Fungicides and Landscape Disease Management

Ken Rudisill
Horticulture Extension Faculty
Bay County Extension

Not all plants die because of diseases. Many plants are killed by environmental stresses which look like disease. For example, overwatering will kill plants by inducing root rots. Disease occurs when a pathogen infects a plant and disrupts its growth or kills the plant over time. Fungi, bacteria and viruses are the most common plant pathogens.

All three sides of the disease triangle must be present for disease to occur: a susceptible host, a pathogen that can cause disease, and an environment favorable for infection and disease development. Photo Credits: Homeowner's Guide to Fungicides for Lawn and Landscape Disease Management, Philip Harmon, Aaron Palmateer, and Rachel Ribbeck

Environmental factors influence disease development in the landscape. When the environment favors the host plant, disease is unlikely to occur. When environmental factors favor growth of the pathogen and infection of the host, disease is more likely to occur. The disease triangle includes the host, the pathogen and the environment. For a disease to occur all three must be present. Some have included “time”, and have called it a disease table.

It is often difficult to determine which disease is causing the problem, because there are a wide variety of problems that occur on many plants in the landscape.

Correctly diagnosing disease problems is critical because treatment recommendations are vastly different for different disease problems. To diagnose your plant disease problem you can:

    • Contact your county extension agent;

    • Consult a Master Gardener or other knowledgeable plant person;

    • Submit a sample to your local Extension Office; or

    • Consult a lawn and landscape professional.

    Only by knowing what is causing the problem, can you determine how to manage and how to prevent the problem from occurring again.

    Proper cultural controls will help prevent disease. Proper irrigation schedules, fertilization schedules and planting the right plant for the right place reduces the chance of some diseases becoming a problem. If new plants are being established in your landscape, take care to choose a strong and healthy plant that is not already infected with a pathogen. To do this, look for signs and symptoms of plant disease on the plant and root system. It is a good idea to only purchase plants from reputable and licensed nurseries. Inspect "bargain" plants carefully.

    Fungicides can be classified by how they work on the fungus and how they work on the plant.

    Contact fungicides are sprayed onto plants and act as a protectant barrier from pathogen infection. They prevent infections from occurring when applied before symptoms are visible, but infections that have already occurred will continue to develop.

    Systemic fungicides move into the plant to some extent depending on the chemical. Some systemic products exhibit curative action which means the disease is stopped at the current state of development.

    Source: Homeowner's Guide to Fungicides for Lawn and Landscape Disease Management document PP233.