Spring Technical Guide: 6 Turf Diseases and Their Cures

EDITOR'S NOTE: For additional articles from the March 1997 Spring Technical Guide from Lawn & Landscape magazine, please click the following links:

• Spring Technical Guide: PHC - Making Plant Health Care Work
• Spring Technical Guide: Managing Plant Health Through Better Root Growth
• Spring Technical Guide: The State Of Biologicals

1. HELMINTHOSPORIUM LEAF SPOT, MELTING-OUT and NETBLOTCH

Pathogens: Several species of Drechslera and Bipolaris.
Primary Hosts: Kentucky bluegrass, creeping red fescue, perennial ryegrass.
Predisposing Conditions: Overcast, rainy weather from early spring to early winter.

Many of the fungi that cause leaf spotting and melting-out diseases once belonged to the taxonomic genus of Helminthosporium. Today, these fungi are more appropriately referred to as a species of Drechslera or Bipolaris, but it will be difficult for most people to recognize names other than “Helminthosporium.”

Perhaps the most important springtime disease of Kentucky bluegrass is Helminthosporium leaf spot caused by Drechslera poae. This disease is not as devastating as it once was due to the development and widespread use of resistant bluegrass cultivars. Many “common” types of Kentucky bluegrass are very susceptible to leaf spot, but these cultivars remain frequently used components of bluegrass blends due to their lower retail price to end-users.

D. poae is a cool weather pathogen, and is most active during the spring (especially April and May) and autumn (especially September and October). D. poae causes disease that may occur in two phases: the leaf spot phase and the melting-out phase. Typically, distinct oval-shaped, purplish-brown leaf spot lesions are produced on the leaves and sheaths of affected turfgrass plants. Heavily infected turf appears yellow or red-brown in color from a standing position.

If overcast, cool and drizzling weather continues, successive layers of leaf sheaths are penetrated by the fungus and crowns are invaded. At this point, the disease enters the melting-out phase. During this phase, entire tillers die, and the turf loses density. Hence, it is the melting-out phase of the disease that is most injurious to plants.

As temperatures increase in late spring and early summer, the activity of D. poae subsides. D. poae may again become active with the advent of cool, moist weather in the fall. Generally, disease severity is greater in the spring than fall. In summer, Kentucky bluegrass, fine leaf fescues and perennial ryegrass may decline due to invasion by Bipolaris sorokiniana. This fungus also may cause a leaf spot and melting-out phase. B. sorokiniana is normally most severe when temperatures exceed 85 F, humidity is high and there is a lot of overcast weather.

Netblotch is a disease of tall fescue and perennial ryegrass caused by another ‘Helmin-thosporium,’ Drechslera dictyoides. D. dictyoides also is a cool, wet weather pathogen that attacks turf primarily during spring and fall. Initially, symptoms appear as minute, purple-brown specks on tall fescue leaves. As the disease advances, a dark brown, net-like pattern of necrotic lesions develops on leaves providing a netblotch appearance. These netblotches may coalesce, and leaves turn yellow or brown and die back from the tip.

On leaves of perennial ryegrass, numerous oblong, dark brown lesions are produced. Under ideal environmental conditions, the fungus may invade crowns of perennial ryegrass, causing a melting-out of the stand. Netblotch is a chronic, but relatively nondestructive, disease of mature tall fescue.

Cultural Management: Raise the mowing height, avoid spring and summer applications of water soluble nitrogen fertilizers; avoid applying broadleaf herbicides or plant growth regulators when these diseases are active and causing thinning of the stand; and overseed in the fall with resistant cultivars.

Chemical Management: One or two applications of a penetrant such as Chipco™ 26019 or Curalan™, and chlorothalonil (i.e., Daconil™ or Thalonil™) provide the longest residual control.

2. RED THREAD

Pathogen: Laetisaria fuciformis.
Primary Hosts: Perennial ryegrass, creeping red fescues, tall fescue, Kentucky bluegrass.
Predisposing Conditions: Overcast rainy periods at any time of year, especially spring.

Red thread has become one of the most common diseases of turfgrass, and it is generally among the first diseases to appear in early spring. Development of red thread is favored by cool (65-70 F), wet weather in the spring and fall, but may also occur during warm or very cool to cold weather in the presence of plenty of surface moisture or at snow melt in February and March. Occasionally, it occurs during prolonged overcast and drizzling, rainy weather in the summer. It can even become widespread among turfgrass species during mild winters.

