Viruses, nematodes, and bacteria can cause plant disease, but fungi are the number one disease-causing organism. As a result, fungicides are frequently used by homeowners when managing plant disease problems.
Fungicides are pesticides that can kill or inhibit growth of fungi on plants, but they are not effective against bacteria, nematodes, or viral diseases. Fungicides can be classified based on if they are absorbed by the plant and if they can move around in plant tissues, their 'curative' properties, how they kill the fungi (mode of action), and type of chemical. Some of the major categories are described below:
- Contact vs systemic: Contact fungicides (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 are able to move from the site of application to other parts of the plant.
- 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 have the ability to stop the disease after the infection has started or after first symptoms are observed.
- Mode of action: This refers to how the fungicide affects the fungus. Fungicides may work by damaging the cell membrane of the fungus, or by inhibiting an important process that the fungi. It's important to incorporate different modes of action by mixture or by alternating products to maintain effectiveness and prevent fungicide resistance.
- Chemical group: this classification may vary depending on chemical composition, structure, and mode of action.
Rules of thumb for fungicide use
For efficient and safe fungicide use, certain rules have to be followed. First, the problem has to be diagnosed correctly. Before applying a fungicide make sure that you know the cause of the disease (that is a diseases caused by a fungus). Second, 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. Third, follow the recommendations of when and how often to apply the fungicide. Timing of the fungicide application can enhance the effectiveness of the product and prevent additional sprays.
Fungicide labels should provide information on recommended use, ingredients, mode of action, and formulation of the product. For more information please see University of Kentucky Extension bulletin ID100.
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.