Lawn Fungus in St Augustine: How to Treat and Prevent it

St. Augustine grass is a widely cultivated warm-season turfgrass known for its resilience and aesthetic appeal. However, even with its natural resistance to fungal disease, St. Augustine grass can still fall victim to various types of fungi under certain conditions. Fungal infections can cause unsightly brown patches, thinning, and even death of the grass if left untreated. In this guide, we will delve into the different types of lawn fungus diseases that can affect St. Augustine grass, their symptoms, and effective treatment methods.

Types of Lawn Fungus Diseases in St. Augustine Grass

St. Augustine grass is commonly affected by several types of fungal diseases, including brown patch disease, fairy ring fungus, grey leaf spot fungus, and pythium blight (also known as grease spot fungus). Each of these fungal infections has distinct characteristics and requires different treatment approaches. Understanding the specific fungus affecting your lawn is crucial for effective treatment.

Brown Patch Disease

Brown Patch Disease

Brown patch disease is a common fungal infection in St. Augustine grass, particularly in areas with high humidity and prolonged periods of wetness. It is characterized by the appearance of irregular brown spots and patches on the lawn, along with the yellowing of the grass blades. Dark brown fungal threads may also be visible on the stolons and crowns of the grass. Brown patch disease can vary in size, ranging from a few inches to several feet in diameter, depending on soil conditions.

Fairy Ring Fungus

Fairy Ring Fungus

Fairy ring fungus manifests as a brown ring of dead grass surrounding a dark green patch. It is typically caused by the presence of white soil sections on the outer parts of the rings. Symptoms of fairy ring fungus include the growth of mushrooms in a circular pattern, overgrown or dead grass, and greasy leaf blades. Treating fairy ring fungus involves removing the affected areas and replacing them with fresh St. Augustine grass sod.

Grey Leaf Spot Fungus

Grey Leaf Spot Fungus

Grey leaf spot fungus is characterized by irregular bleached-out patches on the turfgrass. It is caused by high humidity levels and over-fertilization. Symptoms of grey leaf spot fungus include the appearance of whitish or grayish patches on the grass blades. Treating grey leaf spot fungus requires the use of high-quality fungicides such as Thiophanate-Methyl and azoxystrobin.

Pythium Blight (Grease Spot Fungus)

Pythium Blight (Grease Spot Fungus)

Pythium blight, also known as grease spot fungus, is a fungal infection that results in greasy and sticky leaf blades on St. Augustine grass. This type of fungus thrives in warm and wet conditions, making it more prevalent during periods of high humidity. Effective treatment involves the alternate use of various anti-fungal products to prevent resistance.

Identifying Fungal Infections in St. Augustine Grass

Identifying the specific fungal infection affecting your St. Augustine grass is crucial for proper treatment. Different fungal diseases exhibit distinct symptoms that can help you determine the type of fungus invading your lawn.

Signs of Brown Patch Disease

  • Stolons turning brown
  • Brown spots and patches on the lawn
  • Augustine’s grass blades are yellow randomly.
  • Dark brown fungal threads cover the stolons and crowns of the grass.

Symptoms of Fairy Ring Fungus

  • Circular pattern of mushrooms
  • Overgrown or dead grass
  • Greasy and sticky leaf blades

Recognizing Grey Leaf Spot Fungus

  • Irregular bleached-out patches on the turfgrass
  • Whitish or grayish patches on the grass blades

Identifying Pythium Blight (Grease Spot Fungus)

  • Greasy and sticky leaf blades
  • The presence of a slimy, water-soaked appearance on the grass
  • The rapid spread of the disease, especially during warm and wet conditions

Causes of Lawn Fungus Diseases in St. Augustine Grass

Various factors contribute to the development of fungal diseases in St. Augustine grass. Understanding these causes will help you take preventive measures to protect your lawn.

Environmental Factors

  • Prolonged periods of wetness, especially in shaded areas
  • High humidity levels
  • Extreme precipitation
  • Mild daytime temperatures of 70-90 degrees Fahrenheit and cool nights

Poor Lawn Maintenance Practices

  • Overfertilization
  • Overwatering
  • Excessive shade
  • Compacted soil
  • Insufficient watering during the spring-summer season
  • High foot traffic leading to soil compaction

Treating Fungus in St. Augustine Grass

Effective treatment of fungal diseases in St. Augustine grass depends on the specific type of fungus. Once you have identified the fungus affecting your lawn, you can apply the appropriate treatment method.

Replacing Affected Grass with New Sod

For fairy ring fungus, which appears as a brown ring of dead grass enclosing a dark-green patch, the best treatment method is to remove the affected areas and replace them with fresh St. Augustine grass sod.

Using Azoxystrobin to Kill Fairy Ring Fungi

If your lawn shows symptoms such as a circular pattern of mushrooms, overgrown grass, or dead grass, treating fairy ring disease requires the use of anti-fairy ring fungicides like azoxystrobin and Flutolanil. These fungicides can be sprayed on the infected lawn to eliminate the fungus.

Treating Grey Leaf Spot Fungus with Thiophanate-Methyl

For grey leaf spot fungus, characterized by irregular bleached-out patches on the turfgrass, high-quality fungicides containing thiophanate-methyl and azoxystrobin are effective treatment options.

Alternate Anti-fungal Products for Pythium Blight

To treat pythium blight, which causes greasy and sticky leaf blades, it is recommended to use a combination of different anti-fungal products. This is because pythium blight tends to become resistant to treatment over time. Consult with a lawn-care professional to determine the most suitable anti-fungal products for your specific situation.

Preventing Lawn Fungal Infections in St. Augustine Grass

Prevention is always the best approach when it comes to fungal diseases in St. Augustine grass. By implementing proper lawn care practices, you can create conditions that hinder fungal infections and minimize the risk of an outbreak.

Reducing Soil Compaction

Compacted soil is more susceptible to fungal diseases. Use a plug aerator to de-compact high-traffic areas and promote healthy root growth. Signs of compacted soil include runoff water and pooling water after rain.

Boosting Soil Nitrogen Levels

Lawn soil rich in nitrogen is less prone to fungal infections. Leave grass clippings on the lawn after mowing to add nitrogen back to the soil. Alternatively, use nitrogen-rich fertilizers specifically formulated for St. Augustine grass.

Proper Watering Techniques

Water your lawn optimally to avoid dry soil conditions that can facilitate fungal growth. Water at dawn during the growing season (spring-summer) and avoid overwatering, as this can create a favorable environment for fungal infections.

Dethatching During Active Growth Season

Thatch buildup can increase the risk of fungal infections. Dethatch your St. Augustine grass lawn during the active growth season (spring to early fall) to remove excessive thatch and promote airflow.


Fungal diseases can be a nuisance for St. Augustine grass lawns, but you can effectively treat and prevent them with proper knowledge and proactive measures. By identifying the specific fungus affecting your lawn and implementing the appropriate treatment methods, you can restore your St. Augustine grass to its lush and healthy state. Additionally, practicing good lawn care habits and creating conditions that discourage fungal growth will go a long way in preventing future outbreaks. Remember, a well-maintained lawn is more resilient to diseases and will provide you with a beautiful and vibrant landscape to enjoy.

Revitalize your St. Augustine grass with our comprehensive guide on identifying, treating, and preventing fungal diseases. Ensure a vibrant and healthy lawn by visiting ECOLawn Florida. Take action now for a lush landscape you’ll love!