A lawn fungicide is a chemical designed to control fungal diseases that damage turfgrass. These products work either by forming a protective barrier on the leaf surface or by being absorbed into the plant tissue to stop an active infection. Effective disease management requires understanding the correct timing and frequency of application. The decision of how often to apply fungicide depends on the strategy chosen, the type of product used, and the environmental conditions present.
Identifying the Need: Preventative vs. Curative Timing
The application frequency begins with determining whether the goal is to prevent a disease from starting or to stop an existing infection. A preventative application is the most effective approach and is applied before any visible symptoms appear on the grass blades. This strategy involves treating the lawn when environmental conditions are ideal for common turf diseases, such as when night temperatures are consistently above 65 degrees Fahrenheit, which favors Brown Patch development. Preventative programs typically use lower product concentrations and aim for a longer, consistent commitment across the growing season.
The goal of a preventative treatment is to create a protective chemical layer that inhibits fungal spores from germinating and penetrating the plant tissue. By starting early, the fungi population is kept low, making it much harder for the disease to take hold.
A curative application, conversely, is necessary when a lawn already shows clear signs of disease, such as discolored patches or visible mold-like growths. Curative treatments must be applied immediately upon the first sight of symptoms to halt the spread of the infection. This approach often requires using the highest labeled concentration or a shorter initial reapplication interval to gain control over the fungus. While fungicides cannot repair grass that has already been damaged, they can stop the infection from progressing and spreading to healthy areas.
Establishing the Reapplication Schedule
Once an application program begins, the fungicide product label provides the guidance for the reapplication schedule, often specifying a range such as every 7 to 28 days. This wide interval exists because the active ingredients have different residual properties and are broken down at various rates. Adhering to this labeled interval is necessary because the grass plant continuously produces new, unprotected tissue, and the chemical protection on the older growth degrades over time.
The type of fungicide formulation directly influences the required reapplication frequency. Contact fungicides remain on the surface of the grass blades, acting as a protective shield against fungal spores. Because they do not penetrate the tissue, they are susceptible to weathering and can be washed away by heavy rain or irrigation. Contact products therefore typically require reapplication at the shorter end of the interval range, sometimes as frequently as every 7 to 10 days, to maintain full coverage.
Systemic fungicides are absorbed by the plant and move within its vascular system, protecting the grass from the inside out. This internal movement makes them less vulnerable to rain wash-off and environmental degradation. Systemic products generally offer a longer residual effect, allowing for reapplication intervals of 14 to 28 days or more.
Environmental factors and disease pressure also affect the schedule, potentially forcing an application at the shortest labeled interval. During periods of persistent high humidity or frequent rainfall, the risk of fungal infection is elevated. If the lawn is experiencing high disease pressure, using the shorter interval ensures the protective chemical concentration remains high enough to suppress the fungus. The frequency of fungicide application will naturally decrease or stop entirely when conditions no longer favor fungal growth, such as during the winter dormancy period.
Preventing Resistance Through Fungicide Rotation
Long-term, repeated use of the same fungicide chemical class presents a significant challenge by leading to the development of disease resistance. Fungi have a high reproductive potential and can undergo genetic mutation, which allows them to become resistant to a specific chemical mode of action. When resistance occurs, the product is no longer effective, and the fungal disease will spread unchecked despite continued application.
The solution to this problem is a strategy known as fungicide rotation, which requires switching products to ensure different chemical modes of action are used sequentially. To simplify this process, the Fungicide Resistance Action Committee (FRAC) developed a coding system that categorizes fungicides based on their mode of action. This FRAC code, a number and/or letter combination, is listed on the product label and indicates the specific biochemical function of the fungus that the chemical targets.
To prevent resistance, a product with one FRAC code should not be applied more than two or three times consecutively. Instead, the user should rotate to a product with a completely different FRAC code number for the next application. For example, if a fungicide with a FRAC code 3 is used for one application, the subsequent application should use a product from a different group, such as FRAC 11. This rotation ensures that any fungi that have developed resistance to the first chemical are eliminated by the second, chemically unrelated product, preserving the long-term effectiveness of the entire fungicide inventory.