Turf Diseases: Warm Weather Diseases

Symptoms with a powdery Appearance:

Powdery Mildew:

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 conditions and air circulation by pruning trees and shrubs will minimize the occurrence of powdery mildew. Planting disease resistant cultivars of Kentucky bluegrass and shade tolerant cultivars, such as fine fescues, will also help reduce powdery mildew.

Red Thread:

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 pink mycelium 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.

Rusts:

From July to November rusts are a problem on lawns with moderate drought stress and low fertility. Heavy dew or frequent, light rains exasperate the problem and lawns look brown-yellow or red.

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. Perennial Ryegrass and Kentucky Bluegrass are more susceptible but all turfgrass species can get rust.

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.

Slime Mold:

Slime mold spores coat the grass and look like cigarette ash on the grass blades. This disease usually appears in patches following warm rains. Although slime mold does not harm the turf, physical removal is sometimes suggested. No chemicals are required to treat this disease.

Symptoms with patches or rings:

Dollar Spot:

Dollar spot 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 will 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 ore 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.

Rhizoctonia Blight:

Formerly know 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 these 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 the 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. As the disease progresses, many of the patches may 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.

Pythium Blight:

Pythium blight is first seen as small, irregularly shaped, purplish areas ranging from 1-4 inches in diameter. The individual leaves in these patches have a dark, water soaked appearance. The color of the affected leaves soon changes to light brown or reddish brown, and they become dry and shriveled.

Specific climate conditions must be met for the disease to occur. Maximum daily temperatures must exceed 80 degrees F and the relative humidity must be higher than 90% for 14 hours, during which time the minimum temperature remains higher than 68 degrees F. Symptoms usually appear in July and August. Prevention is very important in controlling pythium blight.

Do not water in the evening and improve soil drainage and air circulation. Fungicides should be used as a preventive measure against pythium blight and should first be applied immediately after the first occurrence of night temperatures that do not drop below 65 degrees F and the relative humidity during the nighttime period under consideration is 85% or higher.   

 

Acknowledgements:
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