Last updated 10/1/98

Managing Lawn Diseases

Richard Latin, Professor of Plant Pathology

This publication addresses the management of the major diseases that occur on turfgrasses planted for midwestern lawns. Most of the diseases occur on above-ground plant parts (leaves and shoots). They are identified by patterns that appear on stands of affected turf and by symptoms and signs of disease that occur on individual plants. Development of all leaf and shoot diseases requires extended periods of surface moisture. Also, careful management of nitrogen nutrition is essential for managing these diseases in the most effective and efficient manner. Root disease pathogens cause root injury as well as a general dieback of individual leaves and decline of the turf stand. Most root disease problems are minimized by management practices that contribute to a healthy and vigorous root system.

Although there are over 70 diseases of turfgrass species worldwide, midwestern lawns are affected by relatively few. The management of these important diseases are discussed below.

Snow Molds

Two types of snow molds, gray snow mold and pink snow mold, are recognized on turf grass species. They are caused by different pathogens but often occur under the same environmental conditions.

Gray snow mold is first noticeable in early spring, after snow melt is complete. It commonly occurs in areas where snow accumulation is the greatest, e.g. along driveways and in areas where drifts develop. It is caused by a fungus that inhabits most soils. The pathogen becomes active at cold temperatures; only under snow cover when soil is not frozen. Special structures are produced by the fungus to ensure its survival during summer months. Turf affected by gray snow mold has a matted, bleached, gray-white appearance. In most cases, infected grass blades are killed, but crowns and roots usually survive.

Pink snow mold (also called Fusarium patch) also is caused by a pathogen that inhabits midwestern soils. Snow cover is not required for the development of pink snow mold. The pathogen may also be active during cool (40º - 50º F) wet weather in spring and fall. In recent years in Indiana, pink snow mold outbreaks occured as late as mid-June. The fungus is able to survive periods of summer heat and drought as mycelium and dormant spores in the soil. Under favorable conditions, growth of the fungus on grass blades results in a pink cast to affected areas (hence the name pink snow mold). The patches of affected turf become brown or tan as warm dry weather returns. Like gray snow mold, pink snow mold usually kills leaf blades only. However, under especially severe conditions, crowns and roots may be severely damaged or killed.

Management options for midwestern lawns are similar for both gray and pink snow mold. Cultivars of perennial ryegrass, Kentucky bluegrass, and tall fescue that exhibit any resistance to snow mold infection are not available. Also, chemical control is neither practical nor necessary. Serious snow mold problems can be minimized by mowing turf at the recommended heights well into the fall to avoid the exposure of long, lush turf to disease favorable conditions. Eliminating snow piles or accumulations that prolong the duration of snow cover in problem areas will help reduce the snow mold threat. Other practices will help reduce snow mold injury to lawns include improving drainage to eliminate wet pockets within the stand, and lightly raking affected areas in the spring to break up and dry out the matted grass. Reseeding may be necessary if infected areas do not recover by early May.

Leaf Spot

Leaf spot is the name given to a fungal disease of cool season grasses that may also result in a thinning or 'melting out' of turf stands in late spring. Environmental conditions favorable for disease spread include a broad range of temperatures (50º - 90º F) accompanied by several consecutive days with wet periods that exceed 10 hours. Severity of disease development is closely related to turf stress (low mowing heights, drought conditions, etc.) and an excess of nitrogen fertility in early spring.

The leaf spot disease is caused by several closely-related pathogens that overwinter with infected plant material in the turf stand and soil. The fungi become active during wet spring and fall weather, and release an abundance of spores into the atmosphere with periodic rain showers. The windborne spores eventually may be deposited onto healthy grass and create new infections. The spores also may be washed down to the soil level and infect crowns and roots, initiating the melting out phase of the disease syndrome.

Distinct, small (1/16" diameter), oval, purple-brown lesions on grass blades are characteristic of the initial phases of the leaf spot disease. The spots enlarge to span the width of the leaf, killing the tips of affected blades. The stand will thin as infections to crowns and roots kill entire plants. As the disease advances, more plants die, giving the 'melting out' appearance. Patches of damaged turf will not recover once the disease progresses to the melting out phase.

Prevention of leaf spot development is based on avoiding the application of nitrogen fertilizer in early spring. Midwestern lawns that appear lush and green in May are prime targets for this disease! Maintenance of Kentucky bluegrass lawns at mowing heights greater than 3" will lessen the threat of leaf spot infection. Replanting dead areas with cultivars that are less susceptible to leaf spot is advised to improve the appearance of severely affected turf. Fungicides can be effective in minimizing the damage. But, for satisfactory results, they must be applied early in the spring, as grass begins to green, or in the fall.

