Gray Leaf Spot in St. Augustine Grass: Identification and Treatment

Gray leaf spot is one of the more distinctive lawn diseases because it shows up as damage to the individual grass blades, not the circular ground-level patches typical of brown patch. If you’re seeing spotted, discolored blades rather than a patch pattern, this is likely what you’re dealing with.

What It Looks Like

Small, diamond or oval-shaped lesions on the grass blades themselves — gray to brown in the center with a darker purple-brown border. Individual lesions are usually less than a quarter-inch, but in a bad outbreak they multiply enough that the whole lawn takes on a scorched, blighted look from a distance, especially in shaded or humid areas.

This is different from brown patch (see Brown Patch and Large Patch), which produces circular patches at the ground level rather than spotting on individual blades.

Why It Happens

Gray leaf spot is caused by the fungus Pyricularia grisea, and it thrives under a specific combination of conditions that Central Texas hits reliably every summer:

• Hot, humid weather — mid-summer, especially after rain or heavy dew
• Shade — worse in shaded St. Augustine areas where blades stay wet longer
• Excess nitrogen — outbreaks often follow a heavy summer fertilizer application, since lush, nitrogen-rich growth is more susceptible
• Evening watering — same underlying issue as brown patch: wet blades overnight are exactly what fungal disease wants

Treatment

1. Cut back on nitrogen during peak summer heat. If you were due for a fertilizer application, hold off or switch to a lower-nitrogen option until the outbreak clears.
2. Switch all watering to early morning (5–10am) if you haven’t already — this is non-negotiable for controlling any fungal issue in St. Augustine.
3. Apply a fungicide containing propiconazole, following label directions, for an active outbreak that isn’t resolving on its own.
4. Improve airflow in shaded areas where possible — thinning overhanging branches helps blades dry faster after dew or rain.

Prevention

• Stick to the recommended fertilizing schedule in our Lawn Care Guide rather than applying extra nitrogen “just in case” — this is the single biggest preventable trigger.
• Morning-only watering, year-round, not just during outbreak season.
• If a specific shaded section of the lawn is a repeat offender, consider whether Amerishade St. Augustine (our deepest shade-tolerant variety) might handle that spot with less disease pressure than a standard variety planted somewhere it doesn’t get enough airflow.

Gray Leaf Spot vs. Brown Patch

Not Sure What You’re Looking At?

A close-up photo usually makes it obvious which disease you’re dealing with — call (936) 510-0964 or reach out through Get a Quote if you’d like a second opinion before treating.