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Frequent information updates for agricultural audiences

Fungicide Use In Corn - from Mike Roegge

Posted by John Fulton -

The old adage "knee high by the 4th of July" has been used to describe corn growth for years. A more modern version would be "knee high by the 4th of June, and tasseling by the 4th of July". For the past number of years, corn tasseling on July 4th has not been uncommon at all. This year, far fewer corn fields are tasseling, but a few of the early planted ones are.

Which brings us to the topic of corn fungicide use. Application of fungicides in corn has commonly been done at tasseling or silking. Dr. Carl Bradley, U of I Plant Pathologist recently provided a summary of his data comparing leaf disease incidence and corn yield with fungicide use. His data strongly suggests that the use of a corn fungicide should be used on corn that exhibits corn leaf diseases. Gee, that kind of makes sense!

He divided his data into 3 groups based upon percent of the ear leaf showing disease lesions: less than 10%; between 10-14%; and greater than 15%. Unfortunately these observations were made about 4 weeks after fungicides were applied.

The data show that for corn with less than 10% ear leaf disease infection, the application of a fungicide increased yields by only 0.1 bushels. For corn between 10-15% disease infection, the yield increase was 4.8 bushels per acre. For corn that had greater than 15% infection, the yield increase was 14.1 bushels.

So to help you determine the benefits of corn fungicide, scout your corn fields for signs of leaf disease (gray leaf spot and northern corn leaf blight). You'll need to speculate as to environmental conditions to account for disease progress during the next few weeks to determine disease levels after application. Carl is suggestion to determine "disease risk factors" to determine potential of disease progress, which include: Susceptibility level of corn hybrid (check with seed provider); previous crop (many disease pathogens survive in corn residue); weather (rainfall and high humidity for the next few weeks increase disease likelihood); and field history.

Scout for foliar diseases in corn at tassel emergence. Current disease management guidelines suggest the following criteria for considering an application of foliar fungicide:

  • For susceptible hybrids--if disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined.
  • For intermediate hybrids--if disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined, if the field is in an area with a history of foliar disease problems, if the previous crop was corn, if there is 35% or more surface residue, and if the weather is warm and humid.
  • For resistant hybrids-fungicide applications generally are not recommended.


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