Red Thread

Red thread may occur on all cool season turfgrasses used for midwestern lawns. Disease development is favored by long dew periods over a broad range of cool temperatures (40º - 70º F). It is generally a disease of spring and fall, with symptoms often disappearing during the heat of the summer. Outbreaks occur in turf stands where nitrogen fertility is at less than optimum levels. The disease may be spread by water or machinery moving over infected turf. Patches enlarge as the fungus grows outward from the initial infection site.

Symptoms initially appear as small (3-8") circular patches of off-colored turf. The patches tend to have a pink cast. Close inspection of affected grass blades may reveal red 'threads' growing from the leaf tips. The threads are structures called sclerotia that enable the fungus to survive in the turf and upper layers of soil. Symptoms can be confused with dollar spot at initial observation. However, the dollar spot pathogen does not produce the red sclerotia on leaf tips, and the red thread pathogen does not produce the bleached appearance of infected leaves nor the hourglass-shaped lesions characteristic of dollar spot infection.

Since perennial ryegrass is especially susceptible to red thread, avoid planting more than 10% ryegrass in a seed mix when establishing a lawn. Relieving nutrient stress (particularly nitrogen) will effectively limit the incidence and severity of red thread outbreaks. Also, scheduling irrigation to avoid prolonged periods of leaf wetness will reduce the extent to which red thread patches expand. Irrigation is not recommended during late afternoon and evening hours (from 4:00 pm through midnight). Some cultivars of Kentucky bluegrass and perennial ryegrass exhibit a moderate degree of resistance to red thread infection. Like many other lawn diseases, fungicides are recommended only as a last resort. Numerous protectant and systemic fungicides are effective against red thread.

Dollar Spot

Dollar spot is common on all cool season turfgrasses used for midwestern lawns. It is caused by a fungal pathogen that survives in local soils and infected turf residue. Dollar spot is generally considered a disease of late spring and early fall, when night time ambient temperatures range between 50º and 60º F and long dew periods prevail. However, symptoms may be apparent throughout the summer, if environmental conditions remain favorable. Outbreaks of dollar spot are more likely to occur in turf that is growing slowly due to environmental stress and nitrogen deficiency. New dollar spot patches arise with the transport of infected grass clippings. Individual spots enlarge as the pathogen grows outward from an infection center.

The disease is called dollar spot because the spots produced on short mown golf course turf are approximately the size of a silver dollar. It reveals how long this disease has been studied, because silver dollars have been scarce for the past 40 years or more. The spots are much larger on turf that is maintained at heights of 2" or greater. On warm, humid mornings, the pathogen produces patches of cottony growth that may resemble signs of Pythium infection. Symptoms on individual leaves are characterized by bleached white lesions with distinct brown margins. These lesions often have an hourglass shape and extend across the width of the leaf blade.

The strategy for preventing intolerable dollar spot outbreaks is based on maintaining vigorous turf with adequate nitrogen fertility during the latter weeks of spring and early fall. Timing irrigation to avoid prolonged dew periods is another factor contributing to disease control. Deep, infrequent irrigation can be helpful in minimizing dollar spot outbreaks. Cultivars of various turf species with slight resistance are available. Dollar spot is readily controlled with a few timely applications of protectant and systemic fungicides if conditions warrant chemical application.

Brown Patch

Brown patch is a disease caused by a common soil fungus that attacks all cool season grasses. Perennial ryegrass and tall fescue appear to be more susceptible than other species planted for midwestern lawns. Conditions that favor brown patch occur in summer months, when dew periods are long (at least 12 hours) and temperatures are above 85º F during the day, and above 65º F during the night. Disease development is more severe under conditions of high nitrogen fertility. Brown patch is considered a foliar disease that does not severely affect crowns in mature stands. Turf stands may be weakened by severe infection but should recover with the return of environmental conditions that do not favor disease.

Because the brown patch fungus is an excellent saprophyte, the pathogen will always exist in the turf/soil environment, and the disease will be an annual threat when turf is vulnerable to infection and favorable weather conditions prevail. Damage generally is more severe on grasses maintained at lower mowing heights (less than 3" for tall fescue and 2" for perennial ryegrass). Individual patches spread by rapidly growing mycelium, and by mechanical operations, such as mowing, that redistribute infested grass residue. Spread also may be facilitated by water moving across the turf surface.

Initial symptoms of brown patch include a thinning of the turf in clustered, roughly circular patches. The areas will expand as suitable weather persists. The distinct patch symptom is more evident on turf maintained at less than 1" in height. Characteristic brown patch lesions (tan blotches with irregular dark brown margins on blades) can be observed on individual infected leaves. Under extremely favorable conditions on shorter mown turf (less than 2" for most lawn grasses), white-gray mycelium may be observed growing at the margins of the symptomatic turf.

