There’s A Fungus Among Us

Factors to consider to determine whether a fungicide is the right answer

Farmers must consider many factors when deciding if fungicide treatments will make or break profitability in your field. The first three of those include the pathogen, the variety of soybean in the field and the environment/weather. This Plant Disease Triangle can help determine if disease will negatively impact yield.


“While foliar fungicides are an import component of integrated disease management in soybeans, these applications are not always a profitable solution,” warns Hillary Mehl, Ph.D., Extension Plant Pathologist at Virginia Tech Tidewater AREC.

In the Focus on Soybean webcast “Integrated Approaches to Fungicide Applications in Soybeans,” Mehl discusses strategies to determine if using a fungicide is the best course of action. She wants farmers to be profitable and encourages growers to consider three numbers before applying a fungicide:

•  Your anticipated yield
•  Cost of the treatment
•  Price per bushel

This chart shows a clear relationship between the cost of chemical application and anticipated yield response in determining the profitability of a fungicide application.

In field trials conducted in Virginia to document the effect of fungicide treatments, only 50 percent of treated plots saw improved yields over the control. The average increase in yield was 2.2 bushels per acre. This study demonstrates that without careful understanding of field conditions, fungicide applications can be a waste of money. Beyond yield, treatment cost and price per bushel, there are other factors that can help you define if a fungicide treatment is needed.

• Know how susceptible or resistant your variety of soybeans is to certain pathogens
• Know your yield potential
• Know what was previously planted and the cropping system used
• Understand what disease is present and if the fungus is resistant to certain fungicides
• Know the weather conditions and what conditions are favorable for pathogen growth

Fungal diseases have the greatest impact on yield if it flourishes during pod development. Once your crop starts developing pods, it is important to scout your fields for signs of disease development. Make sure to check the foliage, stems and roots for disease, and be prepared to take proper samples so that pathogen identification can be made. Be aware of crop advisories from the Extension service, as these can be a heads up to be on the lookout for certain diseases in your area.

If the presence of a known disease is detected, then consider the following when developing a plan for fungicide treatment:

• Proper timing of the fungicide application – remember, applications that protect the plant during pod development are desired. Fungicides typically protect plants from infection, they do not “cure” existing disease symptoms.
• Weather conditions – many pathogens flourish in extended periods of warm, moist weather. The forecast must be conducive to fungal growth, or a fungicide may not be needed.
• Fungicide selection – use appropriate treatments for the fungus that is present, and follow the instructions on the label. Failure to follow the label may support development of fungicide-resistant fungal populations.