The recent and severe outbreaks of red thread can be attributed to the more widespread use of perennial ryegrass. Before 1980, perennial ryegrass was not commonly used as a turf; however, the advent of the improved “turf-type” cultivars has led to its widespread use a turfgrass species. Unfortunately, perennial ryegrass is very susceptible to red thread, as well as brown patch disease.

More recently, red thread even has become a severe problem in tall fescue lawns in humid regions. This is surprising, as tall fescue was regarded for years as having good thread resistance. Red thread is also quite damaging to the fine leaf fescues and some cultivars of Kentucky bluegrass.

The symptoms and signs of red thread are distinctive and unmistakable. In the presence of morning dew or water from rain, a coral pink or reddish layer of gelatinous fungal growth (mycelium) can easily be seen on leaves and sheaths. Upon closer inspection, the green leaves of invaded plants develop a water-soaked appearance.

When leaves dry, the fungal mycelium becomes pale pink in color and is easily seen on the straw-brown or tan issues of dead leaves and sheaths. During the final phases of disease activity, bright red, hard and brittle strands of fungal mycelium called ‘red threads’ or sclerotia may be seen extending from leaf surfaces, particularly leaf tips.

Red thread is often more injurious to poorly nourished turfs, but it is on the rise in professionally maintained lawns. Given the levels of nitrogen used in lawn care, it would appear that the pathogen has adapted and is now a problem in properly nourished turf.

Cultural Management: If turf has not been fertilized in a long time and is not vigorous, an application of 0.5 to 1.0 pounds of N per 1,000 square feet will help stimulate recovery. Rely mostly on fall applied, slow release nitrogen; and apply phosphorus, potassium and limestone per a soil test.

Chemical Management: In most situations a single application of a labeled fungicide for red thread would be expected to limit foliar blighting to acceptable levels. Effective fungicides include Banner™ MAXX, Bayleton™, Chipco 26019, Curalan, Eagle™, ProStar™, and Sentinel™, and chlorothalonil-based products (e.g., Daconil and Thalonil).

3. DOLLAR SPOT

Pathogen: Sclerotinia homeocarpa.
Primary Hosts: Kentucky bluegrass, fescues, perennial ryegrass, bentgrass.
Predisposing Conditions: Periods of warm days and cool nights from spring to late fall.

Dollar spot is widespread and destructive to most turfgrass species. It has become troublesome in tall fescue, a species thought to be resistant to this disease. The symptomatic pattern of dollar spot varies with turfgrass species and management practices.

Under close mowing, the disease first appears as small, circular, straw colored spots of blighted turf about the size of a silver dollar. With higher mowing and coarser textured grasses such as Kentucky bluegrass, perennial ryegrass or tall fescue, the straw colored blighted areas are 3 to 6 inches in diameter and tend to be irregularly shaped. Affected patches frequently coalesce and involve large areas of turf.

Grass blades often have straw colored or bleached white lesions shaped like an hourglass, with leaves typically dying back from the tip. The hourglass banding on the leaves is often made more obvious by a definite narrow brown band, which borders the bleached sections of the hourglass from the remaining green portions. In tall fescue, however, the pathogen causes a die back from the tip and produces irregularly shaped leaf lesions with brown borders that are not usually hourglass shaped.

When the fungus is active and moisture is present a fine, cobweb-like mycelium may cover the infected area during early morning hours. Dollar spot tends to be most damaging in poorly nourished turfs, particularly when humidity is high or a heavy dew is present. The disease, however, can be destructive to lawns fertilized in spring with water soluble nitrogen sources. Hence, like red thread, dollar spot is not exclusively a disease of poorly nourished turf.

Cultural Management: An application of 1.0 pound of N per 1,000 square feet to poorly nourished turf stimulates recovery. Use primarily fall applied slow release nitrogen in balance with phosphorous and potassium; apply limestone according to soil test recommendations. Schedule mowing early in the morning to promote the drying of foliage and leave clippings on the lawn. Avoid a drought stress situation, increase mowing height and control thatch and soil compaction.

