Summer Lawn Diseases
Brown patch
Example of Brown Patch. Photo by Dr. Van Cline, Agronomist, The Toro Company.
This disease is prevalent during moist, hot weather on over-fertilized lawns. Brown patch, also known as rhizoctonia blight, is most active when grass remains wet and temperatures reach 80 to 90 degrees Fahrenheit.
Look for: Dark, water-soaked looking grass turning into browned-out circular areas several inches to several feet in diameter. Some green leaves may persist within the patch, and roots remain intact. In addition, blades may have irregular ash gray lesions with a dark brown margin running along one side. On short turf, a 2-inch “smoke ring” of gray mycelium may encircle the patch in early morning.
Management: Water deeply but infrequently, mow high, remove excess thatch, and improve aeration and drainage.
Dollar spot
Example of Dollar Spot. Photo by Dr. Van Cline, Agronomist, The Toro Company
This disease affects lawns during heavy dew periods that are low in nitrogen and stressed by drought. Dollar spot, named after the silver-dollar sized spots that appear on closely mowed lawns, signal nutritionally deficient turfs.
Look for: Mottled, straw-colored 4 to 6 inch wide patches on lawns with taller grass. Grass blades have light tan bands with reddish brown margins spanning across them. Patches may merge to form large, irregular areas. Grayish white cobweb-like mycelium may also be present in early morning.
Management: Maintain adequate nitrogen and potassium fertility, water deeply when necessary, and remove excess thatch. If your grass is prone to dollar spot, remove morning dew by dragging a hose across the lawn. Alternatively, overseed with a blend of improved cultivars.
Pythium blight
Pythium blight, also known as grease spot or cottony blight, spreads rapidly and causes die-off, often in less than a single day. It is a serious disease involving the entire grass plant, and occurs on poorly drained soils that have a wet grass canopy. Look for it when nighttime temperatures (Fahrenheit) plus relative humidity equals 150.
Look for: The sudden appearance of 1 to 6 inch wide reddish brown wilted patches, which turn to streaks as they enlarge along drainage patterns. In early morning, the grass will be slimy, dark, and matted. White cottony mycelium may be present when grass is wet. As it dries, the grass turns light tan and shrivels.
Management: Improve drainage and air circulation, avoid overwatering, aerate, reduce excess thatch, and avoid nitrogen fertilizer during warm weather. Check calcium levels, and add lime if deficient. Observe closely for spread, and consult your Cooperative Extension Service (CSREES) if the disease progresses.
Rust
Rust is a self-limiting disease aptly named for the color your grass turns when it is present. A closer look at grass that has been infected with rust shows orange, red, brown, and yellow spores that rub off when touched. Photo by Dr. Van Cline, Agronomist, The Toro Company
When growth slows during hot, dry weather, rust appears on compacted, shady, or low fertility lawns.
Look for: Initial small yellow flecks that develop into pustules releasing yellow, orange, red, or dark brown spores. From a distance, the turf appears orange or yellow, and the colored spore residue will rub off when touched.
Management: Provide appropriate fertilization and irrigation, prune low-hanging tree branches to reduce shade, maintain aeration, mow frequently, and bag clippings. Upgrade your lawn with a rust-resistant cultivar if the disease persists.
