5917, C6007, C6056, C6713, C7120, and C7427 are rated Very Good for GLS. Planting these can give you a head start.

· Scout Diligently: GLS starts low in the canopy, so check lower leaves regularly. If you see those telltale lesions, report them to our team. This helps us refine hybrid ratings and tailor recommendations.

Looking Ahead

GLS is a persistent threat in humid conditions, and with Southern Rust already active, we could see a one-two punch that hurts yields. August will also bring risks from Northern Corn Leaf Blight and Tar Spot, so don’t let your guard down.

Act Now

Download the Product Guide to find the best hybrids for GLS resistance and soil types in your fields. Scout early, apply long-residual fungicides at R1, and contact us at Cornelius Seed for personalized advice. Let’s keep GLS in check and protect your corn crop!