7/25/2023 - By Dr. Paul Giordano

Gray leaf spot can ravage turfgrass, killing large areas quickly if left unchecked. It is a common disease caused by the fungus Pyricularia grisea that affects many grass species. Including tall fescue, Kentucky bluegrass, perennial ryegrass, and St. Augustine.

Identifying the Lawn Disease Gray Leaf Spot

The disease can sometimes be confused with other issues or maladies, including drought or stress (biotic or abiotic). Below are some key descriptions of how to identify gray leaf spot on lawns:

1. Lesions: The disease starts as small, water-soaked spots on the leaf blades. These spots gradually enlarge and develop into elongated tan/brown lesions with dark-colored borders. The lesions can range in size from a few inches to several inches long.
2. Coloration: The lesions initially have a water-soaked appearance and later turn tan or light brown. As the disease progresses, the affected areas may take on a grayish coloration, hence the name "gray leaf spot."
3. Shape: The lesions are typically elongated and often have a rectangular or cigar-like shape. They can run parallel to the leaf veins.
4. Borders: Gray leaf spot lesions have distinct dark borders or margins that separate the infected tissue from the healthy grass. These dark borders are more noticeable as the lesions mature.
5. Distribution: Gray leaf spot infection may appear scattered or in clusters across the lawn. Disease often starts in lower, older leaves and gradually moves up the grass blades as the disease progresses.

How and Where Does Gray Leaf Spot Develop

Gray leaf spot typically occurs in warm, humid conditions and thrives in areas of high humidity and prolonged leaf wetness. It is most severe during periods of hot and humid weather, typically in the mid-to-late summer months. The disease can spread rapidly, especially when the environmental conditions are favorable.

The disease weakens the affected grass, making it more susceptible to other stresses, such as drought, heat, and other diseases. If left untreated, gray leaf spot can result in significant damage to the turfgrass stand, leading to bare patches and an overall decline of large areas.

Combatting the Lawn Disease Gray Leaf Spot

Effective management of gray leaf spot involves a combination of cultural practices and, if necessary, the use of fungicides. Proper lawn maintenance, including regular mowing, appropriate watering, and good air circulation, can help reduce the risk of gray leaf spot. Fungicides labeled for gray leaf spot control are best used in a preventive manner before widespread disease symptoms appear. Curative treatments can help stop the spread of gray leaf spot to new areas but are only marginally effective at curing the disease that has occurred.

When it comes to selecting fungicides for gray leaf spot, it's essential to consider the specific products labeled for use on gray leaf spot and those labeled specifically for residential lawn care. The availability of fungicides may vary depending on your location and local regulations.

Remember to always follow the application recommendations on the product label or otherwise reach out to your Harrell's Representative for application advice.

Fungicides for Gray Leaf Spot

Your Harrell’s rep can provide you with tailored solutions and implement Harrell's Balanced Approach programs to address lawncare issues like insects, weeds, and diseases while optimizing overall plant health. Harrell's ProtectMAX® line of products includes some commonly used fungicides that have shown effectiveness against gray leaf spot on fescue grass:

1. Azoxystrobin: This active ingredient is commonly found in fungicides used for controlling gray leaf spot. It provides both preventive and curative action against the disease.
2. Propiconazole: Another active ingredient used in fungicides for gray leaf spot control. It is a systemic fungicide that helps protect the grass by inhibiting fungal growth.
3. Thiophanate Methyl: This fungicide is effective against various fungal diseases, including gray leaf spot. It provides preventive and curative activity against the disease.

You can learn more about these products in our Lawn Care Disease Trips article, which includes quick facts about common lawn care diseases.

For Gray Leaf Spot Diagnosis and Management, Contact Your Harrell’s Rep

It's important to note that the appearance of gray leaf spot can vary slightly depending on the specific turfgrass species and environmental conditions. If you suspect gray leaf spot on your lawn, consult with your Harrell's Rep, who can provide a definitive diagnosis and appropriate management recommendations.

Not all products are registered in all states. Always read and follow the instructions on the fungicide label, including application rates, timing, and safety precautions.