Sunken Patches in Your Lawn: What Fungal Disease Is Usually to Blame

A patch of dead turf that sits slightly lower than the rest of your lawn is more than an aesthetic problem — it’s a diagnostic clue. Sunken or depressed dead patches in North Texas lawns almost always indicate that structural damage has occurred below the surface, not just to the blades. When a lawn patch collapses below grade, it means the crowns, stolons, or root system have been destroyed and the plant structure that was physically supporting that turf layer is gone. That’s the signature of several serious fungal diseases, and getting the right lawn disease and fungus control response quickly matters because the exposed bare soil becomes a weed magnet almost immediately in North Texas.

Why Fungal Disease Creates Sunken Patches

Healthy turf has physical mass. The dense network of roots, crowns, stolons, and thatch hold the lawn surface at a consistent grade. When a root-rotting or crown-rotting fungal disease kills that substructure, the organic material collapses as it decomposes, and the soil settles slightly. The result is a patch that sits a quarter inch to a full inch below the surrounding turf — sometimes dramatically visible when the lawn is viewed at a low angle in morning or evening light. This physical depression is distinct from simple blade death, which leaves the lawn level even as the grass turns brown.

Take-All Root Rot: The Primary Suspect in DFW

Take-all root rot (TARR), caused by the fungus Gaeumannomyces graminis var. graminis, is the most common cause of sunken, depressed dead patches in St. Augustine and Bermuda grass lawns in North Texas. It is also one of the most frequently misdiagnosed lawn diseases in the region because its above-ground symptoms closely resemble drought stress, iron deficiency, and overwatering — leading to treatments that actually accelerate the disease.

Take-all root rot destroys the root system first, then progresses up through the crown. By the time the turf above is visibly yellow and dying, the roots below have often already turned dark brown to black and decayed extensively. Pull a handful of runners from a suspected TARR area and you’ll find:

• Black, shortened, rotted roots: Healthy St. Augustine roots are white to tan and extend several inches into the soil. TARR roots are dark, sparse, and snap off at or near the crown with almost no resistance.
• Runners that detach easily from the soil: Because the root system is gone, you can rake or pull runners from TARR-affected areas with very little force — the grass is essentially sitting on the soil surface unanchored.
• No living root material below one inch: In severe TARR, scraping the soil surface in the patch reveals no healthy white roots whatsoever — just decomposing dark organic matter.

North Texas Conditions That Trigger Take-All Root Rot

TARR thrives in specific soil and environmental conditions that North Texas delivers routinely:

• High soil pH: Alkaline soils above pH 7.0 — extremely common in Arlington and the broader DFW region — strongly favor Gaeumannomyces activity. The pathogen suppresses its own soil competitors in alkaline environments, allowing it to colonize root systems with little biological resistance.
• Overwatering or poor drainage: Waterlogged root zones provide ideal anaerobic conditions for TARR development. Many homeowners over-irrigate St. Augustine lawns in summer trying to keep them green through heat — this inadvertently creates the soil conditions the pathogen loves.
• Phosphorus deficiency: Research has linked phosphorus-deficient soils with increased TARR severity. Many North Texas lawns are phosphorus-sufficient but not universally so, and a soil test revealing P deficiency alongside fungal symptoms strongly suggests TARR.
• Cool, wet periods followed by heat stress: TARR tends to become most visible in late spring and early summer — the fungus infects during the mild, wet conditions of spring, and the resulting root damage becomes apparent when summer heat demands a root system the grass no longer has.

Spring Dead Spot: Sunken Patches in Bermuda

In Bermuda grass lawns specifically, sunken dead patches that appear in spring are almost always spring dead spot (SDS) rather than TARR. The two diseases produce similar visual results — sunken, collapsed dead zones with rotted root systems — but they affect different grass species and have different seasonal timing and treatment approaches. If your Bermuda has circular sunken patches that appeared as the lawn greened up in March or April, see our companion post on circular dead rings and how to diagnose the fungal cause for a detailed look at distinguishing SDS from other ring-forming diseases.

Pythium Root Rot: When Sunken Patches Develop Rapidly

Pythium root rot is a water mold (oomycete) rather than a true fungus, but it causes similar sunken patch symptoms and responds to some similar fungicides. Pythium root rot is triggered almost exclusively by saturated soil conditions — standing water, excessive irrigation, or heavy rainfall that keeps the root zone flooded for extended periods. In DFW, this typically means patches that develop rapidly during periods of heavy summer thunderstorms in low-lying areas or near irrigation heads with poor coverage uniformity.

Pythium patches tend to have:

• A greasy, water-soaked appearance at the margin when actively spreading
• Extremely rapid development — visibly enlarging within 24 to 48 hours during wet weather
• A foul or fishy odor from the rotting tissue when the patch is fresh
• White, cottony mycelial growth visible in early morning at the patch border — though this disappears once conditions dry slightly

What Not to Do When You See Sunken Patches

Several common homeowner responses to sunken dead patches make the underlying disease significantly worse:

• Increasing irrigation: If the cause is TARR or Pythium root rot — both of which thrive in wet conditions — adding more water accelerates the disease. If the lawn is wilting despite adequate soil moisture, the problem is almost certainly root-based, not drought.
• Heavy nitrogen fertilization: Applying nitrogen to stressed turf attempting to force recovery pushes succulent growth that the compromised root system can’t support, making the turf more susceptible to additional pathogen attack.
• Applying the wrong fungicide: Products labeled for brown patch or dollar spot have little to no activity against Gaeumannomyces. TARR treatment requires specific chemistry — particularly phosphonate fungicides and products containing azoxystrobin or thiophanate-methyl applied to the root zone, not just the foliage.

Recovery and Prevention

Once sunken patches from TARR or SDS have developed, recovery requires both treating the pathogen and addressing the soil conditions that enabled it. Soil pH reduction through sulfur applications, improved drainage through core aeration, and corrected irrigation timing — combined with the right fungicide chemistry — gives the turf the best chance to recover and reduces recurrence the following season. In severe cases where the root system is completely gone, sodding or plugging the dead areas is necessary, paired with a preventive fungicide program to protect the new turf.

Hamann has treated sunken dead patches and root rot diseases across Arlington and DFW since 2006. Our lawn disease and fungus control program starts with identifying what’s actually happening below the surface — because that’s where these diseases live and where treatment has to reach.