Protect the tools that protect your yield MFA agronomists share best practices for managing fungicide resistance

Fungicides have become an essential tool for protecting yield and crop quality in today’s high-input row crop systems. However, as fungicide use has increased, so has the risk of fungicide resistance. Managing that risk is critical to preserving the long-term effectiveness of these tools.

Fungicide resistance occurs when a population of plant pathogens adapts in a way that allows it to survive fungicide applications that were previously effective. Over time, repeated use of fungicides with the same mode of action can favor resistant strains, reducing product performance and limiting disease control options.

Unlike insect or weed resistance, fungicide resistance can develop quickly because fungal pathogens often reproduce rapidly and in large numbers. Once resistance becomes widespread in a field or region, it is extremely difficult—if not impossible—to reverse.

Corn and soybeans are grown on millions of acres each year, often in simple crop rotations with large fields planted to a single crop. Many acres receive fungicide applications annually in a proactive move to protect plant health and yield. This combination of widespread use and repeated exposure increases selection pressure on fungal populations.

Frogeye leaf spot is a well-documented example of a pathogen that can develop resistance to certain fungicide classes if not managed properly.

Fungicides remain a valuable yield-protecting tool in crop production, but their long-term usefulness depends on responsible management. Here are some strategies to mitigate fungicide resistance in your fields:

Rotate modes of action — This is easier said than done. For foliar applications, there are really two modes of action commercially available and three sites of action. Though we don’t want to use a single site of action year over year, fungicide groups available for corn, soybean and wheat applications are pretty limited. As a result, the next strategy becomes all the more important.

Use multiple modes and sites of action — Tank-mixing or applying premix fungicides that contain multiple effective modes and sites of action can significantly reduce resistance risk. Multi-mode products help ensure that if a pathogen is less sensitive to one active ingredient, another active ingredient can still provide control. It’s important that all modes of action in the mix are effective against the target disease. Including an ineffective partner does not contribute to resistance management and may give a false sense of security.

Properly time applications — Correct timing plays a role, not only in disease control but also in resistance management. Applications made too early or too late may provide limited disease suppression, exposing pathogens to sub-lethal doses that can accelerate resistance development. Proper application techniques—such as adequate carrier volume, correct nozzle selection and sufficient canopy penetration—also help ensure that fungicides work as intended and reduce the chance of survivors.

Integrate non-chemical practices — Fungicides should be part of an integrated disease management system rather than the sole line of defense. Crop rotation, residue management, hybrid and variety selection, planting date and fertility management all influence disease pressure. Using varieties with strong disease resistance traits helps reduce reliance on fungicides and lowers overall selection pressure. Likewise, rotating crops can reduce pathogens and slow resistance development.

Watch for performance issues — Reduced disease control, inconsistent results or disease progression shortly after application may indicate emerging resistance. While many factors can influence fungicide performance, suspected resistance issues should be evaluated closely. Frequent irrigation can drive the environment that favors disease development. These acres need special attention.

Fungicides are a critical tool we want to protect. Not only do they control diseases, but they also enhance crop performance and help keep other strategies such as hybrid and variety disease tolerance from developing resistance. 

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