Lawn fungus compromises the health and appearance of turfgrass, often turning a vibrant lawn into a patchy landscape. The proliferation of fungal pathogens is typically a symptom of an underlying imbalance in the host, the pathogen, and the environment. This “disease triangle” indicates that fungus takes hold when grass is susceptible and environmental conditions, such as prolonged moisture, are favorable. Controlling an outbreak requires a two-pronged approach: halting the active infection and implementing long-term cultural and environmental corrections to make the lawn inhospitable to future growth.
Identifying the Type of Fungal Disease
Effective treatment begins with correctly identifying the specific fungal disease, as different pathogens require different management strategies. One common issue is Brown Patch, which appears as circular or irregular patches of brown, wilted grass ranging from a few inches to several feet in diameter. Caused by the fungus Rhizoctonia solani, this disease is most active in warm, humid weather when nighttime temperatures remain elevated, often between 70 and 90 degrees Fahrenheit.
Another prevalent turf disease is Dollar Spot, distinguished by its smaller, silver-dollar-sized circular spots of straw-colored grass. These spots often have a tan center and a reddish-brown border on the blade lesions. Dollar Spot thrives in moderate temperatures (60–85°F) and periods of low soil moisture combined with high humidity or heavy dew, often exacerbated by low nitrogen fertility.
A third type, Red Thread, presents as pink or reddish patches up to two feet wide, characterized by visible, thread-like fungal structures (sclerotia) that bind the grass blades together. This fungus prefers cool, wet conditions, typically in spring and fall, and indicates slow turf growth due to insufficient nitrogen.
Immediate Treatment Options (Fungicides)
Once a fungal disease is identified, immediate application of a fungicide can quickly halt the spread of the active infection. Fungicides are broadly categorized as either contact or systemic treatments based on how they interact with the plant. Contact fungicides form a protective barrier on the grass blade surface, killing fungal spores upon contact. They have a short residual life, requiring frequent reapplication, and offer no protection to new grass growth.
Systemic fungicides are absorbed into the plant’s vascular system, moving throughout the grass tissue to protect the entire plant from the inside out. These products offer longer-lasting protection, often 7 to 25 days or more, and provide curative action against infections that have already entered the plant. For an active outbreak, a systemic product is preferred for its ability to treat existing infections.
To maximize effectiveness and slow the development of pathogen resistance, it is recommended to alternate between fungicides with different modes of action. Application timing is also important; preventative applications are made when environmental conditions, such as high heat and humidity, favor disease development. Whether using synthetic chemical fungicides or biological/organic options (like those containing Neem oil), strict adherence to the product label for mixing rates and safety precautions is mandatory.
Adjusting Cultural Practices for Long-Term Health
While fungicides treat the symptoms, adjusting routine maintenance practices is the only way to achieve long-term prevention by controlling the environmental side of the disease triangle. Proper watering is paramount, as excessive moisture on the grass blades creates the ideal environment for fungal spores to germinate. The lawn should be watered deeply and infrequently, applying about one inch of water per week to encourage deep root growth.
The timing of irrigation is equally important. Watering should occur in the early morning, just before sunrise, so the sun and rising temperatures can quickly dry the grass blades. Watering in the late afternoon or evening should be avoided entirely, as prolonged leaf wetness through the night creates a humid microclimate for fungal growth.
Mowing height must be maintained at the optimal level for the specific turfgrass species, as cutting too short stresses the plant and makes it vulnerable to infection. When mowing, no more than one-third of the blade height should be removed in a single session. Dull mower blades rip and damage the grass tips, creating open wounds for pathogens to enter, so keeping blades sharp is a preventative measure. Proper fertilization with a balanced nutrient plan, especially avoiding excessive nitrogen application during periods of high heat, strengthens the grass’s natural defenses.
Correcting Underlying Environmental Issues
Beyond routine cultural practices, structural and environmental issues must be addressed to permanently reduce disease susceptibility. Soil compaction is a major contributor to fungal problems because it restricts the movement of air and water, leading to poor drainage and waterlogged conditions that favor pathogens. Aerating the lawn by removing small plugs of soil improves gas exchange and allows water to penetrate the root zone more effectively, reducing surface moisture.
Poor surface drainage can also lead to standing water, which sustains the humid conditions that fungi require. Solutions include correcting the grade of the lawn to ensure water slopes away from structures or improving soil composition through topdressing with organic matter. Excessive thatch—a layer of dead and living organic material between the grass blades and the soil—can hold moisture and should be managed through dethatching to improve airflow and water percolation.
Managing shade is another structural correction, as areas with reduced sunlight and poor air circulation often remain wet for longer periods, promoting fungus growth. Pruning lower tree limbs or shrubs can increase the amount of sunlight reaching the turf and improve air movement across the grass surface. This helps the blades dry more quickly after irrigation or rain events, making the lawn a less hospitable environment for fungal pathogens.