What’s Killing Your Grass? A Guide to Lawn Disease Diagnosis and Prevention
- horticulturist and gardening expertJuly 19, 2025
Your lush green lawn suddenly develops dead spots. You don’t have a dog or haven’t recently fertilized or sprayed a total vegetation killer on or near the grass. Despite all your efforts to grow a healthy lawn, the weather was just right for a disease to move in and damage your lawn.
Once you discover a disease and have it properly diagnosed, correct your lawn care practices to speed recovery and avoid problems in the future.
Quick Lawn Disease Prevention Tips
- Mow high to encourage deeply rooted grass that is more drought-tolerant and resistant to disease problems.
- Mow often removing no more than 1/3 the total height with each cutting. This reduces stress on the lawn, and the small clippings can be left in place to return moisture, nutrients, and organic matter to the soil. A season’s worth of clippings is equal to one fertilization.
- Water thoroughly and only when needed to encourage deep roots. Set the sprinklers to run early in the day so the grass blades dry quickly and less water is lost to evaporation.
- Apply the right type and amount of fertilizer at the proper time to limit the risk of diseases. Regular fertilization, three to four times yearly, encourages better results since most soils do not contain the essential nutrients for optimum growth.
- Avoid high nitrogen quick-release fertilizers that promote lush succulent growth that is more susceptible to disease and less drought-tolerant. Instead, use a low nitrogen slow-release fertilizer like Milorganite that promotes steady growth and is more drought-tolerant and disease-resistant.
- Next, take a close look at the dead areas on the lawn. Note the damage pattern and check the green grass blades on the edge of the dead patches for more clues.
Seasonal Lawn Diseases
Review the Seasonal Lawn Disease Guide below to help narrow down the possibilities and determine if you have northern (cool) season grass or southern (warm) season grass. Once you have gathered this information, visit our lawn disease page for more detailed information and photos to help with diagnosis.
Snow Mold (cool-season grasses only)
As the snow recedes, watch for circular, gray-to-straw-colored areas of matted grass caused by grey snow mold. Pink snow mold, however, does not require snow cover to develop. Snow mold is most often a problem when snow freezes and thaws on the grass and ice accumulates. Use a leaf rake to lift the matted grass, remove leaf litter, and reduce the risk of this disease. Keep mowing throughout the fall and avoid heavy fertilization late in the year.
Brown Patch (cool and warm-season grasses)
Monitor lawns for Brown Patch fungal disease when temperatures and humidity rise and grass remains wet for long periods of time. Infected lawns will have somewhat circular patches of thin, light brown grass. Look for white cottony strands of fungal mycelium early in the morning on dew-covered lawns. Check grass blades for small, irregular tan spots with dark brown borders on the individual grass blades. Shaded lawns, those growing on poorly drained soil, in areas with limited airflow and are overfertilized, overwatered or watered late in the day typically suffer more severe damage. Correct poor growing conditions whenever possible, look for more brown spot resistant varieties and avoid heavy fertilization with fast-release fertilizer in early spring and summer.
Dollar Spot (cool and warm-season grasses)
Look for dollar size to 6” diameter spots of bleached or light tan grass if you suspect Dollar Spot fungal disease. Infected leaves have white lesions with reddish-tan margins that often resemble an hourglass. Over and under fertilization, drought, water on the grass blades for an extended time, and mowing too low all increase the risk of this disease. Have a soil test to determine the best type and amount of fertilizer for your lawn. Mow high and if you opt to water, water thoroughly and only when needed.
Rust (cool and warm-season grasses)
Closely examine lawns with a reddish hue to confirm the presence of Rust fungal disease. Rust-infected lawns are covered with an orange or yellowish powder, the fungal spores, that can leave an orange residue on your shoes. Newly seeded and lawns weakened by inadequate fertilization and drought are most susceptible. When seeding and overseeding your lawn select a quality grass seed mix with several cultivars of each species included. This along with proper care, especially after seeding, will help reduce the risk of rust developing in your lawn and recovering from the disease appears.
Leaf Spot (cool and warm-season grasses)
Stay alert for Leaf Spot diseases that can attack lawns. These warm-weather diseases are most evident when temperatures reach 70°F and are more severe as temperatures rise above 90°F. There are some leaf spot-resistant bluegrass varieties that can be used for overseeding dead patches. Avoid excess fertilization and watering in the late afternoon and evenings.
Once you diagnose the problem, correct lawn care practices as needed. Proper care and reseeding of dead areas with the most disease-resistant grass varieties is often enough to manage the disease. Be sure you need a fungicide and that the one selected will provide the desired results before applying. These chemicals are costly, the results can be disappointing, and when used improperly, they can be harmful to pollinators and the environment. So, if you opt to treat your lawn, read and follow label directions for the safest and most effective control.
Further speed recovery with a change in mowing habits. Continue to mow high, but cut the healthy portions of your lawn first. Then, cut the grass in the diseased areas. Once done, use a disinfectant to clean the mower blades, and then rinse with clear water. This, along with collecting and disposing of clippings from the diseased areas of the lawn, reduces the risk of spreading the disease the next time you mow.
Provide proper care throughout the year to reduce the risk of disease. This is much cheaper and often as effective as fungicide treatments. Monitor your lawn throughout the growing season for signs of disease. Discovering problems early means you’ll achieve better results in managing the disease with less effort and money spent.
