Grass fungus, or fungal disease, frequently affects turfgrass, turning healthy lawns into patchy, discolored expanses. Identifying these issues requires close attention to visual symptoms, which vary widely depending on the specific fungus involved. Signs can manifest as broad patterns visible from a distance or as distinct markings on individual grass blades, sometimes including the physical presence of the fungus itself. Recognizing these different forms of damage is the first step in understanding lawn health.
Identifying Common Lawn Patches
Fungal diseases often create distinct, circular patterns across the lawn area. These large, noticeable patches are typically the first indicator of a problem.
Brown Patch, a common summer disease, appears as roughly circular, sunken patches of discolored grass, ranging from a few inches to several feet in diameter. The infected grass turns a light brown or straw color. A unique “smoke ring”—a grayish-purple border—may be visible at the patch’s edge during periods of high humidity, especially in the early morning.
Necrotic Ring Spot creates a “frog-eye” pattern: a ring of dead or dying grass with a healthy tuft of green grass surviving in the center. These rings can be several inches to a few feet wide. The affected grass often turns a straw-like yellow color.
Fairy Rings manifest as large, expanding circles or arcs of grass that can be either darker green or dead and tan. The fungi responsible often cause the soil beneath the ring to become hydrophobic, or water-repellant, which starves the grass of moisture. Occasionally, small mushrooms or puffballs, which are the fruiting bodies, appear along the outer edge of these rings after rainfall.
Recognizing Specific Leaf and Blade Symptoms
While some fungi create large patches, others primarily attack the individual grass blade, requiring a closer look for accurate identification.
Dollar Spot disease creates silver-dollar sized spots across the lawn. On the grass blade, it is characterized by distinct hourglass-shaped lesions. These lesions are tan or straw-colored in the center but feature a clear, reddish-brown border.
Red Thread is identified by the pinkish or reddish tint it gives to the grass patches from a distance. Upon close inspection, the actual sign is the presence of fine, thread-like fungal structures, called sclerotia, that extend from the tips of the grass blades. These structures can be pink, red, or gelatinous in appearance.
Leaf Spot diseases show up as small, distinct lesions on the blades. These often have a purple or dark brown edge surrounding a lighter, tan-colored center. As the disease progresses, these lesions can merge, causing the entire grass blade to wither and die back from the tip.
Visible Signs of Fungal Growth
In certain conditions, the fungus itself, rather than just the resulting damage, becomes physically visible on the grass.
Mycelium, the vegetative body of the fungus, appears as a delicate, white, cobweb-like growth stretched across the turf surface. This fuzzy growth is typically observed in the early morning when the grass is covered with dew. It is a common sign associated with Brown Patch or Pythium Blight.
Powdery Mildew presents as a light gray or white coating that looks like the grass blades have been dusted with flour. This powdery substance is composed of millions of fungal spores. It is most often found in shaded areas with poor air circulation.
Slime Molds, while not true fungi, often appear on lawns as slimy, mucilaginous blobs sitting on top of the grass blades. These can be brightly colored—yellow, white, or gray—and may resemble scrambled eggs or dog vomit before they dry out and turn crusty and gray.
Ruling Out Other Causes of Lawn Damage
Discolored grass is not always the result of a fungal infection, so distinguishing between disease and other common stressors is important for treatment.
Drought stress causes a more uniform wilting and browning across the lawn area, where the grass blades curl or fold lengthwise to conserve moisture. Unlike fungal patches, which have distinct or circular boundaries, drought damage affects large, irregularly shaped areas that lack adequate water coverage.
Insect damage typically presents with irregular patterns. Visual evidence often includes chewed edges on the blades or visible tunnels and holes in the soil. Fungal diseases rarely target the grass in a linear or streaked fashion.
Chemical burn, such as from improperly applied fertilizer or herbicide, appears as streaks or defined lines of rapid browning, following the pattern of the application equipment. Fungal damage, in contrast, tends to spread outward in radiating patches or rings rather than in straight lines.