Lawn disease is a condition that harms turfgrass, typically caused by a biotic agent that infects the plant. These diseases often manifest as areas of discoloration, wilting, or thinning, indicating that the plant’s healthy functions have been disrupted. The majority of turfgrass diseases result from fungal pathogens that attack the blades, stems, or roots. Recognizing early signals is the first step toward effective management and maintaining a vibrant, healthy lawn.
The Pathogens and Environmental Triggers
The presence of a lawn disease requires the interaction of three specific components, a concept known as the disease triangle. For an infection to occur, a susceptible host (the grass), a pathogen (the disease-causing agent), and a favorable environment must all be present simultaneously. If any one of these three elements is missing, the disease will not develop.
Most turfgrass diseases are caused by ubiquitous fungal spores that are already present in the soil and thatch layer of the lawn. These spores remain dormant until the environmental conditions become ideal for them to infect the grass. While fungi are the primary culprits, some less common lawn diseases may be caused by bacteria or viruses.
Environmental factors often trigger disease by creating the perfect setting for the pathogen to thrive. Excessive moisture is a significant factor, whether from over-watering, poor soil drainage, or prolonged periods of high humidity and dew. Improper temperature, such as high nighttime temperatures above 65°F for some diseases, is another major trigger. Lawn stress caused by mowing the grass too short, soil compaction, or nutrient imbalances also increases susceptibility.
Visual Signs of Infection
Identifying a lawn disease begins with observing the visual evidence, which can be categorized into symptoms and signs. A symptom is the grass plant’s reaction to the infection, such as a change in color or structure. Common symptoms include general thinning, discoloration ranging from yellowing to browning, or the formation of discolored patches.
These symptoms often appear in distinct patterns across the lawn area. Patches are common, starting as small, isolated spots that can merge into larger, irregular dead areas. Other diseases create circular or arc-shaped patterns, sometimes referred to as “fairy rings,” where the grass inside the circle is discolored or dead.
Examining individual grass blades can reveal specific symptoms, such as lesions or spots. These spots may be dark brown, black, or purplish with lighter centers, indicating a leaf spot disease. Signs, in contrast to symptoms, are the visible parts of the pathogen itself. These include the whitish, cottony threads of fungal growth, called mycelium, visible on the grass surface during periods of high humidity and moisture.
Specific Examples of Common Lawn Diseases
Brown Patch, caused by the fungus Rhizoctonia solani, is widespread and favors hot, wet conditions. It appears as circular patches of discolored grass, ranging from a few inches to several feet in diameter. The grass blades within the patch often have irregular, tan lesions with dark brown borders. This disease is active when daytime temperatures exceed 80°F and nighttime temperatures remain above 65°F, especially when the turf stays wet.
Dollar Spot is a common fungal disease named for the small, sunken, straw-colored spots it creates, roughly the size of a silver dollar. These spots can coalesce to form larger, irregular areas of damaged turf. Dollar spot is often linked to low nitrogen fertility and is prevalent in late spring or early summer when temperatures are between 60°F and 90°F.
Rust disease is identified by the presence of orange, red, or yellowish-brown dust that coats the grass blades and can rub off on shoes or clothing. This powdery substance is a mass of fungal spores. Rust affects slow-growing turf, often occurring in late summer or early fall during periods of drought stress or low nitrogen.
Maintaining Lawn Health to Prevent Disease
Preventing lawn disease relies on cultural control practices that reduce grass stress and eliminate favorable environmental conditions for pathogens. Proper watering is the most effective preventative measure; water deeply and infrequently. Watering should be done in the morning, ideally between 6 AM and 10 AM, to allow the grass blades to dry completely before evening, preventing prolonged leaf wetness.
Mowing practices also play a role in disease prevention. Mowing at the correct height for the grass species and never removing more than one-third of the blade at any one time helps maintain the grass’s strength. Using a sharp mower blade is necessary to prevent tearing the grass, as ragged cuts create easy entry points for fungal pathogens.
Managing the soil environment is equally important for a healthy lawn. Aeration and dethatching help to reduce thatch buildup and improve soil drainage, which prevents the saturated conditions that many fungi prefer. A balanced fertilization program is also needed, as both excessive and insufficient nitrogen can make the lawn more susceptible to different diseases.