Gray Leaf Spot: A Common Fungal Disease on St. Augustinegrass

Gray leaf spot, caused by the fungus Pyricularia grisea, is a prevalent disease that primarily affects St. Augustinegrass (Stenotaphrum secundatum) but can also infect other grass types like tall fescue, bermudagrass, centipedegrass, and ryegrass in residential lawns. This fungal infection manifests as small brown spots on the leaves, which quickly expand into large, tan to gray oval or elongated spots with purple or brown borders. Some spots may have a yellow halo, and overall leaf yellowing may occur.

When warm and humid conditions persist, the fungus develops a grayish mycelium (cottony fungal growth) and releases spores on dying foliage, giving the spots a grayish appearance. Severe infections can cause the grass blades to wither and die, resembling extreme drought stress. The disease is most severe during rainy, humid periods and thrives when temperatures range between 77 and 86 °F during the day and above 65 °F at night.

To effectively manage gray leaf spot, homeowners can adopt various strategies:

Reducing the Thatch Layer

Reducing the thatch layer is crucial in controlling the disease. Thatch consists of accumulated dead grass and debris between the soil and the actively growing grass. Regular dethatching helps create an environment less favorable for fungal growth.

Proper Irrigation Practices

It is important to water deeply but infrequently. Watering once a week with one inch of water is generally sufficient. Always water in the early morning to promote quick drying of the foliage. This reduces moisture and minimizes the ideal conditions for disease development.

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Avoiding Weed Killers During Active Infection

While the disease is active, it is advisable to avoid using post-emergent weed killers on the lawn. These chemicals can further stress the grass and hinder its ability to recover from the disease.

Moderate Nitrogen Fertilization

High levels of nitrogen fertilization can promote lush, succulent growth, which is more susceptible to gray leaf spot. It is advisable to use medium to low nitrogen fertilizer levels to avoid creating a favorable environment for the fungus.

Improving Air Circulation and Sunlight Exposure

Improved air circulation and sufficient sunlight exposure can help control the disease. Trimming overhanging tree branches and pruning nearby shrubs allows more airflow and sunlight to reach the grass, making it less susceptible to gray leaf spot.

Proper Mowing Practices

Mow the lawn at the recommended height and only when the grass is dry. If the disease is present, bag and dispose of the grass clippings to prevent further spread and reinfection.

Controlling Chinch Bug Infestations

Chinch bugs can weaken the grass and make it more susceptible to diseases like gray leaf spot. Regular monitoring and appropriate pest control measures can help manage chinch bug populations.

Fungicide Treatments

In severe cases, fungicide treatments may be necessary. Several brands and products are available for controlling gray leaf spot. It is essential to follow the instructions and use the appropriate fungicide for the specific type of grass and disease. Always consult with a professional or refer to the product label for proper application guidelines.

For more information on maintaining a healthy St. Augustinegrass lawn, visit Ames Farm Center. You can also find additional resources on other turfgrass diseases, such as leaf diseases and brown patch, for comprehensive lawn care.

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Gray Leaf Spot
Gray Leaf Spot on St. Augustinegrass

Remember, preventing and managing gray leaf spot requires a combination of good cultural practices, proper irrigation, and timely intervention when necessary. By following these tips, you can maintain a vibrant and disease-resistant lawn throughout the growing season.