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Lawn Diseases: Prevention and Management Published 1/02 In this Guideline:
Identifying the problem Selecting a suitable lawn grass Sampling for commercial laboratory diagonsis See also UC Guide to Healthy Lawns Cultural practices to reduce lawn disease About Pest Notes Publication Glossary

Maintaining a healthy, vigorously growing lawn is the best way to prevent a severe disease outbreak in a turfgrass. A 5,000 square foot lawn contains about four million turfgrass plants, each requiring optimum amounts of water and fertilizer, the right mowing regime, and an aerated, well-drained soil. About 75 to 85% of common lawn diseases can be avoided altogether just by optimizing these practices to avoid stressed grass, which is much more susceptible to disease outbreaks than healthy grass.

For a disease to occur, all three sides of the "disease triangle" must be present. Even if a disease-causing pathogen is present, infection will not occur unless the environment (temperature, quantity of water, etc.) is conducive to disease development and a susceptible host (species of grass) is available. Homeowners can prevent major disease infestations from occurring by planting locally adapted lawn grasses and providing optimal care. Selecting a turfgrass species that is adapted to the climate and intended use and following through with cultural practices that favor the grass rather than the pathogen are important steps a home gardener can take to avoid severe lawn diseases. Many common diseases reduce the quality of the lawn for only a short time and do not result in adverse long-term impacts. Often, when the weather becomes more favorable to growth of the turfgrass, the lawn will recover on its own if proper cultural practices are maintained. Few, if any, fungicide applications should be necessary under these conditions.

IDENTIFYING THE PROBLEM

The primary cause of lawn damage is often difficult to identify, especially if a long period of time has elapsed between when the damage actually occurred and when the problem was recognized. If the damage is severe, secondary pests or problems such as insects, pathogens, weeds, or environmental stress may be contributing to the observed symptoms. For this reason, it is a good idea to inspect your lawn once a week and immediately identify the cause of any damage.

Damage that resembles disease symptoms may result from incorrect watering, fertilizing, or mowing practices; dog urine and pesticides and other chemicals; soil characteristics that result in poor drainage and compaction; vertebrate or insect damage; extremely high or low temperatures; competing vegetation; or thatch that exceeds 1/2 inch in thickness. Irrigation problems are the most common cause of discolored lawns. Fixing broken sprinklers and conducting "can tests" (described in the irrigation section) to insure even water coverage may be all that is necessary to improve the health and appearance of the lawn. No amount of fungicide will control a problem that results from poor watering practices.

Lawn diseases are usually the result of pathogenic fungi that infect the blades, stems, or roots of turfgrass plants. Typical signs and symptoms include leaf spots, white powdery growth, thin grass, and small to large areas of discolored or dying lawn. Diseases are often diagnosed by identifying symptoms of the disease and signs of the causal agent. Visible parts of the pathogen (called signs), such as whitish cottony growth or small, hard, dormant structures (sclerotia), are very useful in the identification process. Other typical symptoms of lawn diseases include frog-eye patterns (i.e., a circular area of dead grass with healthy grass in the center), leaf spots, rotted crowns and roots, yellow leaves, stunting, and wilting. Affected lawn areas can become discolored and lose density quickly. Appendix 1 lists and describes the most common diseases occurring in home lawns in California. Refer to Turfgrass Pests listed in "References," and the UC IPM Web site for photographs of several common lawn diseases.

SELECTING A SUITABLE LAWN GRASS

All types of turfgrass have positive and negative characteristics. There is no one perfect turfgrass suitable for all lawns. The type of grass you choose for your lawn should be compatible with your climate, anticipated use and maintenance level, and aesthetic desires; it also should have some resistance to common diseases.

Make every effort to choose a grass that grows well under your conditions. For instance, too much shade causes stress that can lead to disease development. Similarly, some species are more heat or drought tolerant than others.

New and improved cultivars of lawn grasses offering greater disease resistance, color, texture, density, and uniformity have been developed over the past several years. Contact a reputable nursery or the UC Cooperative Extension office in your county for specific recommendations. Also see the publication Turfgrass Selection for the Home Landscape listed in "References."

