Fungicide and the economics of the disease triangle


Sometimes, the decision of whether to apply fungicide, or any other pesticide, is easier than others.  When making the decision at the margin, or considering whether to apply fungicide in the absence of disease, the calculation can be more challenging.  The MSU Extension Fungicide Application Calculator is one tool in your toolkit to help make that decision.

To find out if chemical application pays off, one must weigh the benefits against costs.  Benefits in this case are the value of any additional yield gained from the application.   Costs include the chemical itself and the costs of applying it.

The value of the crop, the chemical cost, and the cost of application are relatively easy to estimate. However, estimating the expected yield benefit of the application is more difficult. The yield benefit estimate must consider how the fungicide will enter into the disease triangle.  The disease triangle is a model that helps explain how different components interact to form disease outcomes.   Its three corners  are pathogen, environment, and host.  Here, pathogen refers to whether the disease is present.  Environment would be upcoming weather conditions.  Host refers to how the crop variety reacts to the disease. The yield benefit must also factor in the efficacy of the product, given the application method and timing.

Disease Triangle; CC BY-SA 3.0/Earlycj5

The MSU Extension Fungicide Decision Tool can help in deciding whether fungicide should be applied by calculating the change in net revenue from application, over a range of wheat prices. An example is shown at the top of this post.

You can experiment with changing the application cost, drive-down (loss from wheel tracks when spraying, if applicable), and expected yield gain.  The line changes color depending on whether the application would bring about a negative or positive change in profit.

In the example here (click on the image to see all of it), the grower’s fungicide application cost is $10 per acre. Note that this includes the cost of chemical, as well as costs to operate equipment used in application. The expected drive-down from the application is 1.3 bushels per acre. (Some guidelines for estimating drive-down are provided on the website.)  If fungicide is applied along with another chemical, the cost of application and drive-down may be less for each as the equipment costs can be split between them. The yield gain from fungicide application is estimated at 4.2 bushels per acre. Note that there are wide ranges in estimates of the yield benefit from applying fungicide. These depend on each of the three corners of the disease triangle and the chemical itself.  For this operation, the benefit from fungicide application exceeds the cost when the price of wheat is $4 per bushel or more.

For more extensive information on fungal diseases, fungicides, modes of action, and resistance, see the Fungicide Use in Field Crops MontGuide, written by Mary Burrows, Jessica Rupp, and myself.


About Author

Kate Binzen Fuller is an assistant professor and extension specialist in the Department of Agricultural Economics and Economics at Montana State University. She holds an M.S. and Ph.D. from the University of California, Davis. Her extension and research program focuses on the economics of farm management decisions, including USDA programs and policies, pest and disease responses, and issues surrounding leasing and land values. Kate’s extension program takes her on the road often, resulting in a rapidly expanding knowledge and appreciation for Montana’s interstates, highways, and (especially) gravel roads.

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