Chemical Management: An application of penetrant-type fungicide (e.g., Banner MAXX, Bayleton, Chipco 26019, Cleary’s 3336™, Curalan, Eagle, Fungo™, Rubigan™, or Sentinel) would be expected to provide from 21 to 28 days of dollar spot control on lawns. Chlorothalonil (i.e., Daconil and Thalonil) also is very effective.

4. BROWN PATCH

Pathogen: Rhizoctonia solani.
Primary Hosts: Perennial ryegrass and tall fescue.
Predisposing Conditions: Warm and humid nights from June to September.

Brown patch symptoms vary according to the host species. On closely mown turf such as bentgrass, blighted patches are generally circular and range from three inches to three feet or greater in diameter. When dew is present in early morning, the outer edge of the patch may develop a one- to two-inch wide smoke ring. The smoke ring is blue-gray in color and is caused by Rhizoctonia mycelium in the active process of infecting leaves. Smoke rings are not always present and patches may have an irregular rather than circular shape.

Close inspection of leaf blades reveals that the fungus primarily causes a die back from the tip down, giving affected turf its brown color. In tall fescue, perennial ryegrass and Kentucky bluegrass affected areas are frequently irregular in shape and smoke rings are often absent. R. solani produces distinctive and often greatly elongated lesions on tall fescue leaves. The lesions are a tan or chocolate brown color, and are bordered by narrow, dark brown bands of tissue.

In perennial ryegrass and Kentucky bluegrass smaller leaf lesions are produced and tip dieback commonly occurs. In the early morning when the disease is active, the mycelium of R. solani may be observed on leaves in the presence of a heavy dew.

Environmental conditions that favor disease development in cool-season grasses are day temperatures above 85 F and high relative humidity. A minimum daily temperature above 61 F in combination with either rainfall or relative humidity above 95 percent for more than 8 hours are the most critical environmental requirements for disease development.

Evening or nighttime thunderstorms from mid-June to late September and a minimum air temperature above 68 F are most conducive for severe brown patch. Weather conditions that result in little or no dew formation (i.e., low relative humidity or low soil moisture) or cool nighttime temperatures (i.e., less than 60 F) will result in a marked reduction in brown patch severity. Late spring and summer applications of nitrogen fertilizer, in particular water soluble nitrogen sources such as urea and ammonium-based products, increase brown patch severity. Evening or nighttime watering or rain showers will greatly increase the potential for disease.

Cultural Management: Apply most of the total annual nitrogen in the fall and rely mostly on slow release fertilizer. Apply phosphorous, potassium and limestone according to a soil test. Maintain turf at a 2 to 3 inch height of cut and remove clippings when the disease is active. Irrigate early in the morning and avoid night irrigation.

Chemical Management: In regions where brown patch is chronically severe, disease control with fungicides may not be practical or economical. Products providing relatively long periods of brown patch control (i.e., 18 to 28 days) would include Heritage™, Sentinel and ProStar. Other effective materials include Chipco 26019, Cleary’s 3336, Fungo and chlorothalonil (e.g., Daconil, Thalonil).

5. SUMMER PATCH

Pathogen: Magnaporthe poae.
Primary Hosts: Kentucky bluegrass and creeping red fescue.
Predisposing Conditions: Sunny days above 90 F and moist or compacted soils.

Summer patch is perhaps the most destructive disease of Kentucky bluegrass and creeping red fescue lawns. Another patch disease of Kentucky bluegrass and creeping red fescue lawns is necrotic ring spot. Summer patch is probably a more common disease than necrotic ring spot. Necrotic ring spot, however, is an important disease, particularly in the more northern regions of the United States, and is discussed below.

Symptoms of summer patch initially appear as wilted, dark green and roughly circular areas of turf that are 1 to 2 inches in diameter. The affected turf soon turns a straw-brown color, leaving patches that initially resemble the symptoms of dollar spot disease. These patches increase in size and may form crescent shapes, elongated streaks or circular patches. The outer periphery of patches may have a yellow or bronzed appearance, and leaves die back from the tip.

Healthy turf may persist in the center of blighted patches producing rings or “frog-eye” symptoms. The most common are circular, straw-colored patches that range from 3 to 12 inches in diameter. There may be living plants in the center of patches, but distinct frog eyes are not always evident. Circular depressions called “crater pits” are occasionally present. Affected regions may coalesce and large areas of turf can be destroyed within a 7- to 21-day period.

