Most turfgrass diseases are caused by pathogenic fungi that invade the leaves, stems or roots of plants, causing various symptoms such as leaf spots, root rots or death of entire plants. Sometimes these fungi produce visible structures such as mushrooms, white powdery mildew or a fluffy, moldy growth.
These fungi are normally present in most lawns, but disease only occurs when environmental factors favor growth of the pathogen and increase the susceptibility of the grass host. This relationship between the environment, host, and pathogen are the key factors in disease causation and control.
Turfgrass management practices alter the environment and therefore have a major impact on disease development. These management factors include mowing, irrigation, fertilization, thatch control, traffic, soil pH and soil compaction.
For diseases with an irregular pattern, be on the lookout for powdery mildew, rust, and leaf spot or melting out. Here are a few quick visual diagnostic tips for these diseases:
Powdery mildew— dusty white to light gray powder on leaf surfaces; leaf yellowing
Rust— From a distance, turf appears orange to rusty in color. On close examination, leaf blades have bright orange to reddish-brown pustules.
Leaf spot or Melting out— elongated to circular spots with purple to black margins and brown, or light tan centers with yellow area surrounding leaf and crown lesions.
Dollar spot— affected areas extend 2" — 6" in diameter, leaf lesions are irregular and bleached white or tan surrounded by dark brown to reddish-purple margins and typically appear in the middle of the leaf.
Fairy ring— an outer ring of dark green grass surrounds dead grass inside. Patch can grow up to 20' in diameter.
Necrotic ring spot and Summer patch— small circles of dead grass surrounding a tuft of green grass ("frog-eyes"), 6" — 12" in diameter. Necrotic ring spot occurs in wet cool conditions (58 — 82F) and summer patch is more active on high summer temperatures (85 — 95F).
Preventing fungal lawn diseases is a lot easier than trying to cure them. This can be done by planting resistant varieties; aerating to improve root growth, soil drainage and air exchange; dethatching, fertilizing properly, and watering deeply but infrequently especially during dry periods of summer and fall.
Chemical sprays may be helpful but are only effective when applied before the disease becomes severe, and may not be necessary when proper cultural practices are followed.