Fungicides are chemical compounds used to control fungal diseases that threaten turfgrass health and appearance. While these products target microscopic pathogens, homeowners often worry about collateral damage to the lawn itself. This concern is valid, as even properly labeled chemicals carry the risk of injuring the grass when application methods or environmental conditions are incorrect. Understanding how these products interact with plant biology helps explain why a treatment meant to save the lawn can sometimes cause damage instead.
Understanding Phytotoxicity in Grass
The physical damage caused to grass by a fungicide application is termed phytotoxicity, which describes chemical injury to plant tissue. This damage occurs when the active ingredients or the solvents within the formulation disrupt the normal biological functions of the grass cells. For instance, some contact fungicides, like those containing copper, can physically destroy chloroplasts, inhibiting photosynthesis and leading to immediate discoloration. Other systemic fungicides, such as Demethylation Inhibitors (DMIs), can interfere with plant hormones, causing temporary discoloration or growth regulation effects. Phytotoxicity can also manifest as an osmotic imbalance, where the chemical draws moisture out of the plant cells, effectively dehydrating the leaf tissue.
Application Errors That Cause Damage
Overdosing and Calibration
The most common reason for fungicide burn is an error in the application process, leading to the chemical injury described above. Applying a concentration that is too strong, known as overdosing, overwhelms the plant’s defenses and is a frequent cause of phytotoxicity. This mistake often happens during the mixing stage or when an applicator fails to calibrate equipment, resulting in uneven coverage and concentrated “hot spots.”
Environmental Stress
Environmental factors during application significantly raise the risk of damage, particularly when temperatures are high. Turfgrass is most likely to suffer injury when liquid applications are made in temperatures exceeding 85°F, as the heat increases the chemical’s uptake rate and volatility. Applying fungicides to grass that is already under drought or heat stress also makes the turf more susceptible to chemical burn. Stressed plants cannot metabolize the chemical compounds effectively.
Plant Vulnerability
Their damaged cellular structure is more vulnerable to disruption when stressed. Certain grass types, such as fine fescues and bentgrass, are naturally more sensitive to particular fungicide classes. Using a non-approved product can result in damage even when following the label directions for other turf types.
Differentiating Fungicide Burn from Disease Symptoms
Distinguishing between the original fungal disease and new chemical burn is crucial for proper lawn care response. Fungicide burn typically appears very rapidly, often within 24 to 48 hours of application, showing a sudden, immediate scorching. This damage frequently follows a pattern related to the application method, such as distinct stripes from a sprayer or defined wheel tracks where the concentration was highest. Chemical damage tends to involve the uniform yellowing or browning of the entire leaf blade or the crown of the grass plant where the spray landed. In contrast, fungal diseases develop more slowly and often present with specific, non-uniform symptoms like distinct leaf spots, lesions with dark borders, or circular patches that expand over several days.
Steps for Prevention and Recovery
Preventing fungicide burn begins with meticulous attention to the product label, which specifies the correct mix ratios and application volumes for different grass species. Before treating the entire lawn with a new product, applying it to a small, inconspicuous area as a spot test can help confirm the turf’s tolerance. To minimize the risk of phytotoxicity, applicators should aim to spray when air temperatures are between 60°F and 85°F, avoiding the peak heat of the day. It is also important to ensure the grass is not suffering from drought stress before spraying, as wilted plants are highly sensitive to chemical injury. If chemical burn is suspected immediately following application, the affected area should be thoroughly watered to dilute the chemical and leach it away from the roots and off the leaf blades.