Creeping red fescue turf develops orange or bronze colored patches prior to affected plants turning straw colored. Crater pits are more common in creeping red fescue than Kentucky bluegrass. Summer patch most commonly occurs in Kentucky bluegrass lawns that are 32 years of age or older. The disease may appear in newly sodded lawns because sod is often two or more years of age before it is harvested. Summer patch, however, frequently appears in the summer following a fall seeding of creeping red fescue.

Summer patch is more severe in full sun than in shaded sites. Symptoms generally appear in late June or early July when day time temperatures above 90 F occur. Soil temperatures above 78 F in the root zone and moist soils are critical factors in predisposition of turf to summer patch. The activity of the disease declines with the advent of cooler weather in mid-to-late September. Summer patch is most severe on sunny, exposed slopes or other very warm areas of a lawn such as those adjacent to walks and driveways. Conversely, it tends to be less severe in shaded sites.

Drought stress following warm and wet periods and compacted soil are other factors that enhance summer patch. Other predisposing factors include: spring applications of high levels of nitrogen fertilizer; accumulation of thatch; and frequent light irrigations or rain storms. Low mowing is the major cultural factor that contributes most to increasing disease severity. Scalping lawns by mowing very low after allowing the turf canopy to grow too high can trigger the appearance of symptoms.

Cultural Management: Avoid excessive nitrogen fertility, especially water soluble, spring applied nitrogen. Use mostly an acidifying, slow release nitrogen source such as sulfur coated urea in the fall. Increase mowing height to 2.5 inches in the summer. Avoid light and frequent applications of water; irrigate at onset of wilt to wet soil to a depth of 3 inches or deeper if possible. Core cultivate compacted soils and control thatch.

Chemical Management: Cultural control may be more practical for lawns than the use of fungicides. Two to three applications of either Banner MAXX, Bayleton, Eagle or Sentinel once monthly beginning in early to mid-May are effective. Curative applications are less effective than preventive applications.

6. NECROTIC RING SPOT

Pathogen: Leptosphaeria korrae.
Primary Hosts: Kentucky bluegrass and creeping red fescue.
Predisposing Conditions: Extended cool and wet periods in the spring and fall.

Necrotic ring spot can occur anywhere Kentucky bluegrass is grown, but is more prominent in northern regions of the United States and Canada.

The symptoms of both summer patch and NRS are similar. While the greatest injury from NRS may not become evident until summer, the NRS pathogen actively attacks roots during cool and wet weather. Hence, if patch symptoms appear in spring or fall, it is very likely that the disease is NRS and not summer patch.

The initial symptoms of NRS appear as 1 to 2 inch diameter spots, and leaves in affected areas may have a burgundy-red color. These initial spots appear in spring, but tend to be inconspicuous. The advent of warm and dry conditions in late spring or early summer will stress the disfunctioning root system of affected plants in a radial pattern and circular patches begin to expand rapidly. Spots of dead or unthrifty turf can expand from 3 inches in diameter to more than 12 inches in less than 30 days. In chronically affected areas, NRS patches tend to be large, often greater than 18 inches in diameter. Distinct frog-eyes are common.

Sodded lawns in newly cleared woodland lots are most severely affected. Very susceptible cultivars of Kentucky bluegrass, however, can develop large patches in the summer following a fall seeding. The disease is severe in both shaded and full sun areas. On older lawns, NRS patches can be evident almost year-round in low lying, wet and shaded sites. Once established at a site, the disease normally recurs at the periphery of the last year’s patches. Over time, the center of these patches is recognized by either the desirable turf, other resistant grasses or weeds.

Cultural Management: Because the root system of affected plants may be severely diminished by the pathogen, daily and light applications of water to affected lawns may be required during periods of heat or drought stress. Fall applied natural organic fertilizers and acidifying nitrogen (e.g., sulfur coated urea or ammonium sulfate) may reduce NRS severity over time.

Increasing mowing height, improving drainage, controlling thatch and alleviating soil compaction will help to reduce disease severity. In most situations, the disease naturally declines over a period of three to five years. It can, however, remain a chronic problem where very susceptible cultivars are grown or in wet and shaded sites.

Chemical Management: Two early spring (usually April and May) applications of Cleary’s 3336, Fungo or Rubigan reduce NRS severity, but fungicides generally do not completely control his disease. Curative applications are not as effective as preventive fungicide sprays.

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