Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001569
01/01/98
Helminthosporium Leaf spot or Melting-out
These diseases can occur at anytime during the growing season. Leaf spot is a problem on fescue while melting-out is more of a problem on Kentucky bluegrass. Both diseases cause small, purple to black spots on the leaves. The center of the spots can turn brown and die while the spot margin remains dark colored.
Symptoms of the disease are the spots until the fungus reaches the plant crown. The plants die when the crown becomes infected causing the green to fade away. The lawn becomes yellowed then brown.
Cultural controls include mowing the grass at 2 inches and avoidance of spring fertilization. Provide moderate nitrogen fertilization during the summer and do not water in the evening or at night. Use resistant varieties. Commonly available fungicides that may be used according to label directions.
Lawn Ring(Patch) Diseases
Several diseases cause rings of dead grass with live green grass in the center. These can occur in both cool and hot, dry weather. Many other problems are similar in appearance and are easily confused with ring diseases. Managing moisture and fertility levels is an important aspect in controlling these diseases.
Symptoms are round circles of dead grass with green grass in the middle. Sometimes there are purple or red grass blades around the margins of the spots.
When possible, water for 20 minutes each day around noon to suppress the symptoms. Avoid moisture stress as drought favors these diseases. Apply 1/2 pound of nitrogen per 1000 square feet in June, July and August and 1 pound per 1000 square feet in September and November. Make the November application after the grass has stopped growing.
Homeowners may find it easier and less expensive to manage the diseases through cultural practices rather than use the fungicides.
Snow Mold
Snow molds are active during cool weather and are most noticeable in the spring after the snow melts. The spots of dead grass killed by snow mold are easily confused with patches of dead bentgrass. A late season application nitrogen, leading to a late flush of growth, may predispose the lawn to attacks of snow mold.
Symptoms are patches of dead, gray to straw colored grass in the spring. A gray or pink fungus may be seen growing at the edges of the patches.
Chemical controls may not be practical for home lawns. Do not apply fertilizer late in the season unless the grass has gone dormant. Rake out the dead grass and reseed if necessary.
Brown Patch
Brown patch is not so common as some other diseases. The fungus that causes the disease can be found in most soils and can live for years without a host plant.
Symptoms are brown patches of grass ranging from a few inches to several feet in diameter. Infected leaves are at first water-soaked but later dry and turn dark brown. A "smoke ring" occurs around the outside of the patch when the humidity is high. Brown patch has a slimy appearance when it infects perennial ryegrass.
Avoid high nitrogen levels during hot, humid seasons and remove the dew in early morning to reduce the disease severity.
Dollar Spot
The dollar spot fungus is most active when the temperature is between 70 and 90 degrees. The disease is spread by infected clippings moved about by mowers, hoses and shoes.
The symptoms are sunken, straw-colored spots about the size of a quarter to a silver dollar. A fluffy, white fungal growth may be seen in the spots in the early morning while the grass is still covered with dew. The lesions on blades of Kentucky bluegrass are yellowed to bleached bands with reddish brown borders on the ends of the lesions.
High nitrogen levels will reduce dollar spot.
Smut
Smut is easily transmitted by people and machinery moving across infected areas. It is hard to detect and may be widely distributed before it has been diagnosed. The disease weakens the grass so that it succumbs to heat and drought stress during the summer.
Symptoms are pale green new leaves with black streaks between the veins. Spores are produced in the black stripes and when the spores are liberated the leaf blade becomes frayed. The tip of the leaf blade becomes segmented, twisted and white.
Reduce spring and summer nitrogen fertilization on infected lawns. Keep watering to prevent summer dormancy, as any infected plants that go dormant will die.