CULTURAL PRACTICES TO REDUCE LAWN DISEASE    UC Guide to Healthy Lawns

To prevent lawn diseases, employ cultural practices that promote a dense, vigorous, actively growing grass with good recuperative ability. Cultural practices that promote healthy lawns and help them to resist major disease outbreaks include irrigation, fertilization, mowing, soil cultivation, and thatch removal. Appendix 1 outlines cultural practices that are recommended to prevent specific diseases.

Irrigation

Much of California has a Mediterranean climate characterized by rainfall in winter and spring and very little precipitation during summer and fall. Throughout the state, lawns require irrigation. It is important to follow sound watering practices (whether hand-watering or employing an automated system) to promote an environment favoring growth of the lawn rather than disease outbreak. Applying too much or not enough water can result in unhealthy, slow-growing grass that is vulnerable to pathogens. Waterlogged soils are poorly aerated, which restricts root growth, promotes some diseases, and allows algae and moss to thrive. In general, a deeply watered lawn develops a deeper and more extensive vertical root system, which provides it with greater drought and disease resistance than a shallowly watered lawn.

Turfgrasses vary in water requirements. Warm-season turfgrasses (bermudagrass, zoysiagrass, and St. Augustinegrass) are more drought resistant than cool-season grasses (tall fescue, perennial ryegrass, Kentucky bluegrass) and require about 20% less water. See Table 1 for information on how many minutes to water warm- and cool-season lawns each week in various parts of California, based on the output of the irrigation system (or hose-end sprinkler). It is best to water the lawn until runoff just begins, and avoid watering each day. The number of times to water each week depends on how long the irrigation system can run before water just starts to puddle or run off the soil surface laterally. For example, if a grass needs 40 minutes of irrigation each week, but runoff begins after 20 minutes, then water twice a week for 20 minutes. In cases where soil takes up water so slowly that runoff occurs before 10 minutes, water cycling is necessary. To cycle, irrigate until runoff just begins, turn the system off, and repeat the process in 30 minutes before the soil surface dries out. Several cycles per day may be necessary to apply the desired amount of water.

To determine sprinkler output, conduct a "can test" by setting small, empty, straight-sided, equal-sized containers such as tuna or cat food cans on top of your lawn every 10 to 15 feet between sprinkler heads operated by the same valve and run the system for 15 minutes. After 15 minutes, turn off the system and measure the amount of water in each can with a ruler to determine the average amount of water per can. (To find the average, add up the measurements from all the cans and divide this number by the number of cans used.) Multiply this number by 4 to calculate the sprinkler output rate per hour. Compare this number to the outputs listed in Table 1 to determine how many minutes you need to irrigate weekly. Conducting "can tests" regularly is also useful to determine how evenly irrigation water is distributed over the area watered and allows for sprinkler-head misalignments and other mechanical problems to be discovered and corrected.

The best time to water is early in the morning, when evaporation rates are lowest and water pressure is at its peak. Irrigating in the afternoon is wasteful because of higher evaporation rates, and prolonged damp conditions in the evening may encourage disease development. Remember that irrigation requirements change from month to month and may not be needed at all if it has rained. Reset your sprinkler system to meet your lawn's changing irrigation needs.

Fertilization

Applying the correct amount of fertilizer is an important aspect of maintaining a healthy, dense lawn with improved disease resistance. Fertilization influences turfgrass growth, which in turn influences the recuperative ability of stressed grass. All turfgrasses require nitrogen, and certain sites may also require phosphorus, potassium, and iron on a regular basis. Applying too much nitrogen, especially in a highly soluble, fast-release form, can result in excessive, succulent leaf and stem growth, leading to increased opportunities for fungal penetration that may result in diseases such as brown patch, Pythium blight, and leaf spot. Over-fertilized lawns also require more frequent mowing and water. Conversely, lawns grown under nitrogen-deficient conditions are prone to dollar spot, rust, and red thread diseases.

