Gray Leaf Spot vs. Dollar Spot in St. Augustine Grass: Side-by-Side Comparison

If you have St. Augustine grass in DFW and you are staring at spots on your blades trying to figure out what is wrong, you are probably dealing with one of two diseases: gray leaf spot or dollar spot. Both create discolored, spotted blades. Both show up during humid, wet weather. And both are routinely misidentified by homeowners — and even some lawn care providers — because they can look superficially similar from a distance. But they are completely different diseases caused by different fungi, they peak in different seasons, and they respond to different treatments. Getting the diagnosis right is the difference between an effective treatment and weeks of frustration. When you are not sure what you are looking at, professional lawn disease and fungus control removes the guesswork entirely.

Gray Leaf Spot: The Summer Destroyer of St. Augustine

Gray leaf spot is caused by Pyricularia grisea (also known as Magnaporthe grisea), a fungal pathogen that is almost exclusively a problem in St. Augustine grass in DFW. While the same species can infect other grasses in other parts of the country, North Texas gray leaf spot outbreaks are heavily concentrated in St. Augustine — which makes it both a regional and grass-specific problem. If you have Bermuda or zoysia and you see spotting, gray leaf spot is far less likely.

The disease is a summer pathogen. In DFW it becomes most aggressive from late June through August, exactly when the heat and humidity peak. The combination of daytime temperatures in the 90s, overnight lows staying warm, and the frequent afternoon thunderstorms that characterize North Texas summers creates near-ideal conditions for rapid spread. A lawn can go from a few spots to widespread, severe damage in less than two weeks during a gray leaf spot outbreak under these conditions.

What Gray Leaf Spot Looks Like: The Diagnostic Details

The early-stage symptoms of gray leaf spot are small — and that is exactly what makes them easy to miss until the disease has already gained significant momentum:

• Initial lesions: Tiny, water-soaked spots on the blade, roughly the size of a pencil point. At this stage they look like small wet dots that might be confused with insect damage or irrigation residue.
• Mature lesions: The spots enlarge and develop a tan or gray center surrounded by a purple, brown, or olive-colored halo. The halo is one of the most reliable visual identifiers of gray leaf spot. Under a hand lens, the center of the lesion may show a velvety gray appearance from the sporulating fungal structures.
• Blade yellowing and death: As lesions multiply, the blade yellows and dies from the tips inward. Severely infected blades turn yellow or brown and may curl or collapse entirely.
• Rapid spread after rain: Gray leaf spot explodes after summer thunderstorms. The disease spreads by spores that travel in water splash, wind, and mowing equipment. A storm event followed by warm, humid nights can trigger a near-overnight increase in infected area. This explosive post-rain spread is one of the clearest behavioral indicators of gray leaf spot.
• Lawn appearance from a distance: A moderately infected St. Augustine lawn looks yellowed or “scorched” across large areas rather than in small circular patches. The damage is diffuse because the disease infects individual blades across the lawn, not in a concentrated circular pattern from a single infection point.

Dollar Spot in St. Augustine: Less Common, Different Pattern

Dollar spot, caused by Clarireedia jacksonii, is far more commonly a Bermuda grass disease in DFW — but it does occur in St. Augustine, and when it does, homeowners often confuse it with gray leaf spot because both create discolored blade tissue. The key differences:

• Lesion appearance: Dollar spot on St. Augustine produces the same hourglass lesion that appears on Bermuda — a bleached or tan area crossing the width of the blade, bordered at the top and bottom of the lesion by a reddish-brown or tan band. The lesion is typically larger and more clearly defined than a gray leaf spot lesion, and it does not have the purple or olive-colored halo.
• Patch pattern: Unlike gray leaf spot’s diffuse, lawn-wide appearance, dollar spot creates more discrete circular patches of bleached grass roughly the size of a silver dollar. From a distance, dollar spot looks like scattered round dead spots; gray leaf spot looks like the whole lawn is yellowing.
• No purple halo: The purple or olive border around the lesion is specific to gray leaf spot. Dollar spot lesions have a reddish-brown band but no purple discoloration and no obvious halo effect under normal viewing conditions.
• Morning mycelium: Dollar spot produces a cobweb-like white mycelium visible across the lawn surface in early morning before it dries. Gray leaf spot does not produce the same surface mycelium display, though sporulation is visible on individual lesions under magnification.

Seasonal Difference: The Most Reliable First Filter

Before you examine a single blade, ask yourself one question: what month is it?

• Gray leaf spot: Peaks in July and August. Active outbreaks in June are possible in wet years. Almost never a significant problem before June or after September in DFW. If you are seeing widespread blade spotting in the heat of summer, gray leaf spot is the far more likely diagnosis in St. Augustine.
• Dollar spot in St. Augustine: More likely in spring (April–May) or fall (September–October) when temperatures moderate and dew periods are long. Summer dollar spot can occur but is less common because extreme heat slows the pathogen. If your blade spotting is appearing when the weather is mild rather than at peak summer heat, dollar spot moves up in probability.

This seasonal filter alone eliminates a lot of diagnostic uncertainty before you ever pull out a hand lens.

Treatment Differences: Why the Diagnosis Matters

Treating the wrong disease does not just waste money — it can actively make the right disease worse:

• Gray leaf spot treatment: Requires fungicides that target blast diseases. Azoxystrobin, pyraclostrobin, and trifloxystrobin (strobilurin-class fungicides) are highly effective. Apply at first sign of active spread and repeat on a 14–21 day interval while summer humidity remains high. Critically: do not apply nitrogen fertilizer during an active gray leaf spot outbreak. High nitrogen pushes lush, soft growth that is extremely susceptible — this is one of the most common mistakes homeowners make. Fertilizing a gray-leaf-spot-infected lawn can rapidly worsen the outbreak.
• Dollar spot treatment: Also responds to strobilurin fungicides, but propiconazole and thiophanate-methyl are also effective choices. Unlike gray leaf spot, dollar spot often improves significantly with a light nitrogen application alongside fungicide treatment. The disease is strongly associated with low nitrogen levels, and correcting the deficiency removes a key vulnerability. Applying nitrogen to dollar spot accelerates recovery; applying nitrogen to gray leaf spot accelerates the disease. This is a critical distinction.

How DFW Homeowners Most Commonly Mix Them Up

The most common misidentification pattern: a homeowner sees spots on their St. Augustine blades in July, assumes dollar spot because they have heard of it, applies a light nitrogen treatment alongside fungicide — and watches the lawn get dramatically worse over the following week. What they had was gray leaf spot, and the nitrogen they applied to address what they thought was a “starvation disease” fed the actual pathogen explosively.

The reverse also happens: homeowners see circular bleached patches in spring, assume it is a summer disease and wait for it to resolve on its own, not realizing they are watching dollar spot spread through nitrogen-deficient turf that could be corrected quickly with the right treatment.

Hamann Lawn Care & Weed Control has been diagnosing St. Augustine diseases in Arlington and DFW since 2006. We look at the blade lesions, the season, the moisture conditions, and the grass history before recommending any treatment — because the wrong diagnosis in lawn disease does not just fail to help, it actively causes more damage. For a related comparison, read about take-all root rot vs. brown patch root system comparison to understand the root-level diagnostic process.