Diagnostic Center

Now is a good time to get the jump on lawn diseases. Besides applying fungicides there are several cultural practices that can help prevent them. Three most common diseases that affect the lawns are: gray leaf spot, brown patch and dollar spot. The beauty of a lawn can be quickly destroyed by these fungal diseases.

Gray Leaf Spot on St. Augustine grass

Gray leaf spot is a fungal disease that occurs most commonly on St. Augustine grass. The initial leaf spots are small and brown, but they expand rapidly into large, oval or elongate, tan to gray leaf spots, which are surrounded by purple or brown borders. A yellow halo may surround some of the spots, or a general leaf yellowing may occur.

During warm, humid weather the fungus produces a grayish mycelium (cottony fungal growth) and numerous spores on dying foliage, which gives a grayish appearance to the spots. Blades of grass will wither and die if leaf spots are numerous. Under favorable conditions for disease development, large areas of turf may appear to be scorched, resembling extreme drought stress.

Gray leaf spot is favored when temperatures are between 77 and 86 °F during the day and above 65 °F at night, and is most severe during rainy, humid periods. The fungus survives dry summer conditions as fungal mycelium and spores on dead foliage and in the thatch layer. When favorable conditions of moisture and high humidity return during the summer, the fungus produces an abundance of spores that are spread by wind, splashing rainfall or irrigation, and the movement of lawn equipment on wet grass.

How to Manage Gray Leaf Spot Disease of the Lawn?

• Reduce thatch layer.
• Irrigate deeply, but infrequently. This generally means one time per week with one inch of water. Always irrigate in the morning, which promotes quick drying of the foliage.
• Avoid using post-emergent weed killers on the lawn while the disease is active.
• Avoid medium to high nitrogen fertilizer levels.
• Improve air circulation and light level on lawn. Limb up over-hanging trees and prune back nearby shrubs.
• Mow at the proper height and only mow when the grass is dry. Bag and dispose of grass clippings if disease is present.
• Control chinch bug infestations.
• Use fungicide treatments as needed along with proper turf grass culture.

Brown Patch and Large Patch Diseases of Lawns

Brown patch and large patch are serious fungal diseases (each caused by different strains of Rhizoctonia solani) that can affect all South Carolina lawn grasses. Disease can develop rapidly when daytime temperatures are warm (75 to 90 °F) and humid, nighttime temperatures are above 60 ºF, and there is an extended period of leaf wetness. Generally, symptoms of brown patch begin on cool-season grasses (tall fescue, ryegrass, bluegrass and bent grass) during the late spring. It may also occur on these grasses during warmer periods of the winter months. Warm-season grasses (St. Augustine grass, zoysia grass, Bermuda grass and centipede grass) most commonly are affected by large patch (formerly also known as brown patch) during the early spring and late fall.

Symptoms of brown and large patch diseases may vary greatly with the type of grass and soil conditions. The diseases usually cause thinned patches of light brown grass that are roughly circular in shape. These areas range in diameter from a few inches to several feet. Often the center of the patch will recover, resulting in a doughnut-shaped pattern.

When disease conditions are favorable, large areas of the lawn may be uniformly thinned and eventually killed with no circular patch being evident. This type of pattern is commonly seen on infected St. Augustine grass grown in shady, moist locations.

All types of warm-season or cool-season lawn grasses grown in South Carolina can be affected by large patch or brown patch, respectively.

Prevention & Treatment of Brown Path Disease of the Lawn

The best way to prevent brown patch or large patch in the home lawn is by following good lawn care practices. This is much easier and less expensive than the use of fungicides and can be very effective.

• Avoid high rates of nitrogen fertilizer on cool-season grasses in the late spring and summer. Avoid high nitrogen rates on warm-season grasses in mid to late fall or in early spring. The disease-causing fungus readily attacks the lush growth of grass which nitrogen promotes. Avoid fast-release forms of nitrogen fertilizer.
• Irrigate grass only when needed and to a depth of 4 to 6 inches (generally 1 inch of irrigation water per week), but do not subject the lawn to drought conditions. Water early in the morning. This disease can spread fast when free moisture is present, especially greater than 10 hours.
• Avoid spreading the disease to other areas. Remove clippings if the weather is warm and moist to prevent spread to other areas during mowing.
• Keep lawns mowed on a regular basis to the proper height for the grass species you are growing. Lower than optimum mowing height can increase disease severity. Do not mow fescue lawns shorter than 2½ inches high, nor higher than 3½ inches. Mow centipede at 1½ inches high.
• Provide good drainage for both surface and subsurface areas. Correct soil compaction by core aeration. Prevent excessive thatch buildup.
• Have the soil tested and apply lime according to test recommendations. Disease may be more severe if the soil pH is less than 6.0. Keep potassium (K) level at upper end of sufficient rating on soil test.

Fungicides can be difficult to rely upon for controlling brown patch and large patch in the home lawn, but regular applications can vastly improve appearance. A good “rule of thumb” to follow on either cool- or warm-season grasses is to initiate fungicide sprays when nighttime low temperatures reach 60 °F. Stop applications when nighttime lows are forecast to be below 60 °F for five consecutive days. Typically, applications are made at 14- to 28-day intervals, depending upon the fungicide. If disease is severe enough to warrant chemical control, select one of the following fungicides listed in Table 1.

It will help in disease control to alternate fungicides used with subsequent applications to prevent a buildup of resistance to a fungicide. Slightly better control may be obtained by a liquid fungicide application rather than by granular application of the same fungicide active ingredient. Granular fungicides must be irrigated after application (follow label directions).

Dollar Spot Disease of the Lawns

Dollar spot is caused by a fungus. It causes straw-colored spots about the size of a silver dollar (2 to 6 inches diameter) to appear on closely mowed turf. Grass in affected areas may die and the spots may merge to form larger, irregular patches. In coarse textured grass that is cut high, the dead spots are larger and more diffuse. Leaf blades have light tan spots with reddish-brown margins that develop across the leaves. Early in the morning you may be able to see a cobweb-like growth of the fungus over the infected area.

Dollar spot most commonly occurs on Bermuda grass, zoysia grass and centipede grass. Dollar spot is most active from late spring through fall. The fungus develops during humid weather, when daytime temperatures are warm (59 to 86 °F) and nights are fairly cool. These conditions result in heavy dew forming on the grass.

Prevention & Treatment of Dollar Spot Disease:

Adequate fertilization will help the lawn overcome this disease. Prevent thatch buildup and remove excess thatch. Avoid drought stress by watering the lawn deeply, and thoroughly when needed, timing irrigation for early morning. Remove morning dew if possible by mowing or irrigating the lawn. Fungicide applications may be needed during moist weather in the spring and fall, when day temperatures are between 70 to 80 °F. For the home lawn, fungicides that give good control of dollar spot should contain propiconazole, azoxystrobin with propiconazole, or triadimefon. Always apply all chemicals according to directions on the product label.

[For more information go to Clemson Cooperative Extension Home and garden Information Center at http://hgic.clemson.edu/]