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Why apply fungicides on peanuts but hesitate on other crops?

Fungicides protect yield and increase profitability. However, unlike in peanut, the need for fungicides can vary from field to field across the region.

Bob Kemerait, Plant Pathologist

May 31, 2024

4 Min Read
Brad Haire

A correction was made to paragraph seven to clarify diseases that overwinter.

"To spray, or not to spray” is a question often asked for fungicides. Can you imagine not applying an herbicide?  Is there usually question about need for an insecticide? Often there is not the same conviction for use of a fungicide, especially for field corn, soybean and cotton. 

Why is there such conviction to apply fungicides to peanuts but hesitation to other row crops in the Southeast?  First, like weeds and insects, we know that leaf spot diseases and stem rot/white mold are almost certainly going to affect every field of peanuts.  We know that if not effectively treated, leaf spot and white mold diseases can easily reduce yields by 50%. 

Corn, cotton and soybeans grown in the southeastern United States are affected by fungal diseases that result in crop loss. Northern corn and southern corn leaf blights, especially where a susceptible variety is planted, southern corn rust, and perhaps, tars spot can all rob a corn crop of yield.  Target spot and areolate mildew can rob cotton growers of yield.  Soybean rust, Cercospora leaf blight and frogeye leaf spot are examples of diseases that can cut soybean yields. 

For these crops and these diseases, fungicides can be used to protect yield and increase profitability. However, unlike in peanut, the need for fungicides can vary from field to field across the region.

Why is there is such variability in the need for fungicides, and why might Bob Kemerait in Georgia and Dr. Tom Allen in Mississippi disagree over a recommendation?  These are valid questions and below are points to consider.

Unlike in peanuts, where fungicide applications often begin 30 days after planting, applications in corn, cotton, and soybeans are typically not initiated until tasseling, mid-bloom, and pod development stages, respectively. (There can be exceptions when considering diseases like northern and southern corn leaf blights.)

Two of the most important diseases, soybean rust and southern corn rust, generally do not overwinter in the Southeast and must be reintroduced each year from somewhere tropical. The fungus causing southern leaf blight does survive in crop debris. The need to protect corn and soybeans from these diseases depends upon when the disease-causing pathogens return.

These diseases can cause yield loss, but they are unlikely to affect every field equally. The earlier a disease appears, the more likely it is to cause significant damage. When a disease first appears is largely driven by the interaction between the growth stage of the crop, the weather, and, often, the previous crops planted.  “Hot and dry” can reduce risk and the need for spraying as can good crop rotation.

Finding a disease in a field is always a reason for concern but may not be a reason to spray. If a disease appears after hard-dough/R6 in corn, after full-seed/R6 in soybeans, or within a month of defoliation in cotton, there really isn’t any need to spray.

If disease has progressed significantly in a field, it may be pointless to spray now. The disease genie is out of the bottle, and you can’t put her back in.

Why would Tom Allen in Mississippi and Bob Kemerait in Georgia differ in recommendations for the same disease and the same crop?  Sometimes it is because Mississippi growers face a different situation (weather, varieties planted, rotation) than do Georgia’s growers. Sometimes disease intensity is more severe in one state than in another. And, sometimes, it is our different experiences and different comfort with risk that drive our recommendations. Still, we both base all recommendations on research. 

To that end, the following are my general recommendations for growers, recommendations which may vary among my colleagues in other states. 

Corn growers should consider applying a fungicide as their crop approaches tassel. The final decision should be based upon yield potential, irrigation, favorable weather and presence of the pathogen. 

Soybean growers should consider spraying at the R3 growth stage, depending upon the disease situation. This is especially true if Dimilin and boron are applied at that same growth stage.  Cotton growers should be prepared to make a fungicide application by the third-to-fifth weeks of bloom, but should hold off on pulling the trigger until the disease is present or is likely present.

Another question I get asked is “Do you actually get paid to do your job?” Yes, yes, I do. I am grateful.  It’s the best job in the world.

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About the Author(s)

Bob Kemerait

Plant Pathologist, University of Georgia

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