Corn Fungicide Decision Guide For Foliar Diseases

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Scout for foliar diseases in corn such as Northern Leaf Blight and Gray Leaf Spot just before tassel emergence and answer the following questions when considering an application of foliar fungicide:

What was the Previous Crop?

  • Many foliar pathogens survive in corn residue, so the risk of foliar diseases (such as gray leaf spot and northern leaf blight) increases when corn is planted into a field that was corn the previous year.

What has the Weather Been Like?

  • Rainy and/or humid weather is most favorable to foliar diseases. In growing seasons when these conditions prevail, the risk for disease development increases.

Does the Field have a History of Disease?

  • Some field locations may have a history of greater foliar disease severity.
  • Fields in river bottoms or low areas or surrounded by trees may be more prone to having foliar corn diseases.

summer fog photo


Gray Leaf Spot

  • High residue farming allows the gray leaf spot pathogen to build up in corn residue over time.
  • Extended periods of leaf wetness (13 hours) allow infection of leaves.
  • Typical lesions are rectangular with straight edges.
  • Lesions can grow together and kill entire leaves.
  • High relative humidity (>90%) can lead to increased disease.

gray leaf spot photo

Northern Leaf Blight

  • Infection occurs when there is free water on the leaf surface for 6 to18 hours and temperatures are 65 to 80 F.
  • Lesions are 1 to 6 inches long and cigar-shaped.
  • Yield losses are most severe when the disease infects plants early and progresses to the upper plant leaves by pollination or early ear fill.

northern leaf blight photo

What is the Resistance Level of the Hybrid?

  • If the disease resistance rating is a 6 or greater, a fungicide application may not provide a benefit.
  • For susceptible products (rating less than 4), spray if disease symptoms are:
    • present on the third leaf below the ear or higher on 50% of the plants examined
  • For intermediate products (disease rating of 5), spray if these conditions apply:
    • disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined
    • the field is in an area with a history of foliar disease problems
    • the previous crop was corn
    • there is 35% or more surface residue, and
    • the weather is warm and humid

Average yield response of Pioneer® brand products to foliar fungicide applications.

avg. yield response chart

aerial application photo

Aerial application is a frequently used method of fungicide treatment in post-tassel corn.

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