Lawn fungal diseases can be frustrating to diagnose and difficult to treat. The symptoms often look similar to insect damage or even damage from animals. Once daytime temperatures are warm and nighttime temperatures are still cool, it is time to take care how often you water, and be cautious about applying fertilizer. Fungus and grass have similar temperature and nutrient needs, so overzealously feeding and watering the lawn may inadvertently invite disease.
The list of lawn fungal diseases can be alarmingly long: brow patch, fusarium blight, fairy rings, slime molds, snow molds, rust ... the list goes on. The good news is that most of these are highly seasonal. Snow molds, for example, are problematic while snows are melting. Brown patch likes it hot, preferring weather between 80 and 90 degrees Fahrenheit. The window of opportunity is so small that some diseases can be "waited out." Proper identification and diagnosis, however, usually requires the help of your local extension service or garden center.
Most varieties of lawn fungus need mild to warm temperatures, ample moisture and a ready supply of food or fertilizer to take hold. Water only as much as your local extension service recommends for your area and time of year, and do not overfertilize. Avoid watering in the evening; water in the morning instead. Correct low-lying areas that hold water by adding soil or digging a drainage swale. The best prevention is starting with disease-resistant varieties of turf grass, or switching to a non-turf grass groundcover.
Once the lawn fungus has been properly identified, avoid watering the grass more than necessary. Fungus prefers moist conditions, and sometimes just denying the disease proper moisture is enough to get your grass through the season. Do not fertilize. This will feed the fungus and speed your lawn to early demise. Composts with alfalfa help suppress certain varieties of lawn fungus if applied 1/4 inch thick. As a last resort, spot-treat the lawn with a fungicide that specifically treats the identified fungus.
Fungicides have a habit of killing beneficial microbes along with fungal diseases. Some of the beneficial fungus and microbes extend or strengthen the roots of your grass or even help prevent the spread of diseases already in the soil. Fungicides can also harm beneficial insects such as predatory nematodes and earthworms. Use fungicides as sparingly as possible to treat lawn fungal diseases.
When your lawn seems past all hope, or if you have an area that is repeatedly beleaguered by fungal diseases, it is time to consider other planting alternatives as a last resort treatment option. Transform spots that hold too much water into bed areas for moisture-loving plants. For large areas, consider creeping vines, low-growing ferns or even clumping grasses such as monkey grass as a groundcover alternative to lawn. Lawns are surprisingly demanding on water, upkeep and fertilizer. Switching to other perennial plants often reduces maintenance and increases the interest of your yard.