Proper Fungicide Use


Multiple organisms (viruses, nematodes, fungi, and bacteria) can cause plant disease. Preventing and managing diseases is best accomplished by combining practices known as Integrated Pest Management or IPM. Management practices include matching the plant with the site, selecting disease-resistant varieties, plant care that prevents stress (irrigation, mulch, fertilization as needed, etc.), and preventative fungicide use when warranted.


Fungicides are pesticides that prevent, kill, mitigate or inhibit pathogen growth on plants, but they are not effective against viral diseases. Fungicides can be classified based on:

A diagram of acropetally mobile fungicides (top) move upwards in the plant while ambimobile fungicides (bottom) can move up and down within the plant.
Acropetally mobile fungicides (top) move upwards in the plant while ambimobile fungicides (bottom) can move up and down within the plant. Courtesy of CPN. 

Mobility in the plant

Contact fungicides (also known as protectants) are not absorbed by the plant but stick to plant surfaces. They provide a protective barrier that prevents the fungus from entering and damaging plant tissues. Systemic fungicides (also known as penetrants) are absorbed by the plant and can move from the application site to other parts of the plant.

Application period

Preventive fungicides work by preventing the fungus from getting into the plant; therefore, the fungicide needs to be in place before the pathogen makes contact with the tissue. Preventative fungicides have to be re-applied frequently to new plant tissues (as leaves or needles expand in the spring) or if the product washes off.

Curative fungicides can halt pathogens after the infection has started or after the first symptoms are observed. Fungicides that can move in the plant can be both preventative and curative. However, most fungicides available to home gardeners are preventive and not curative.

Fungicide (FRAC) Code

  • Mode of action: This refers to how the fungicide affects the fungus. Fungicides may work by damaging the cell membrane of the fungus, inhibiting a critical process that the fungi, and pinpointing single or multiple functions in the fungus. It's important to incorporate different modes of action by mixture or alternating products to maintain effectiveness and prevent fungicide resistance.

Best Practices for Fungicide Use

The following questions should be considered to make a safe and effective fungicide application:

  • Are the symptoms I see in my plant a disease?

Many abiotic conditions or insect problems can look like diseases caused by pathogens, see Plant Diagnosis: The Deceptive Nature of Symptoms.

  • What kind of pathogen is causing these symptoms? Virus, Fungi, Bacteria or nematodes?
  • Are the other disease or problems that may look like this? (look-alikes)
  • What are weather conditions conducive to disease development?
  • When is the most effective time to treat?
  • How often should I apply the fungicide?

Answers to these questions will depend on the pathogen causing the disease, as their lifestyles and overwintering habits vary. Various abiotic disorders may present symptoms similar to a biotic disease, therefore, it is important to properly identify the cause of the symptoms before making a fungicide application. A misdiagnosis of a plant disease can lead to symptoms persisting even after a fungicide application.

Inspect your plant often and systematically for symptoms, to early detect symptoms of plant problems, see Scouting for Landscape Plant Problems.

Need help with plant problems? The Plant and Insect Diagnostics Clinic can help!

When you have determined that a fungicide is needed to prevent further damage from your particular plant problem, read the label and follow instructions. Any application of a fungicide that is not consistent with label directions is against the law. Always apply fungicides using the appropriate PPE, proper application equipment, and at the recommended application rate listed on the label. Following the label instructions will not only protect your plant but will also protect your health and the environment.

Fungicide labels provide information on recommended uses, active and inert ingredients, mode of action, and product formulation. For more information, please read the Extension publication on Understanding Pesticide Labels.  

The best management strategy against plant diseases is to promote proper plant health. Before planting, make sure that soil and light conditions are ideal for your plant. Continue to care for your plant; water and mulch are critical during the first establishment years and during stressful dry weather conditions. Once the plants have been established, make sure to use appropriate sanitation in the fall, fertilize as needed, and regularly prune to enhance plant health. 

Other Resources

Last reviewed:
December 2021

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on December 3, 2021. The information contained within may not be the most current and accurate depending on when it is accessed.