According to the Purdue University Extension, there are more than 70 diseases that affect lawns. Most lawn diseases are caused by fungi. Usually, lawn infections afflict leaves and shoots, which are the above-ground parts of grass. Common signs of a lawn disease are particular patterns appearing on grass strands. Many lawn diseases can be avoided or minimized by using proper management practices contributing to strong root systems.
Fungi are thread-like, microscopic organisms, spread by spores traveling through air or water. These spores produce new diseases in favorable environments or whenever a host is vulnerable, notes North Dakota State University Extension. For example, an infection can develop when a fungus or pathogen is on a susceptible host when undesirable environmental factors such as temperature, inadequate light, poor nutrients, moisture and stress are present.
Powdery mildew, a common fungal disease that looks like dusted flour on leaf blades, typically occurs in shaded areas. It usually attacks in moist, cool weather. Powdery mildew rarely kills a plant, but it looks unattractive. The University of Rhode Island Extension does not recommend using fungicides.
Dollar spot, which is named for the disease's silver-dollar shaped patches, is common on putting greens, fescues and bluegrasses.To avoid the disease mow no more than one-third of a grass's height, advises Cornell University Extension.
Signs of rust infection are yellow lesions, with spores later covering the blades. To prevent rust, water in early morning hours and mow grass to proper heights.
When determining a disease, it helps to recognize key factors such as the shape and size of dying or dead plants. Note the quality of a lawn's root system. Growth characteristics and leaf color are other considerations, according to North Dakota State University. Consider the temperature and time of year when an infection occurs. Keep records of lawn treatments such as mowing height, fertilizers, herbicides and watering amounts.
Often homeowners believe their lawns suffer from a disease, but damage it's actually due to dog urine. Dog urine contains salt that can kill grass. To determine if it's a dog urine problem, look for dead spots that have greener grass around edges, advises Cornell University.
Some diseases afflicting fescue and St. Augustine turfgrass can look like insect damage, warns Life Tips.com. A careful inspection of soil, roots, stems and leaves, where the damaged area borders healthy grass, helps to determine the problem. Symptoms of disease may include blotchy leaves that are also greasy, moldy or slimy. Another sign that a lawn is suffering from a disease, rather than an insect problem, is leaf yellowing that easily detaches from the sheath.