Brown Patch vs. Large Patch: How to Tell Them Apart in a DFW Lawn

Two of the most damaging lawn diseases in DFW share the same genus of fungus but are completely different diseases — and treating one with the wrong approach makes the other worse. Brown patch and large patch are both caused by Rhizoctonia solani, yet they attack different grasses, thrive at different temperatures, and require different treatment timing. Confusing them is one of the most common mistakes homeowners and even some lawn care companies make in North Texas. Here is how to tell them apart and why getting it right is the difference between a healthy recovery and a wasted season. When either disease is active, professional lawn disease and fungus control is the fastest way to accurate diagnosis and effective treatment.

Same Fungus, Different Genetic Groups

Both brown patch and large patch are caused by Rhizoctonia solani, but they belong to completely different anastomosis groups — the genetic subgroups that determine which hosts the fungus can attack and under what conditions. Brown patch is caused by Rhizoctonia solani AG 2-2 IIIB, a strain that can infect a wide range of grasses including both cool-season varieties like tall fescue and warm-season grasses like St. Augustine and Bermuda. Large patch is caused by Rhizoctonia solani AG 2-2 LP, a strain that attacks warm-season grasses exclusively — particularly zoysia, St. Augustine, and centipede.

This distinction matters because the diseases are genetically different organisms that happen to share a family name. The fungicides that work against one will work against both, but the timing, triggers, and target conditions are entirely different. Applying the right product at the wrong time accomplishes little.

Which Grasses Each Disease Targets

Understanding which grasses are vulnerable helps you narrow down which disease you are dealing with before you even start looking at symptoms:

• Brown patch (AG 2-2 IIIB): Can infect St. Augustine, Bermuda, zoysia, tall fescue, and ryegrass. In DFW, it is most destructive on St. Augustine during the summer months. It is one of the few turfgrass diseases that can attack both cool-season and warm-season grasses.
• Large patch (AG 2-2 LP): Strictly a warm-season grass disease. In DFW, zoysia is the primary victim, followed by St. Augustine and centipede. It does not infect tall fescue or ryegrass. If your lawn is zoysia and you have circular damage in fall or spring, large patch should be your first suspicion.

Temperature Triggers: The Key Differentiator

Temperature is the most reliable way to distinguish which disease is active when symptoms first appear:

• Brown patch temperature range: Most active when nighttime temperatures are between 70 and 85°F with daytime temperatures in the 80s and 90s. In DFW this means brown patch outbreaks peak from mid-June through September. The disease slows dramatically when nighttime temperatures drop below 65°F.
• Large patch temperature range: Becomes active when soil temperatures drop below 70°F, which in DFW typically means October through November in the fall and February through April in the spring. Large patch is a cool-weather disease in a warm-weather grass — it strikes when zoysia and St. Augustine are at their weakest, going into or coming out of dormancy.

This temperature difference means the two diseases almost never compete for the same weather window. If damage appears in the heat of summer, brown patch is far more likely. If damage appears when nights are cooling off in fall or as the lawn is greening up in spring, large patch becomes the primary suspect.

Visual Differences in the Field

Both diseases create roughly circular patches of damaged turf, but the visual details are different enough to help confirm your diagnosis:

• Brown patch on St. Augustine or tall fescue: The hallmark visual is the “smoke ring” — a darker, water-soaked, slightly grayish outer margin at the actively expanding edge of the circle. The center of the ring is tan or straw-colored. Individual blades at the edge show irregular brown lesions where the sheath has rotted. The damage tends to be lighter in the center as the fungus advances outward.
• Large patch on zoysia: No smoke ring. Instead, the outer perimeter of the patch has a distinctive bronze or orange color where actively dying tissue meets healthy grass. The center of the patch is typically a bleached tan or straw. The orange border is caused by the way large patch attacks the stolon and leaf sheath at the soil level — blades die from the base up rather than developing the leaf lesions typical of brown patch.
• Large patch on St. Augustine: Appears similar to brown patch but occurs in fall and spring rather than summer. The affected blades yellow and pull free easily from the stolon because the sheath has rotted at the base.

How to Pull a Core Sample to Confirm Your Diagnosis

Visual inspection is a good starting point, but pulling a core sample from the margin of the patch gives you much better information:

• Use a pocket knife or a hand trowel to dig up a small plug of turf from the actively damaged edge of the patch — not the center, which may already be dead and offer no useful clues.
• Examine the leaf sheaths — the area where the blade meets the stolon. Both diseases cause sheath rot, but large patch typically shows more pronounced rotting at the base of the plant, with blades that pull free with almost no resistance.
• Look at the roots. Brown patch typically leaves the root system largely intact, with damage concentrated in the blade and sheath. Take-all root rot (often confused with large patch) causes black, rotted roots that crumble when pulled. If you see blackened roots, large patch is less likely — consider take-all root rot as an alternative diagnosis.
• Check the stolon color. Large patch often discolors the stolon a brown or reddish color at the point of infection. Brown patch damage is usually concentrated above the stolon.

Fungicide Timing for Each Disease

The most important takeaway from understanding these two diseases is that preventive fungicide timing differs completely:

• Brown patch prevention: Apply fungicide in late spring or early summer before nighttime temperatures consistently exceed 70°F. In DFW, this means late May through early June. A second application may be needed in mid-summer if conditions remain favorable.
• Large patch prevention: Apply fungicide in early fall when soil temperatures drop to 70°F, typically late September through October. A second application in early spring (February or March) catches the spring flush of activity before the lawn breaks dormancy. Preventive timing is essential because large patch is much harder to stop once it is actively spreading.

Why Misdiagnosis Leads to Wasted Treatment

A homeowner who sees circular brown patches on their zoysia lawn in October and assumes brown patch will apply fungicide at summer rates and timing — and wonder why the lawn keeps declining. Large patch requires fall-timed preventive treatment, and a curative application in the middle of an active outbreak is far less effective than stopping the disease before it starts. Similarly, treating large patch with a product timed for summer conditions does nothing to address the cool-weather infection cycle.

Hamann Lawn Care & Weed Control has been diagnosing and treating North Texas lawn diseases since 2006. We identify whether you are dealing with brown patch, large patch, or a completely different problem before any product goes down — because the right treatment at the right time is everything. Read more about lawn disease in shaded St. Augustine after summer rain for another common DFW diagnosis challenge.