Good Growing: Sick plants? Rotate fungicides to prevent resistance
By Chris Enroth, Horticulture Educator
University of Illinois Extension
We have hit the other side of the precipitation pendulum. In 2023 and 2024, we experienced drought conditions. Now in 2025, the sump pump sales have surely hit a high note. For instance, Macomb’s total July 2024 rainfall averaged around two inches. In 2025, many Macomb rain gauges recorded over ten inches during July.
Outside our waterlogged basements, most of our landscapes have been enjoying the abundant rain. However, with more moisture comes increased pressure from plant disease. In the last month, there has been an increase in plant questions stemming from fungal infections.
Disease management in the home landscape can seem daunting. According to Michigan State University Extension, around 85 percent of plant diseases are caused by fungi or fungus-like organisms, which may lead to the use of fungicides. Yet at your local garden center, there may be over a dozen fungicide products to choose from to help control plant disease. How do you know which fungicide is right for your situation?
Plant diseases encountered in your backyard must be treated on a case-by-case basis. First, you must identify the disease. Through proper identification, you will gain knowledge of the disease’s lifecycle and when it can be treated. Knowing the plant species will inform what cultural practices can be done to minimize the use of chemical application, and what should be used if it comes to a fungicide.
For fungal infections, several cultural practices should be done first to help prevent the disease from overwhelming landscape plantings. Improve air and light within plants through proper pruning, make sure areas drain properly, and correctly space plants so they are not overcrowded.
When cultural practices fail, sometimes a fungicide application for fungal diseases is warranted. Avoid using the same chemical repeatedly to treat plant disease. Like all pesticides, fungicides should be rotated to keep the disease organism from building up resistance to the product. And verify that you are dealing with a treatable plant disease. Your local Illinois Extension office can help with diagnosis. Because field diagnosis can be tricky, with many plant diseases resembling other issues, we often recommend submitting samples to the University of Illinois Plant Clinic.
Following a disease diagnosis, it is time to determine treatment. When selecting what fungicides to rotate, first check to see what active ingredient(s) will successfully treat the fungal pathogen. The active ingredient should be listed on the front of each product, and the product label will specify what pests can be controlled with the active ingredient.
Once you have identified a list of products whose active ingredient will treat the diagnosed fungal disease, check the active ingredients’ FRAC code. FRAC stands for Fungicide Resistance Action Committee, a worldwide consortium of scientists representing fungicide manufacturers. Their goal is to prevent plant disease from building up resistance to chemical treatments. To rotate fungicides, select products from your list that have different FRAC codes.
The FRAC code distinguishes the mode of action of different active ingredients. The mode of action is how the active ingredient controls the other living organism, in this case, fungal organisms. Different chemicals may share the same mode of action against the target organism. FRAC codes help to separate active ingredients in how they work on the target pest. The more FRAC codes you can alternate, the less likely resistance will develop.
Some products, such as chlorothalonil, copper, or sulfur, are listed under the FRAC code ‘M’. This means these active ingredients target multiple biochemical sites and are low-risk for resistance development. Nevertheless, it is always advisable to incorporate a rotation of any pesticide you use in the home landscape.
Good Growing Tip of the Week: Systemic fungicides are absorbed by the plant tissue to protect from infection. While this may sound advantageous, systemic fungicides have the highest risk of developing resistance.
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