Fall is an ideal time to check your lawn for signs of disease. Many lawn diseases for warm season grasses are the most active in cooler weather, although some can appear any time during the growing season.
Diagnosing a turfgrass disease, however, can be tricky because symptoms similar to disease can be caused by environmental conditions and turf management practices, such as:
- Insect infestations.
- Temperature extremes.
- Competition from tree roots.
- Excessive shading.
- Toxic soil substances.
- Wrong herbicides applied for the grass type.
- Incorrectly applied herbicides.
- Lack of essential nutrients, water or oxygen.
- Soil compaction.
- Inappropriate irrigation scheduling.
- Improper mowing height or frequency.
Diseases for Warm-Season Grasses
Below are some of the common diseases for warm-season grasses in Texas, along with signs to help you identify them.
General information: Large patch is a common disease caused by a fungus (Rhizoctonia solani) that affects most Texas warm-season grasses. It appears in the fall, and is active until temperatures drop and remain below 60°F. It becomes active again in the early spring until the weather warms above 75°F, and the soil begins to dry out. This disease is also called brown patch when it occurs in cool-season grasses (caused by a different strain of the same fungus) and has a different life cycle than the strain responsible for large patch.
You can spread large patch by mowing and then walking on infected areas. The disease can be a problem each year unless measures are taken to control or eliminate it.
Grasses affected: Most warm-season grasses, with St. Augustine and zoysia especially affected.
Symptoms: Symptoms vary greatly depending on the grass types, environmental conditions, and lawn maintenance practices.
Large patch begins with small circular or irregular patches of blighted grass, which can grow to over 20 feet in diameter. The outer edges may be red or brown — St. Augustine grass will turn yellow at the edges — while the rest of the patch is light brown. New leaves may appear in the center of the patch, giving the diseased areas a donut-shaped appearance.
Infected leaves often have a slimy, dark brown lesion at the base of the leaf. The leaf sheaths become rotted, and the leaf easily pulls out.
Take-All Patch, Bermuda Decline, Take-All Root Rot
General information: Take-all patch, also known as Bermuda decline and take-all root rot, is caused by the fungus Gaeumannomyces graminis var. graminis or avenae. It’s active in the fall and winter whenever there’s a lot of moisture, but its symptoms don’t show up until late spring or early summer when the affected grass is stressed by high temperatures and dry weather. However, the disease can appear anytime during the growing season when the grass is stressed.
Grasses affected: Bermuda, St. Augustine, zoysia.
Symptoms: The first visible symptoms are circular or irregular patches of yellowing and wilting grass blades, and darkening of the roots. As the grass dies, it thins, with the patches becoming bare. The patches can grow up to 20 feet in diameter. With severely infected grass, the stolons (slender stem that grows horizontally along the ground) can be easily pulled up from the ground.
Take-all patch is often mistaken for large patch — however the leaves can’t be easily separated from the sheaths, as is the case with large patch.
General information: Leaf spot is a common name for a number of lawn diseases caused by the same family of fungii (Helminthosporium). Different species of the fungus attack different grass types. Leaf spot has two different phases, one for the cool weather, and one for the warmer weather of midsummer.
Grasses affected: All warm season grasses can get the disease, but Bermuda and zoysia are most affected.
Symptoms: In the spring and fall, small, brown-to-purple spots with tan centers appear on leaf blades. Leaves that are severely affected will turn reddish-brown, then wither and die. During the hot, humid weather the “melting out” phase of the disease begins with sheath and crown rot, killing the grass in irregular patches.
If the environmental conditions change, the disease usually stops, and the grass can recover.
General information: Rust is a common fungal disease (Puccinia cynodontis) that occurs on grasses when their growth is slowed. For warm-season grasses this would be late summer through late fall, as well as during periods of stress (e.g. drought, low nitrogen).
It usually starts in shaded areas first, during warm, humid weather. Any grass that remains wet for long periods of time is prone to the disease.
Grasses affected: Bermuda, St. Augustine, and zoysia (most affected).
Symptoms: The disease begins as tiny yellow spots on the grass blades. Over time, the spots grow longer, and enlarge to form orange or yellow raised pustules. Eventually the pustules rupture and spread a powdery mass of rust-orange spores across your lawn. When all the spores have been released, the pustules turn black.
In heavily rust-infected areas, the grass thins out and turns a yellow-orange to reddish-brown color. If the turf is disturbed, clouds of rust spores rise into the air and spread (e.g. mowing). The spores that are left on your lawn will re-infect it once the optimal environmental conditions for the disease return.
Conditions for Lawn Disease
A number of conditions must be present for a lawn disease to develop:
- There must be a pathogen present (such as fungi, bacteria, or viruses) in high numbers.
- The grass type must be susceptible to that pathogen.
- Environmental conditions have to be favorable to that disease.
If you’re starting a new lawn with sod or seed, one way to give it every chance to be healthy is to choose a grass type suitable for your geographical location and site conditions (e.g. amount of sun and shade, soil type), and preferably a disease-resistant variety.
Control and Management
Once your lawn is established, effective management practices can discourage, and control or eliminate fungal diseases. Proper management will also help turfgrass recover. Such practices include:
- Get your soil tested so you know what amounts and types of fertilizers to use. Under and over-fertilization can create conditions for disease (e.g. too much nitrogen helps activate large patch, while too little helps activate rust disease).
- Irrigate deeply and evenly to encourage deep root growth. The amount of watering required varies with soil type, but usually water no more than an inch at a time, enough to soak the soil to a depth between 6-10 inches.
- Irrigate early in the day so moisture can reach the roots, but doesn’t remain too long on the grass.
- Maintain the proper mowing height for the grass type.
- Ensure good soil drainage above and below the surface.
- Remove thatch over 1/2 inch thick.
- Aerate warm-season lawns in late spring or very early summer.
- Apply correct herbicides for your grass type to kill weeds and avoid the overuse during disease activity. Follow the label instructions so as not to cause damage.
The best time to apply fungicides is just before any symptoms appear (e.g. there’s a disease history), or very early in the disease development. Once you’ve identified the infection, choose the correct fungicide for treatment. You can purchase many types at garden centers and nurseries, but a number of effective fungicides are only available to licensed lawn care professionals.
If you’re having difficulty identifying the fungus, you can ask a lawn care professional, or you can contact your Texas A&M AgriLife Extension County Office to speak to a master gardener. He’ll be able to answer questions on plant diseases and tell you where to send diseased samples for diagnosis.
When applying fungicides yourself, follow directions exactly, especially those listing application rates and environmental conditions. Make sure you have the right fungicide for your grass species.
You may see disease symptoms worsen immediately after an application when unseen infections start to develop. Fungicides won’t cure turfgrass already dead or dying, but it will slow the disease development, and prevent it from spreading to unaffected areas.
Correct irrigation is indispensable to a healthy lawn. Make sure your sprinkler system is fully functional. The licensed professionals at South Austin Irrigation can diagnose and repair any irrigation system problem you may be experiencing. Call us at (512) 534-7449 or contact us online for timely on-site service.