Careful management of water and nitrogen fertility will contribute significantly to brown patch prevention. Nitrogen fertilizer applications should not exceed 1/2 lb N per 1,000 sq ft per month during June, July and August. Dew periods may be minimized by avoiding irrigation during late afternoon and evening hours. Overseeding with cultivars that are less susceptible should be considered where brown patch is a chronic problem. A variety of fungicides are registered for brown patch control. If a fungicide is necessary, its spectrum of activity should be considered to protect against other potential threats at the same time.

Pythium Blight

Pythium blight can be very destructive to stands of perennial ryegrass during the hot, humid days of summer. Infection and disease spread are favored by daytime temperatures that exceed 90º F and extended dew periods (more than 12 hours) during which the night temperatures exceed 65º F. It is most severe in ryegrass lawns with high nitrogen fertility and lush leaf growth. Pythium infection will kill leaves, stems and crowns. Unlike other diseases that permit turf recovery after conditions favorable for disease development change, Pythium-infected turf collapses and dies without the possibility of recovery.

Initial symptoms of Pythium blight include small patches of web-like mycelium that appear early in the morning while the grass is still wet with dew. The patches can enlarge very quickly resulting in large areas of dead and dying turf. Pythium kills grass right down to the crowns; the centers of the patches are matted down, usually with a brown or tan color.

The pathogen produces survival spores (called oospores) in infected tissues. The spores may survive indefinitely in soil by themselves or associated with infested residue. Individual patches spread by rapidly-growing mycelium during disease-favorable conditions. New patches may be established by mechanisms that can transport infected grass blades from the infection source, such as foot traffic, mowing or moving water.

There will be some seasons where Pythium infection cannot be avoided. However, serious damage can be minimized with proper maintenance of turf. Areas where soils are poorly drained should be repaired. Irrigation should be applied during the day to reduce the chance of prolonging the dew period. Turf should not be mowed when wet. Pythium threatens lush stands of turf, therefore nitrogen fertility should be maintained at moderate levels during weather conditions that favor Pythium outbreaks. There are a few cultivars of perennial ryegrass that exhibit limited resistance to Pythium blight. If the disease is a chronic problem, then these cultivars should be used when overseeding to restore areas of blighted turf. Fungicides may be required to maintain high-value turf. Several effective systemic products are available. Diagnosis of Pythium should be confirmed before applying these fungicides because they are very expensive and are effective against no other turfgrass pathogens.

Rust

Rust can occur on all cool season turfgrasses, but is a greater concern on perennial ryegrass and Kentucky bluegrass. It is normally observed during or after the heat of summer. Infection occurs over a range of temperatures (68º-86º F) when long dew periods occur. Rust can be a serious problem on turfgrass maintained under stressful conditions such as low fertility, abnormally low mowing height, and drought conditions. Severe infections weaken the turf and may predispose the stand to winter injury and weed infestation.

Rust infections initially occur in clusters. The turf appears off color (an orange-yellow cast) when observed from a distance. Spores produced by the rust fungus will turn one's shoes a similar color after walking through the affected grass. Close inspection of rusted turf shows small orange pustules on individual grass blades. Infected blades turn yellow and die, resulting in a general thinning of the turf stand.

The rust fungus is an obligate parasite and requires living tissue (actively growing or dormant) for every part of its existence. In mild winters the pathogen may survive locally on infected plants. Otherwise, it survives in warmer climates year-round. When conditions are suitable, spores are produced within the pustules on infected leaves. The spores are dispersed into the atmosphere where they may travel hundreds of miles before being deposited with rain showers in new locations.

The most effective and efficient means of limiting damage due to rust involves minimizing stress caused by lack of nitrogen fertilizer during the summer months. Maintaining adequate fertility levels throughout the summer will reduce the threat of rust outbreaks. Application of nitrogen at the first sign of a problem will limit the damage due to rust infection. An application of 1.0 lb N/1000 sq. ft. in late August - early September will significantly reduce the extent of rust development. Some cultivars of perennial ryegrass, Kentucky bluegrass and tall fescue are less susceptible to rust infection. Use of disease resistant cultivars for overseeding should be considered as part of an integrated approach to disease management, especially if rust is a chronic problem. Fungicides can be applied for effective rust control but should only be used as a last resort. The judicious use of nitrogen and a little patience will do as well as any fungicide application.

Slime Molds

Slime molds are close relatives of fungi. They do not cause infectious disease on turfgrass, but their sudden and striking development can change (temporarily) the appearance of a lawn. Slime molds survive in the soil and become active in cool, wet weather, usually appearing on very healthy stands of turf. The mold grows on the surface of grass blades, including clippings, using available decaying organic matter for sustenance. It can produce a bloom of millions of spores in various colors (orange - reddish brown - gray - black) within a few days. The spores are easily dispersed into the wind after they are formed on grass blades. Other than the colored appearance of the active slime mold, the only effect on the turf might be a brief period of chlorosis or yellowing where a dense colony had grown. Dry weather and regular mowing usually result in disappearance of any signs of slime mold within days of its initial development.

