Lawn fungus often transforms healthy, green turf into an unsightly patchwork of brown and yellow. Infections are caused by fungal pathogens that thrive when the lawn’s environmental balance is disrupted. Prolonged leaf wetness, high humidity, and improper maintenance create ideal conditions for spores to germinate and spread. Understanding the specific infection and taking immediate action is important to halt the disease and restore the lawn’s health.
Diagnosing the Problem
Treating lawn disease begins with accurately identifying the specific fungus, as different pathogens require different environmental adjustments. The three most common fungal diseases are Brown Patch, Dollar Spot, and Red Thread, each presenting distinct visual symptoms.
Brown Patch typically appears in circular or irregular patches ranging from four inches to several feet in diameter. The grass blades within the patch turn brown, often exhibiting a grayish or purplish-brown border known as a “smoke ring” on the outer edge of the patch. This fungus favors high heat and humidity, generally appearing when daytime temperatures exceed 80°F and nighttime temperatures remain above 65°F.
Dollar Spot is recognized by small, distinct, sunken patches of bleached grass that are roughly the size of a silver dollar, though they may merge to form larger, irregular areas up to a foot wide. Individual grass blades often show light tan spots with a darker reddish-brown margin. The presence of fine, white, cobweb-like fungal growth, called mycelium, can often be seen on the dew-covered grass in the early morning.
Red Thread is characterized by irregular patches of tan or bleached-white grass, which can range from two inches to three feet across. The most distinguishing feature is the presence of coral-pink or reddish, thread-like strands, known as sclerotia, that bind the tips of the grass blades together. This disease is most active during cool, wet conditions in the spring and fall and often targets turf that is deficient in nitrogen.
Immediate Remediation: Fungicide Application
When a fungal outbreak is actively spreading, immediate fungicide application acts as a curative treatment to stop the infection and prevent damage. Fungicides are categorized based on how they interact with the plant, typically systemic or contact types. Systemic fungicides are absorbed by the plant and move throughout its vascular system, providing protective and curative action from the inside out.
Systemic treatments are longer-lasting, often effective for several weeks, and highly effective against existing infections. Contact fungicides remain on the surface of the grass blades, creating a protective barrier against fungal spores. While they offer rapid initial defense, they are primarily preventative and require more frequent reapplication, typically every few days to a week.
To manage an active disease, a systemic product is preferred for its curative properties. Rotating between different chemical classes (modes of action) is important to prevent the fungus from developing resistance. Follow the product label instructions for application rate and safety protocols, including wearing protective gear. Applying nitrogen fertilizer to an actively diseased lawn should be avoided, as the nutrient can stimulate fungal growth.
Long-Term Cultural Control
The most effective strategy for preventing fungal recurrence involves adjusting the lawn’s environment to make it inhospitable to pathogens. These long-term cultural control practices focus on reducing leaf wetness duration, improving soil conditions, and maintaining proper turf nutrition.
Proper watering practices are paramount in fungal management, as prolonged moisture on the leaf surface triggers spore germination. Water the lawn deeply and infrequently, aiming for one to one and a half inches of water per week to encourage deep root growth. Watering must be done in the early morning (ideally 4:00 AM to 8:00 AM), allowing the grass blades to dry completely as the day warms. Avoid evening or nighttime watering because it leaves the leaf blades wet for extended periods, creating the perfect environment for fungal activity.
Mowing practices also influence disease susceptibility. Sharp blades are necessary to avoid tearing the grass tips, which creates open wounds for pathogens to enter. The grass should be mowed at the highest recommended height for the specific turf type, as longer blades promote deeper root systems and healthier plants. Never remove more than one-third of the grass blade’s total height in a single session, as this causes significant stress and weakens the turf.
Managing soil health is another important component, focusing on reducing compaction and controlling thatch buildup. Core aeration removes small plugs of soil, which relieves compaction, improves water drainage, and increases air circulation around the root zone. Excessive thatch, a dense layer of organic matter between the soil and green leaves, harbors fungal spores and moisture, and regular aeration helps break down this layer.
Fertilization must be carefully managed, particularly concerning nitrogen application. While a nitrogen deficiency can make the lawn susceptible to diseases like Dollar Spot and Red Thread, over-fertilizing with nitrogen during hot and humid summer periods can encourage Brown Patch development. Applying a balanced, slow-release fertilizer provides steady nutrition that supports healthy growth without creating soft, vulnerable tissue that many fungi attack.