Fungicides and How to Use Them Effectively

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Multiple organisms (viruses, nematodes, fungi, and bacteria) can cause plant disease.  Preventing and managing disease is best accomplished by a combination of 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.) as well as fungicide use when warranted.

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

  • Mobility in the plant: Contact vs. mobile (types of systemics). Contact fungicides (AKA protectants) are not absorbed by the plant and stick to plant surfaces. They provide a protective barrier that prevents the fungus from entering and damaging plant tissues. Systemic products (also known as penetrants), are absorbed by the plant and can move from the site of application to other parts of the plant. Movement in the plant varies by fungicide, form moving to old and new tissues (amphymobile or true systemic), new growth (acropetally or xylem mobile), moving from the top to the bottom of the leaf surface (translaminar). For more information see the publication “Fungicide Mobility for Nursery, Greenhouse, and Landscape Professionals.”
  • Preventive vs. curative:  Preventive fungicides work by preventing the fungus from getting into the plant. The preventive fungicide must come into direct contact with the fungus, and they have to be re-applied to new plant tissues (as leaves or needles expand in the spring) or if the product washes off. Curative fungicides affect the fungus after infection. This means they can stop the disease after the infection has started or after first symptoms are observed. Fungicides that can move in the plant can be both preventative and curative.
  • Mode of action: This refers to how the fungicide affects the fungus.  Fungicides may work by damaging the cell membrane of the fungus, inhibiting an important process that the fungi, pinpointing a single or multiple processes in the fungus. It's important to incorporate different modes of action by mixture or by alternating products to maintain effectiveness and prevent fungicide resistance. Stay tuned for our article "what is fungicide resistance?".

Rules of thumb for fungicide use

For efficient and safe fungicide use, certain rules have to be followed:

The problem has to be diagnosed correctly: Before applying a fungicide make sure that you know the cause of the disease (is it a disease? If so what is causing it? Fungi? which one?) and when (spring, fall etc.) and how often to apply the fungicide. The timing of the fungicide application can enhance the effectiveness of the product and prevent additional sprays.

When ready to use the recommend fungicide for the particular problem your plant is facing, read the label and follow instructions. This will not only protect your plant, but it will also protect your health and the environment. Remember always to apply fungicides using the appropriate equipment at the recommended application rate.

Fungicide labels provide information on recommended use, ingredients, mode of action, and formulation of the product. For more information please see What You Need to Know about Reading a Pesticide Label 

Remember that the best management strategy against plant diseases is by promoting plant health in the first place. Before planting, make sure that soil, water, and light conditions are ideal for your plant. Once the plants have been established, make sure to use the appropriate sanitation, fertilization, and pruning practices to enhance plant health. Plant Problems? The Plant and Insect Diagnostics Clinic can help!

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Lina Rodriguez Salamanca Extension Plant Pathologist and Diagnostician (Program Specialist II)

Dr. Lina Rodriguez Salamanca is an extension plant pathologist and diagnostician with the Iowa State University Plant and Insect Diagnostic Clinic, a member of the North Central Plant Diagnostic Network (NCPDN) and National Plant Diagnostic Net...