Fact Sheet Number: PP-06
Extension Plant Pathologist
In Delaware, lawn diseases are caused by pathogenic fungi that infect the leaves, stems, and roots of turfgrass plants. As a result of this infection, leaves may show symptoms such as leaf spots, white powdery mold growth, or thin or dead areas in the lawn. The fungi that cause lawn diseases are normally present in most lawns, but disease occurs only when environmental factors favor growth of the fungus and increase the susceptibility of the grass host. In order to manage diseases in home lawns, it is important to understand the interaction among the fungus, the grass host, and the environment. How you manage your lawn affects the overall health of your lawn, which influences its resistance or susceptibility to lawn diseases.
Management Practices that Reduce Diseases
Turf selection. When establishing a new lawn or renovating an existing lawn, select a turf species or mixture of species that is adapted to your site conditions and the level of management that you are willing to do to keep the turf growing well. Delaware is in the transition zone where the cool-season grasses (Kentucky bluegrass, perennial ryegrass, tall fescue, red fescue) are stressed during our hot, dry summers, and the warm season grasses (Bermuda grass, zoyzia grass) which turn brown after a frost and are invasive. Most people prefer the cool-season grasses, and the cool-season grasses turf-type tall fescue under most situations is the best adapted for normal lawn use. It is more drought tolerant, disease resistant, insect resistant and requires less fertilizer than bluegrass, ryegrass and fine fescues. It can be seeded or sodded. Turf-type tall fescue sod usually contains about 10 percent bluegrass for sod strength and ability to repair bare spots. Perennial ryegrass can be added when seeding for quick germination, because tall fescue is slower to germinate. Plant only high-quality seed or sod from a reputable supplier.
Soil and fertility management. Soil conditions including fertility are crucial for establishing and maintaining a lawn successfully. A balanced fertility program based on soil test results every 3 to 5 years and before seeding a new lawn will improve the vigor of plants and their resistance to diseases. Establishing lawns on new construction sites where topsoil may have been removed or only a thin layer is present is particularly difficult. The addition of organic matter in the form of topsoil, leaf mold, compost, or other source of organic matter is recommended. Soil compaction can be a serious problem in a newly constructed lawn as well as in older lawns. Compacted soil excludes air and impedes water movement, which reduces root function, thus causing a decline in plant vigor and disease resistance. In new lawns, it is important to prepare the soil well. Make sure the seed bed is firm but not compacted. In established lawns, core aeration will relieve compaction and reduce thatch accumulation.
Nitrogen management is the most significant influence on disease severity. Excessive applications of highly soluble nitrogen can favor diseases, such as brown patch, Pythium blight, and leaf spot. The succulent grass has thinner cells walls, which are more easily penetrated by fungi. Conversely, turf grown in nutrient-poor soil can be prone to dollar spot, rust diseases, and red thread. Light applications of nitrogen to turf in this condition will stimulate turf to produce leaves faster than the fungus can infect them. Most of the nitrogen requirements for your lawn should be applied in the fall or early spring before April 30. Most lawns require a total of 2-3 pounds of actual nitrogen per 1,000 sq. ft. per year. Kentucky bluegrass lawns are heavy feeders and require the upper limit, while tall fescue lawns require the lower suggested rate.
Mowing. Regular mowing is necessary to maintain quality turf. Mowing favors fungal infection by creating wounds and, in some cases, disseminating the fungus. Height of cut also influences disease susceptibility. Mowing too closely predisposes turf to several diseases by removing the youngest, most photosynthetically productive tissues. This causes a depletion of food reserves in the plant, which are needed for disease resistance and recovery from stress and injury. Remove no more than 1/3 of the leaf tissue at one time. Keep mower blades sharp. When grass is mowed regularly, clippings can remain on the lawn; they do not contribute to thatch and can reduce the nitrogen requirements of the lawn.
Water management. Water is necessary for spore germination and fungal growth. Proper use of irrigation can influence turfgrass diseases. Frequency, timing, and duration of irrigation are important factors to avoid predisposing turf to diseases. At the extreme, over-watered turf is succulent and prone to infection. Waterlogged soils are not well aerated and root growth is hindered. Algae and mosses thrive in waterlogged soils, particularly where turf density is poor. On the other hand, droughty soils predispose turf to infection as well. Irrigate deeply but infrequently to avoid drought stress. This maintains the turf in good vigor and reduces the impact of diseases. In the summer, morning or afternoon is the preferred time to irrigate so that the turf has a chance to dry by nightfall. An alternative approach is not to water and let the lawn go dormant and recover on its own when rainfall returns.
