Fungicide Use in Field Crops: Benefits and Risks

Mary Burrows, Extension Plant Pathologist
Jessica Rupp, Extension Potato, Sugarbeet, and Pulse Crop Pathologist

Contents

1. Treatment Options
2. Fungicides for Disease Management
3. Other Recommendations
4. Economic Value
5. Further Information

Figure 4. Fungicides are effective tools for disease management. Left, fungicide treated; Right, untreat- ed. Diseases: Stem rust and stripe rest of wheat, inoculated trials in Montana. Photo by Mary Burrows.

Plant diseases are caused by many different organisms including fungi, bacteria, viruses, nematodes, and other organisms. Fungicides are pesticides used for controlling fungal and fungal-like diseases and can be applied as a seed treatment, in-furrow application or foliar spray (Figure 4). They include both synthetic and non-synthetic options such as copper, sulfur and oils.

There are also biological control options and products that induce the immune system of the plant, but here we will focus on synthetic fungicides. Fungicides work, in general, by blocking a specific metabolic pathway in the fungus that prevents spore germination or hyphal growth. These different mechanisms are called “modes of action” (MOA).

Before deciding to apply a fungicide, you should ask yourself several questions:

1. Are the symptoms I'm observing due to a fungal disease?
2. Is the fungicide I'm considering effective on the plant disease of concern?
3. Do the economics of the system justify the application?

Plant diseases can be difficult to diagnose. Diagnosis is easier with experience, but comes down to a combination of familiarity with symptoms and look-alike symptoms, an investigation of the pattern and timing of symptom appearance, and, when needed, testing for the pathogen of interest for confirmation. When you need assistance with disease identification, contact your local county or reservation Extension agent or the Schutter Diagnostic Laboratory (406-994-5150).

Treatment Options

Ascochyta blight on chickpea (pictured) has developed resistance to strobilurin fungicides in Montana. Photo by Mary Burrows.

After you have identified the plant disease you are concerned with and confirm that it is fungal, treatment options can include fungicides. However, not all fungicides are effective against all fungal diseases. Good sources of information about fungicide efficacy and rates include the product label and Extension sources. We have tables that list the products that are currently registered and their efficacy or registration status on some widely planted crops in Montana such as wheat, pulses, and potatoes. These can be found on the Regional Pulse Crops Diagnostic Laboratory website, the MSU Plant Pathology website, the MSU Plant Sciences Plant Pathology website, and the MSU Seed Potato Certification Program website.

Other sources for identifying what products are registered on your crop are available from CDMS, Greenbook, the NDSU Fungicide Guide, the Pacific Northwest Plant Disease Management Guidebook and other sources. A list of products approved for organic production can be found at the Organic Materials Review Institute (OMRI).

Fungicides for Disease Management

When using fungicides for disease management, the principles of Integrated Pest Management (IPM) should be used to avoid resistance development, including:

1. Preventative Cultural Practices
   • Use best management practices including using high quality, pathogen-free seed, crop rotation, using an adapted crop variety, optimal seeding rate, planting date, irrigation practices, fertilization, sanitation including breaking the ‘green bridge,’ etc.
2. Monitoring
   • Scout your crop for pests regularly and get pests accurately identified. Use degree-day models where available to determine when the pest is likely to reach medium-high to high risk.
3. Acceptable Pest Levels
   • Determine what level of the pest you are willing to tolerate.
4. Mechanical Controls
   • Remove infected plants from the system to prevent reproduction and spread of the pathogen (rogueing).
5. Biological Controls
   • Natural biological systems can mitigate pest damage. Beneficial insects that predate on or parasitize insect vectors of plant viruses and biological controls.
6. Responsible Use
   • When a pesticide is needed, follow all label restrictions and use the best application methods possible to target the disease of interest. If lack of efficacy is suspected, leave an untreated strip to compare with treated areas.

If level of disease is the same, then a symptomatic sample should be sent to a diagnostic clinic.

Other Recommendations

Practices to prevent fungicide resistance:

Economic Value

Figure 5. Example of the Fungicide Decision Tool user interface, titled Change in Net Revenue per Acre from Fungicide Application. The largest feature is a line graph; its Y-axis represents change in net revenue per square acre, and the X-axis is Wheat Prices per Bushel. Below the graph are three horizontal slide selectors: Cost, Drive Down, and Yield Gain.

The economic value of a fungicide application needs to take into consideration the value of the crop, the price of the application, and the expected yield benefit of the application. The MSU Extension Fungicide Decision Tool can help inform the choice of whether to apply fungicide by calculating the change in net revenue from fungicide application over a range of wheat prices. An example using the decision tool is shown (Figure 5).

Further Information

If you have questions about the use of fungicides on your crop, please contact your county or reservation Extension agent or one of the plant disease specialists with MSU Extension.

Originally published in the Spring 2017 issue of Montana IPM Bulletin. For more information, contact Mary Burrows, Extension Plant Pathology Specialist.

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