For moderate, even growth, apply a total of 4 to 6 pounds of actual nitrogen per 1,000 square feet of lawn area annually. Avoid applying more than 1 pound of actual nitrogen per application. Sandy soils require the same amount of nitrogen as clay soils, but at lower rates and applied more frequently. Fertilizer should be applied during the active growing season of the grass (generally during spring, summer, and early fall for warm-season grasses and during fall and spring for cool-season lawns).

Some soils also benefit from the addition of phosphorus and potassium. Potassium in particular may help to prevent disease because it increases the turfgrasses' resistance to adverse environmental conditions (heat, drought, etc.).

Mowing

Maintaining a lawn at the recommended mowing height will improve its ability to resist diseases as well as give it greater aesthetic appeal. The frequency with which the lawn is mowed should be based on the growth rate of the grass. Mow lawns often enough so that no more than one-third the length of the grass blade is removed at any time. Removing too much of the grass blade can increase the susceptibility to several diseases by depleting food reserves in the plant, making it difficult for the plant to recover from stress and injury. Repeated scalping greatly reduces the vigor of a turfgrass. Maintain sharp mower blades to avoid fungal infections that result from pathogen entry into created wounds. When grass is mowed regularly, clippings can be left on the lawn, a practice called "grasscycling." Grasscycling has not been found to significantly increase thatch or disease incidence. For additional information, see Mowing Your Lawn and 'Grasscycling' listed in "References."

Soil Cultivation and Thatch Removal

Soil compaction reduces root growth as well as the grasses' recuperative ability, thus increasing a lawn's relative susceptibility to diseases. Soil cultivation, such as coring or aerification, will improve shoot and root growth and recuperative ability, and decrease the likelihood of disease and insect damage.

Thatch is a partially decomposed layer comprised of roots, stems, rhizomes, crowns, and stolons situated above the soil surface. Up to a 1-1/2-inch layer of thatch is beneficial and provides insulation to roots, reduces soil water evaporation, cushions playing surfaces, and may prevent soil compaction. However, thatch layers greater than 1/2 inch should be removed to avoid restricted water entry into the root zone, resulting in drought stress.

Several turfgrass pathogens can survive in the thatch layer, including those that cause summer patch, leaf spot, and melting-out diseases. Bermudagrass, Kentucky bluegrass, and kikuyugrass produce more thatch than most other turfgrasses and require regular dethatching. Equipment rental businesses often carry dethatching (verticutting) machines that are specifically designed to remove thatch from home lawns.

WARNING ON THE USE OF CHEMICALS

REFERENCES

Gibeault, V. A., R. L. Baldwin, and K. D. Gowans. 1984. Lawn Aeration and Thatch Control. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 2586.

Grebus M., and J. Hartin. Mar 2000. Diseases from UC IPM Pest Management Guidelines: Turfgrass. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3365-T.

Harivandi, A., and V. A. Gibeault. 1999. Mowing Your Lawn and 'Grasscycling.' Oakland: Univ. Calif. Agric. Nat. Res. Publ. 8006.

McCain, A. H., R. M. Endo, and H. D. Ohr. 1989. Fungal diseases. In A. D. Ali and C. L. Elmore, comps. Turfgrass Pests. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 4053.

Harivandi, M. A., V. A. Gibeault, M. J. Henry, L. Wu, P. M. Geisel, and C. L. Unruh. 2001. Turfgrass Selection for the Home Landscape. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 8035.

PUBLICATION INFORMATION

Pest Notes: Lawn Diseases: Prevention and Management
UC ANR Publication 7497        

Authors: J. Hartin, UC Cooperative Extension, San Bernardino Co.; P. Geisel, UC Cooperative Extension, Fresno Co.; and M. A. Harivandi, UC Cooperative Extension, Alameda Co.
Editor: B. Ohlendorf
Technical Editor: M. L. Flint
Produced by IPM Education and Publications, University of California Statewide IPM Program

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