Lawn diseases are primarily caused by fungal pathogens that live naturally within the soil and thatch layer of the turf. These microscopic organisms typically become active and cause visible damage when environmental conditions, such as temperature and moisture, favor their growth over the grass’s health. The resulting changes in color, texture, and growth pattern provide the visual evidence necessary to identify a problem. Learning to read these visual clues is the first step in diagnosing what is affecting your turf and distinguishing a fungal infection from other common lawn issues.
General Visual Symptoms of Lawn Disease
Fungal activity often creates distinct, organized patterns that stand out against the surrounding healthy turf. The most common sign is the appearance of discolored patches ranging from a few inches to several feet in diameter. These patches represent areas where the fungus is actively infecting the grass blades, leading to colors like yellow, straw-brown, or reddish-purple.
A strong indicator of disease, rather than simple stress, is the geometric shape of the damage. Many fungal infections manifest as circular or irregularly shaped rings that expand outward over time. For example, some diseases leave behind a “frog-eye” pattern, which is a ring of dead grass with a healthy green center.
The texture of the grass can also change, sometimes appearing matted, slimy, or having a dusty or cottony coating, especially in the early morning dew. The edges of these affected areas often show a sharp, distinct line separating the diseased turf from the healthy grass. If individual blades are examined, they may show spots, streaks, or lesions rather than a uniform color change.
Detailed Appearance of Major Lawn Diseases
Brown Patch/Large Patch
Brown Patch (or Large Patch in warm-season grasses) creates large, circular areas of thinned, brown, or straw-colored turf, often ranging from a few inches to several feet across. When the disease is active, these patches may have a distinct, dark, smoky-gray ring around the perimeter.
The most specific symptom is found by inspecting the grass blades at the edge of the patch. Individual leaves exhibit tan lesions with a dark brown or reddish-brown border. This infection often rots the leaf sheath near the soil line, causing the leaves to pull away easily from the stem.
Dollar Spot
Dollar Spot is named for the small, sunken, circular spots of straw-colored grass, typically the size of a silver dollar or up to six inches across. These spots can merge together, creating larger, irregular patches of dead turf.
The individual grass blades within these spots reveal characteristic lesions. These lesions often look like hourglass shapes, appearing as tan bands with a distinct reddish-brown margin on the middle of the leaf blade. In the early morning, fine, white, cobweb-like fungal threads (mycelium) may be visible across the spots.
Rust
Rust disease is easily identified by the powdery, colored spores it produces on the grass blades, causing the turf to take on a yellowish-orange or reddish-brown hue. If you walk across an infected lawn, the microscopic spores will rub off easily, leaving a dusty orange or reddish residue on shoes, clothing, or mower blades.
These spores are released from pustules that form on the leaf surface, significantly reducing the grass’s ability to photosynthesize. This reduction in function causes the blades to appear thin and weak.
Red Thread
Red Thread is recognized by the appearance of pink or red gelatinous strands extending from the tips of the grass blades. This disease creates irregular, pinkish or reddish patches in the lawn, typically ranging from two to twenty-four inches in diameter.
The distinctive red threads are fungal structures, called sclerotia, which look like small, antler-like extensions on the leaf tissue. Although the patches themselves are often light tan or bleached, the presence of these fine filaments provides definitive identification.
Ruling Out Non-Disease Problems
Before assuming a fungal infection, it is important to compare the visual symptoms against other common lawn stressors like drought, nutrient deficiency, or insect damage. Drought stress causes a uniform, widespread fading across large areas, often turning the grass a dull, blue-gray color before browning.
Unlike the defined spots of disease, a drought-stressed lawn will also show “footprinting,” where compressed grass blades remain flat after being stepped on because they lack the moisture needed to spring back.
Insect damage leaves behind physical evidence of their feeding. Grubs feed on roots, causing the turf to lift easily like loose carpet. Other pests, like sod webworms, leave ragged, chewed edges on the grass blades, while chinch bugs cause irregular patches that mimic drought but do not respond to watering.
Nutrient deficiencies typically result in a uniform discoloration across the lawn, not the distinct patches or spots seen with disease. A nitrogen deficiency, for example, causes the entire lawn to appear pale green or yellow, starting with the older, lower leaves. Stripes on the leaf blade, where the tissue between the veins is yellow while the veins remain green, is a specific sign of iron or manganese deficiency.