Fungal Diseases of Lawns
Click for larger image Snow mold (Typhula spp.) on turfgrass. W. M. Brown, Jr., Bugwood.org.

Several fungal diseases attack lawn grasses. Following is a short description of the most common diseases to help you sort out which one may be causing your problem. The diseases have been categorized as patches (cause dead spots or patches in the lawn), rings (cause rings of dead or affected grass), or leaf blight (affect individual blades of grass, often over a wide area and no distinct patch or ring pattern may be noticeable). Since most grass diseases affect leaves, patches and rings describe certain diseases in more advanced stages of development.

Result in Dead Spots or Patches

BROWN PATCH (Rhizoctonia solani) Round patches of turf up to several feet in diameter appear watersoaked and eventually turn brown. Dark gray smoke ring is sometimes visible on lower mowed turf.

DOLLAR SPOT (Moellerodiscus spp. & Lanzia spp. or Sclerotinia homeocarpa) Rounded, straw-colored, slightly sunken patches up to 2 inches in diameter on bentgrass greens, 4–6 inches in diameter on other lawns. Spots have a reddish margin and expand until they girdle the leaf blade. Cottony mycelium may be evident in the morning growing over dead tissue.

RED THREAD (Laetisaria fuciformis) Pinkish patches from 3 to 15 inches in diameter develop during rainy weather. The grass in the patches is pink and dead and contains coral-red masses of mycelium. The disease is more common in fescues and grows best from 60 to 70 degrees F.

SNOW MOLD (Gerlachia nivalis and Typhula spp.) Grayish white patches up to 3 feet in diameter. Grass blades are collapsed and matted. Several different fungi are responsible for snow mold disease. Dead areas appear in the turf after the snow has melted.

PYTHIUM (Pythium spp.) Rounded sunken patches up to 1 foot in diameter. Leaves appear water-soaked and shiny at first, turning tan when dry. White mycelium grows over all leaf tissue, not just dead tissue as in dollar spot.

Result in a Ring in the Lawn

FAIRY RING (Marasmius oreades and others) Dark green ring with lighter green center, 2 ft to 100 ft in diameter. A mycelium starts in one spot and spreads in all directions. At certain times of the year, the fruiting bodies, mushrooms, may develop near the outer borders of the year’s growth of mycelium. The grass within the ring may appear to be healthy but later may collapse and die.

NECROTIC RING SPOT (Leptosphaeria korrae) 1–3 patches of red or purple leaves that bleach with water stress. Many patches grow greater than 12 inches in diameter with weeds or resistant grasses forming frog-eyes in the center of the ring.

SUMMER PATCH (Magnaporthe poae) Small tan or straw-colored spots appear on the leaves in early summer. These spots enlarge and coalesce until large areas of the lawn are killed. This disease is active only during the warmest part of the summer.

Affects Scattered Leaves

POWDERY MILDEW (Erysiphe graminis) Powdery mildew appears as dirty spots or blotches on the leaf sheaths, varying in color from light gray to brownish. It is especially prone to develop in wet weather and on poorly drained lawns. The infection spreads by the numerous white spores, which form on infected leaves.

LEAF SPOTS (also called Melting out, various organisms) Many grasses are affected by leaf spot diseases. General symptoms include tan or strawcolored spots on leaves usually bordered by a darker color. Black spore-producing structures may be seen in the center of the spots of some diseases.

RUST (Puccinia graminis var. agrostis) The disease appears as small, reddish spots on the leaves. Merion bluegrass, a special strain of Kentucky bluegrass, is very susceptible to this disease.

ANTHRACNOSE (Colletotrichum graminicola). This disease is common on creeping bentgrass and annual bluegrass. The fungus causes individual grass blades and/or plants to yellow and die when it invades crown and root tissue. Black, hairy, and spiny fruiting bodies are abundantly produced on dead tissue but may also be seen on green leaves as well.

More images:

Click for larger image
Symptoms of red thread on bluegrass (Poa spp). M. A. Hansen, VPISU, Bugwood.org
 
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