Thatch management. Thatch accumulation of more than ½ inch can restrict root growth and predispose turf to drought damage and diseases. Many turfgrass pathogens can survive as saprophytes in the thatch layer. Summer patch, leaf spot, and melting-out diseases are a few of the diseases favored by excessive thatch accumulations. Regular core aeration in the spring or fall will reduce thatch accumulation and open compacted soils.
Disease Identification and Management
Turfgrass diseases are diagnosed by identifying symptoms and signs of infection. Symptoms are the expression of what is happening to the plant as a result of an infection. These include leaf spots, rots, yellowing, stunting, and wilting. Signs are the visible parts of the pathogen–white cottony growth of a fungus, fruiting bodies of a fungus (mushrooms, pycnidia), or the resting bodies of a fungus (sclerotia of gray snow mold). A description of the major lawn diseases follows and identification depends on the presence of symptoms and signs. It is recommended that you manage home lawn diseases without using fungicides. While there are situations when fungicides are needed, for most cases, fungicides should be applied by lawn care professionals, not by the homeowner. If a disease is present at a level that the homeowner notices, it is usually too late for a fungicide to be effective. To be effective, fungicides need to be applied preventively (before disease occurs, in most cases). Homeowners typically do not have the proper equipment to make an effective application to prevent diseases unless granular formulations of an effective fungicide are available. If a disease occurs regularly and it can kill the turf, preventative spraying is recommended. Some diseases such as brown patch just disfigure the lawn for a time. When the weather that favored disease development stops, the disease stops, and the turf will recover without a fungicide. Knowledge of the diseases that occur on your lawn and their potential for turf damage can help determine what control measures are needed.
The major diseases that are described below are in the order that they normally occur under Delaware conditions.
Pink snow mold. This disease usually occurs in very early spring or late winter; it can, however, occur in late fall as well. Symptoms. 1 to 5 inch patches of matted leaves that have a pinkish or red-brown color. Center of patches may be bleached white. Occurs often in snow melt, but can also occur during cool periods with plenty of moisture. All turfgrasses are hosts, but are not generally killed. Management. Avoid high nitrogen and thatch. Remove leaves in the fall and mow late into the fall.
Red thread. This foliage disease occurs primarily in the spring and fall, sometimes during prolonged rainy periods in summer. Symptoms. Circular or irregular 3-12 in. patches are seen. Blades are often water-soaked and covered with the diagnostic pink gelatinous fungal growth. Dead leaves in thin, affected areas are straw brown, tan or slightly pinkish. Red, brittle thread like strands can be seen extending from the tips of dried infected grass blades. All turfgrasses are hosts especially perennial ryegrass and fine fescues. Affects only the blades does not kill turf. Management. Avoid low nitrogen, excess thatch, and drought stress.
Leaf spot and melting-out. The leaf spot disease occurs in spring summer and fall. Melting-out occurs primarily in summer and fall. Symptoms. Leaf spots can be brown or purple brown, oval-shaped or elongate. Turf may be thinning in irregular patterns. Found primarily on common cultivars of bluegrass. All turfgrasses can be infected. Common bluegrasses can be killed by the root and crown phase (melting-out). Management. Over-seed bluegrass lawns with a resistant cultivar, and avoid high nitrogen, leaf wetness, and low mowing.
Necrotic ringspot. This patch disease occurs in spring and fall. Symptoms. Rings of dead turf 6 inches to several feet in diameter with green healthy turf or weeds in the center. Roots and crowns will be decayed. Bluegrass and fine fescues are affected. Plants are killed. Management. Avoid high nitrogen and drought stress. Over-seed or replace with tall fescue or ryegrass that are resistant.
Dollar spot. This is a leaf spot that produces small spots 2 to 6 inches diameter in affected turf. Dollar spot usually occurs in late spring through fall. Symptoms. Leaf spots are hour-glass-shaped, extend across the blade with bleached centers and brown, purple or black borders. Early in the morning cottony fungal growth can be seen on infected blades. Bluegrass, ryegrass, and fine fescues are hosts, especially in poorly nourished turfs. Plants are rarely killed, primarily a leaf blight. Management. Avoid low nitrogen, mowing too low, thatch, leaf wetness, and drought stress.
