What’s Wrong With My Lawn?
The presence of lawn diseases is often indicative of the overall health of your lawn. Environmental factors and maintenance practices that you are employing may be contributing to promoting these diseases. Soil borne fungi account for almost all of the diseases affecting turf. These fungi are more likely to infect a stressed plant. Fungicides suppress the damaging fungi’s effects, but also suppress beneficial fungi. Excessive watering and foliar feeding will weaken the turf root system, while herbicides and fungicides destroy the life of the soil. Both effects allow pathogens to get the upper hand. In addition, frequent low mowing especially stresses the grass. Most turf diseases can be eliminated by employing proper cultural practices.
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Common Lawn Diseases
Helminthosporium Leaf Spot
This disease first appears as leaf lesions, small purplish spots that turn brown in the center as they increase in size, finally fading to a light tan with purplish-brown borders.
A severe crown and root rot – which appreciably reduces both vigor and drought tolerance – frequently develops in conjunction with the leaf lesion phase of this lawn disease.
There is a direct relationship between air temperatures and length of leaf wetness required to produce maximum disease infections. When the leaf surface temperature is 70° F, the leaves must be wet continually for 48 hours in order for a high incidence of infection to develop. However, when leaf surface temperatures are in the 80-90° F range, the same amplitude of infection occurs within 24 hours.
Most turfgrass species are affected by gray and pink snow mold.
Pink snow mold is usually seen in early spring at the edge of the spring snow thaw. Small to large patches of gray or white matted turf covered with fungus growth will appear as snow melts. Black or brown specks can be found embedded in the grass blade.
Gray snow mold appears as 1 inch to 3 feet in diameter patches. Grass appears as light yellow, straw colored or grayish-brown turf at snow melt. Leaves appear matted together, sometimes covered with sparse to dense white to gray mycelium.
Mow into late autumn to ensure that snow does not fall onto a tall grass canopy. Avoid applications of heavy nitrogen fertilizer in the late fall. If patches appear in spring, rake affected areas to dry turf and encourage new growth. Lightly fertilize affected areas in spring to help promote new growth. Severely affected areas may need to be aerated and over-seeded.
Powdery mildew is a lawn disease that appears as a white powder or film on grass blades of Kentucky bluegrass and fine fescues. It is common in the spring and fall, but more common in the fall, and in cool, shaded lawn areas. Heavily infected leaves turn yellow and then tan or brown as they die.
Increasing light and improving air circulation by pruning trees and shrubs helps minimize the occurrence of powdery mildew. Planting disease resistant cultivars of Kentucky bluegrass and shade tolerant grasses, such as fine fescues, also helps reduce powdery mildew.
From a distance, red thread symptoms appear as circular patches of tan or pink turf about 4-8 inches in diameter. The pink color is caused by the sclerotia or flocks of pinkmycelium on leaf blades.
Red thread most commonly affects Kentucky Bluegrass, perennial ryegrass, and tall fescue.
Nitrogen fertilizing may help suppress outbreaks, as may thorough irrigation of the turf when dry. Be sure to avoid prolonged wettings. Regular mowings will remove infected leaf tips. Chemicals do not provide consistent results, even those recommended for red thread.
From July to November rusts are a problem on lawns with moderate drought stress and low fertility. Heavy dew or frequent, light rains exacerbate the problem and lawns look brown-yellow or red.
Perennial Ryegrass and Kentucky Bluegrass are more susceptible, but all turfgrass species can get rust.
To distinguish rusts from other turf diseases, look for a red colored soot that rubs off when touched. If it becomes serious, the grass will turn yellow, wither and die.
Cultural control include proper fertilizing, thorough but infrequent watering (not at night), planting resistant turf varieties, and mowing frequently to remove grass leaf tips. If the disease develops in late summer, a fungicide application may be necessary.
Dollar spot is a lawn disease that prevails when soil is dry, nitrogen levels are low, days are warm and nights are cool.
The disease may occur from June through September. Optimal temperatures for the disease range between 59-86 degrees F.
Affected individual leaves at first show yellow green blotches, which progress to a water-soaked appearance, and finally bleach to a straw-colored tan with reddish brown borders. Irregularly shaped, straw-colored patches form ranging in size from .75-6 inches across. Disease development is favored by warm, humid weather and cool nights that produce heavy dews.
Water thoroughly and as infrequently as possible without allowing moisture stress to occur. Do not irrigate turf in the late afternoon or evening, since this prolongs leaf wetness and may intensify the disease on cool night when dew is likely to form. Maintain mowing height of 3 inches and never remove more than 1/3 of the leaf blade when mowing. Aerify soil to reduce compaction and thatch.
Known as brown patch, rhizoctonia blight is common in bluegrasses, fescues, and ryegrasses. Bluegrass is more resistant, but not immune.
This disease usually occurs in July and August and is brought on by hot, humid weather with temperatures ranging from 80-85 degrees. Rhizoctonia patch appears as circular patches 1-3 feet in diameter, with center sections of green, unaffected plants. The individual leaf symptoms first show as small, dull tan lesions. Under favorable weather conditions, lesions continue to enlarge and develop reddish-brown margins.
If the outbreak of this disease was brought on by poor drainage, fungicides will not be effective. Improve drainage by incorporating organic matter as a top dressing. Withhold nitrogen fertilizers during hot, humid weather. Drag a rope or a garden hose over the lawn in the morning to remove dew.
Necrotic Ring Spot
Necrotic ring spot usually attacks lush, vigorous lawns. If dead spots appear on a previously healthy lawn, suspect necrotic ring spot.
The shapes of the individual patches of dead grass are usually more or less circular in outline, ranging in size from 2-3 inches to 2-6 feet in diameter.
As the disease progresses, many of the patches develop center tufts of apparently disease-free grass, referred to as the “frogeye” effect.
This disease usually strikes during periods of high temperatures and dry weather, preceded by wet conditions.
Avoid fertility and moisture extremes. Irrigate susceptible turf more frequently than other lawns especially during a dry spell. Top dressing, aerification, and de-thatching will encourage root development, and minimize the duration of the outbreak.
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Diseases of Turfgrass, Houston B. Couch
Compendium of Turfgrass Diseases, Richard W. Smiley, Peter H. Dernoeden, Bruce B. Clarke
University of Purdue Extension Service: G. L. Work, Warren Schultz