Patch Diseases: Summer Patch and Necrotic Ring Spot

Summer patch and necrotic ring spot are root diseases that primarily attack Kentucky bluegrass, although other species including annual blue grass and tall fescue may be affected. They are caused by different pathogens, but because they are indistinguishable from based on visual symptoms, summer patch and necrotic ring spot are referred to as 'patch disease' out of convenience. Patch disease often occurs in mature stands of turf that are three or more years old, but occasionally affect younger stands. When ample moisture is present, infection can occur over a broad range of temperatures. Summer patch is favored by warm wet conditions; necrotic ring spot is favored by cool wet conditions. Symptom expression of summer patch is associated with heat and drought stress. Symptoms of necrotic ring spot may occur throughout the growing season.

The pathogens survive in the soil and in infested plant tissues. The most significant mechanism for their spread is movement of infested root, crown, or stem tissues during turf maintenance practices such as core aeration and dethatching. The benefits of such maintenance practices outweigh the drawbacks in most cases.

Symptoms on Kentucky bluegrass are similar, beginning with a gray-green wilting of small patches 1 - 3" in diameter. White banded lesions characteristic of summer patch development may occur on leaf blades of affected plants during periods of heat stress. Patches may expand to more than 12" in diameter, with leaves turning tan or brown. Circular patches may not be conspicuous on grass maintained at tall (2" or more) mowing heights. Sometimes, the centers of patches of affected grass are recolonized, resulting in a patch that resembles more of a ring of symptomatic turf. Close inspection of affected plants by patch disease will reveal vascular discoloration in root and crown tissues.

Relieving stress in Kentucky bluegrass stands will help avoid serious patch disease damage. Poorly drained or compacted sites should be aerified to reduce compaction, increase rooting, and decrease the level of symptom expression. Mowing height should be maintained at 3" or more for Kentucky bluegrass to alleviate plant stress. Deep infrequent irrigation and avoidance of thatch layers greater than 1/2" will reduce the extent of symptom expression. Patch disease can be a problem in newly sodded lawns. Therefore, proper soil preparation prior to laying sod is a must. Frequent aerification of the lawn for the first 5 years after establishment will help the turf root into the soil, and lessen the effect of summer patch infection. Replanting, or re-sodding with more resistant Kentucky bluegrass cultivars or, overseeding with perennial ryegrass will help minimize the extent of damage and improve the lawn appearance. Control of summer patch and necrotic ring spot with fungicides is always expensive and often erratic. Fungicide application should be considered only as a last resort and should be handled by lawn care professionals.

Fairy Ring

Fairy rings are caused by various species of fungi that inhabit most midwestern soils and obtain their nutrients from dead and decaying organic matter. Their activity is most noticeable during spring and fall, especially when turf is under conditions of environmental stress, particularly moisture stress and nitrogen deficiency. The rings can enlarge from a few inches to several feet in diameter each year. Sometimes the rings grow together, giving them a scalloped, rather than a ring appearance. Fairy rings are classified according to 3 types based on their symptoms. Type I rings are identified by a ring of dead grass, sometimes with an additional ring of dark green grass. Type II rings are characterized by a ring of lush, dark green grass that appears to be growing faster than other turf. Type III rings exhibit only a ring of mushrooms.

Regardless of their classification, fairy rings are impossible to eliminate. They occur among stands of all grass species, and attempts at chemical control through fumigation or soil fungicide drenches are almost always unsuccessful. Fairy rings are best managed by alleviating stress to reduce their prominence and disguise their appearance. Because fairy ring fungi create a hydrophobic soil environment, deep watering, perhaps in combination with aerification and a wetting agent, will reduce the extent to which they develop. More frequent mowing and the application of moderate amounts of nitrogen fertilizer will help mask their appearance.

Additional Resources

Compendium of Turfgrass Diseases. Second Edition. 1992. This is a 98 page comprehensive resource that addresses all turfgrass diseases. It includes 186 color plates with diagnostic features and numerous illustrations that provide a more complete understanding of infectious diseases. The cost is approximately $35.00 and is available from:

American Phytopathological Society
3340 Pilot Knob Road
St. Paul, MN 55121-2097

Turfgrass Diseases: Diagnosis and Management CD-ROM. 1995. This is an interactive CD that addresses more than 65 diseases and disorders of turfgrass. It includes over 350 quality images, numerous figures and other helpful features. Some information from the Compendium (described above) appears in this CD. The cost is approximately $169.00 and is available from:

American Phytopathological Society
3340 Pilot Knob Road
St. Paul, MN 55121-2097

Purdue University

Cooperative

Extension Service

West Lafayette

Indiana 47907

9/98