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How Much Gray Leaf Spot Pressure Warrants Fungicide Spraying?

In the past week I have started to see some corn leaf diseases, specifically Gray Leaf Spot (GLS), in many corn fields. The GLS has been seen in both both corn/corn on corn after soybean rotations. The picture below was taken from a field that was corn after soybeans and was not pollinating yet. As you can see there was plenty of disease on this leaf.


Why are we seeing pressure this early? Humidity levels have been high. When we look at the past few years (chart below) you can see that relative humidity is just as high this year as it was last year and well above 2010-2012. High relative humidity coupled with later planting in many areas (at least later than 2013), has given us more disease pressure.


If you check your fields, how do you determine if there is enough pressure to warrant fungicide sprays? Here are a few tips:

•    If lesions have been spotted 3 leaves below the ear leaf during pollination (or before) on 50% of the plants then pressure is high enough to spray.

•    Check the Gray Leaf Spot rating for your hybrid. If you have a susceptible hybrid, then you have a greater chance for an economic crop response.

•    Corn on corn rotations should have greater pressure than corn after soybeans so they should be monitored more closely.

•    Fields that have a tendency to stay wetter longer (such as river bottoms and low-lying ground) or areas where fog hangs around in the morning (you usually see this in August) are more likely to have greater pressure and see a greater response to fungicides.

The picture below shows how different genetics can handle GLS pressure. The leaf to the left is from a different hybrid than the leaf to the right (it was in a plot), but you can see how the hybrid to the right is more susceptible to GLS.


I have seen Northern Corn Leaf Blight (picture below) in small amounts, but not to the same extent as GLS.  


With the continuous rain in many areas of Indiana and Ohio the disease pressure is likely to increase. Fungicide applications are a more difficult decision this year due to lower corn prices. Contact your dealer or seed advisor to see if fungicide applications are right for you.

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