Slime molds. These molds produce a gray or black crusty material on blades of turf. It can be rubbed off easily and appears after a prolonged rainy period. This moldy residue may form on plants in rings or arc patterns. These slime molds are not parasitic and do not harm the plants. They are usually found from late spring to fall. Management. The spore masses can be removed by mowing, raking, or washing with a hose. Control thatch.
Brown patch. This disease occurs during the summer when night temperature are above 68 F, high day temperatures and high relative humidity. Symptoms. Brown patch causes a leaf spotting that results in circular brown patches 6 inches to 2 feet and thinned turf. Leaf spots are often irregular and have a thin reddish-brown border. Small patches can coalesce to blight large areas quickly when weather conditions for the disease are ideal. All turfgrass are affected, especially tall fescue and ryegrass. Mature turf is rarely killed. One-year-old stands of tall fescue under severe conditions can be killed. Management. Avoid high nitrogen in spring, leaf wetness, and excessive thatch.
Summer patch. This root and crown rot occurs during the summer, especially during periods of high temperature and drought. Symptoms. Diseased areas are first light green then fade rapidly to a straw color (may be confused with wilting). These areas are often circular or sometimes irregular patches often with living grass or weeds in the center. Sometimes smaller patches can merge and blight large areas of turf. Patches can be sunken and leaves at the borders may have a bronzed appearance. Tip dieback of leaves gives the turf a straw-brown color. Bluegrass and fine fescues older than two years are killed by this disease. Management. Reseed with tall fescue or plant resistant cultivars of bluegrass. Avoid high nitrogen, wet soil, compaction and low mowing.
Pythium blight. This is primarily a leaf disease which occurs during the summer. It is favored by hot, humid periods and poorly drained soils. Seeding in the late summer can fail due to Pythium damping-off and blight when over-watered. Symptoms. Circular small spots are matted, grayish, or water-soaked. Patches will follow surface water flow patterns. Infected blades collapse rapidly, are greasy-looking, and wilt rapidly. Fungal growth can be seen in early morning. Plants will often die in 24 hours. Dead plants are often red-brown to brown in color and matted together. Bluegrass, fescues, and especially perennial ryegrass are killed by this disease. Management. Avoid high nitrogen, leaf wetness and thatch. Improve drainage.
Powdery mildew. This minor disease occurs primarily from late summer to fall especially in shady areas. Symptoms. The leaves of infected pants have a white to gray powdery growth that gives the leaves the appearance of having been dusted with flour or lime. Infected blades may yellow, wither and die. All turfgrasses can be infected, especially in shady conditions, blights only the leaves. Management. Avoid high nitrogen, low mowing, increase light levels or plant shade tolerant grasses.
Rust. This foliar disease is usually seen in the fall. Symptoms. Rust-infected turf becomes reddish-brown or yellow. Rust begins as yellow-orange flecks on individual grass blades and develops into orange or brick-red pustules. Spores within the powdery pustule easily rub off when touched. A heavy rusting can cause leaf blades to die and thin stands of susceptible turf. Primarily perennial ryegrass and bluegrass are infected; most turf including zoyzia can be infected. Turf can be thinned but rarely is killed. Management. Avoid low nitrogen and leaf wetness. Plant resistant cultivars.
Fairy rings. This disease can be seen at any time of the year. Symptoms. Rings or arcs of dead grass bordered by inner and outer zones of dark green grass, or rings of very green grass without a dead zone. Rings can be from 1 to 4 feet in diameter or range up to 20 feet. Mushrooms may or not be present in rings. All turfgrass can be affected, especially at dry locations and poorly nourished turf. Management. Avoid thatch, buried organic debris, and drought stress. Fertilize and rake mushrooms to mask symptoms. Core aeration of stimulated and dead zone, and drenching with an organosilicone wetting agent may alleviate symptoms. Eventually the symptoms will disappear.
Caution: The information and recommendations in these fact sheets were developed for Delaware conditions and may not apply in other areas.
Disclaimer: Reference to commercial products or trade names does not imply endorsement by University of Delaware Cooperative Extension or bias against those not mentioned.