Managing Turfgrass Diseases
Turfgrass Diseases
| Damping-off Diseases, Pythium, Rhizoctonia, and Fusarium spp. |
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Damping-off diseases occur as seed rots and blights of seedlings. This disease seldom occurs on grass planted in early fall or early spring. However, in seedings from May to August, especially if hot, wet weather persists, seed may decay prior to germination, or new seedlings may become diseased and die.
Symptoms and signs
Seedlings become blighted, then collapse and die, forming circular or irregular patches. Affected plants often appear water soaked and slimy. Sometimes mycelium can be seen on dew-covered grass in early morning. The dead tissue may mat together somewhat like paper-mache and form a crust over the soil surface.
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| Mycelium of Rhizoctonia solani on perennial ryegrass. |
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| Dense, white mycelium of damping-off fungus on seedling turf. |
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Disease cycle
Most soils contain fungi that can attack seeds and seedlings. When conditions are favorable for grass seed germination and growth, as in the early fall and spring, these organisms usually are of little significance. Warm, wet weather, however, is more favorable for damping-off fungi and less favorable for seedlings. Under these conditions, fungi can severely damage seed and seedlings.
Damping-off outbreaks are triggered by high temperatures and humidity, water-logged soils, excessive fertilizer, or an excessive seeding rate. In warm weather, higher-than-normal rates of seed may produce a very dense stand of seedlings that will hinder the escape of moisture from the soil surface. Such wet conditions over long periods of time are ideal for invasion by damping-off fungi. High rates of fertilizer can produce a succulent turf that is more susceptible to attack by damping-off fungi. With new seeding techniques, such as hydroseeding and hydromulching, seeding is done all year round, and as a result, damping-off has become a more frequent cause of stand failure. Stand loss to seed decay or seedling blight makes successive attempts to reseed the dead areas more difficult.
Cultural control
Cultural control of damping-off diseases is based on providing favorable conditions for germination of the grass seeds and growth of the plants while minimizing the conditions that favor the fungi. Often, this can be accomplished by planting in spring or fall when temperatures are cool and grass grows well. For summer seedings, prepare a good seed bed, select a seeding rate sufficiently low so that the stand is not too dense, do not apply excessive amounts of fertilizer, and prevent free water from standing on the soil surface. Avoid using excessive quantities of mulch following seeding.
Chemical control
Three types of chemical control methods may be used with new turf seedings: fungicide seed treatment prior to seeding, fungicide drenches or sprays on the soil after seeding, and/or fungicide sprays on the young seedlings. Each method can be effective, and the choice offers the possibility of chemical protection at various intervals during turf establishment. Selection of fungicide control practices depend on the site, the type of grass seeded, the seeding rate, and the environment at establishment.
| Dollar Spot, Sclerotinia homoeocarpa |
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With regular watering of home lawns throughout much of the growing season, many turfgrass diseases, once problems only on golf courses and recreational turf areas, now appear commonly. Perhaps the most important of these is dollar spot.
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| Symptoms of dollar spot disease on creeping bentgrass putting green. |
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Symptoms and signs
On golf course greens cut at or below 3/16 inch, this disease appears as white or tan spots of dead turf about the size of a silver dollar. Hence the name dollar spot. On home lawns cut at 1 to 3 inches, dead areas may reach 2 to 4 inches in diameter. These spots may run together, producing large areas of dead turf. Affected leaves initially show yellow-green blotches, which progress to a light straw color with a reddish-brown margin. Occasionally, white mycelium can be seen covering affected leaves in early morning on dew-covered grass. Dollar spot symptoms occur anytime from early to late summer. The disease usually reaches peak activity when air temperatures are in the 80° F range and under high humidity. Symptoms also may appear in the fall. The most severe cases of dollar spot occur on turf receiving closely-spaced summer irrigation. The disease may also occur on nonirrigated turf when humidity is high from prolonged muggy summer weather. Dollar spot is more severe under nitrogen deficiency or when grass grows slowly.
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| Dollar spot lesions on Kentucky bluegrass leaves. |
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Disease cycle
The causal fungus spreads via mycelium on infected plant parts (mostly clippings). Mycelium may be carried by the wind, water, or on turf equipment and shoes. This fungus does not produce spores. Sclerotinia homoeocarpa is thought to survive as thick-walled crusts of mycelium called pseudosclerotia in dormant infected plant tissues.
Cultural control
Late spring nitrogen-fertilizer applications can help to minimize dollar spot severity, since growth will be stimulated during the period (early summer) when dollar spot infection begins. On lawns, fertilizer that releases nitrogen slowly over an extended period is more advantageous than a quick-release product at this time of year. On golf courses, quick-release nitrogen fertilizers can be applied frequently at very light rates instead of using a slow-release fertilizer. Irrigate deeply, infrequently, and early in the morning to minimize moisture accumulation on leaves. Also, remove dew by early morning mowing or by dragging the turf with a water hose.
Chemical control
On irrigated turf with persistent dollar spot problems, fungicides are very effective against most forms of the causal fungus. Fungicides are almost always applied on a curative basis for controlling dollar spot. Both contact and systemic fungicides are effective in controlling this disease. Because resistance to certain systemic fungicides has occurred with Sclerotinia homoeocarpa, using contact fungicides in a control program is suggested.