One of the most common calls Hamann Lawn Care & Weed Control receives in late spring and early fall involves homeowners describing lesions on individual grass blades — a tan or brown center with a yellow border or halo surrounding it. This pattern is visually distinctive and consistent enough that it tends to point toward one of two diseases: helminthosporium leaf spot or dollar spot. Both produce halo-type lesions on grass blades. Both occur in North Texas lawns. And both are treated with entirely different fungicides, which is why getting the diagnosis right before you purchase anything matters enormously. For an accurate identification and treatment, professional lawn disease and fungus control removes the guesswork. Here is how to tell them apart.
The Classic Halo Lesion: What You’re Actually Seeing
A halo lesion on a grass blade is the result of fungal colonization spreading outward from an infection point. The center of the lesion is the oldest, most advanced part of the infection, where the tissue has already been killed — which is why it appears tan, straw-colored, or bleached. The yellow border represents the zone where the fungus is actively attacking living tissue: cells are stressed, dying, but not yet fully dead. The contrast between the bleached center and the yellow perimeter creates the halo effect that makes these lesions visually recognizable even to untrained eyes.
Both helminthosporium leaf spot and dollar spot produce this general pattern, but the details differ in ways that allow diagnosis by close inspection of individual blades.
Dollar Spot: The Band That Crosses the Full Blade
Dollar spot is caused by Clarireedia jacksonii (formerly Sclerotinia homoeocarpa) and is one of the most widespread turfgrass diseases in the United States. In North Texas, it appears most aggressively on Bermuda grass — which is the dominant lawn grass in many DFW neighborhoods — though Zoysia can also develop dollar spot under the right conditions.
The signature blade lesion for dollar spot is a bleached tan band that crosses the entire width of the grass blade, with a reddish-brown or yellow margin at both edges. This band-across-the-blade pattern is the most reliable visual identifier. On narrow Bermuda blades, these lesions are small but unmistakable when examined closely. The lesion is symmetrical: both margins of the blade show the yellow-to-reddish border, and the center band is distinctly lighter than the surrounding healthy tissue.
Dollar spot patches on the lawn surface appear as silver dollar–sized circles of bleached or tan grass (which is where the name comes from), usually three to six inches in diameter. Multiple patches can merge into larger affected areas as the disease spreads. Morning dew is critical to dollar spot development: the fungus produces visible white cobweb-like mycelium on blade surfaces in early morning dew conditions, which is another diagnostic clue specific to dollar spot.
Helminthosporium Leaf Spot: Irregular Lesions on the Blade Surface
Helminthosporium leaf spot is caused by a complex of Bipolaris and Exserohilum species and behaves differently from dollar spot in several important ways. On St. Augustine — the most susceptible grass in Tarrant County and surrounding areas — helminthosporium produces irregular, elongated lesions that appear on the blade surface rather than crossing the full width of the blade. This distinction is key. A helminthosporium lesion sits within the blade surface, often with darker borders and an irregular or water-soaked outline, while a dollar spot lesion bands completely across the blade from margin to margin.
The yellow halo around a helminthosporium lesion tends to be less uniform and more diffuse than the distinct reddish-yellow border of a dollar spot lesion. Helminthosporium lesions also tend to be darker in the center — brown to gray-brown — compared to the straw-colored or bleached center of a dollar spot lesion.
On Bermuda grass, helminthosporium can also cause melting-out, a more severe form of the disease where the pathogen moves from blade lesions into the crown and root zone, causing widespread thinning and collapse. This is more likely when the disease occurs in combination with stressors like drought, compaction, or very low mowing height.
DFW Grass Type Preferences
Grass type is one of the most useful diagnostic shortcuts in North Texas:
- Bermuda grass with halo lesions: Dollar spot is the primary suspect, especially in spring (April–May) and fall (September–October) when temperatures are in the 60–80°F range and humidity is moderate to high. Bermuda is the preferred host for dollar spot in our region.
- St. Augustine grass with halo lesions: Helminthosporium leaf spot is far more common than dollar spot. St. Augustine’s wide blades are more susceptible to this pathogen, and the lesion pattern — irregular surface spots rather than full-width bands — is usually clear when you look at individual blades carefully.
- Zoysia grass: Both diseases can occur, but dollar spot is more common during periods of low nitrogen and moderate temperatures. Zoysia’s denser growth habit can make patch identification on the lawn surface harder.
Seasonal Timing Differences in North Texas
Seasonal timing is another useful distinguishing tool. Dollar spot in DFW is most active in spring and fall, when overnight temperatures drop into the 50s and 60s°F and morning dew is heavy. It slows significantly during the peak summer heat of July and August when temperatures consistently exceed 90°F. Helminthosporium leaf spot, by contrast, has two active windows in North Texas: spring (March–May) and a summer window when temperatures are warm but lawns are under stress from drought, heat, or both.
If halo lesions appear on your Bermuda lawn in late April after several cool, dewy nights, dollar spot is the most likely culprit. If similar lesions appear on St. Augustine in June during a stretch of cloudy, humid weather, helminthosporium is more probable.
The Nitrogen Connection
Both diseases have a nitrogen management dimension, but they respond in opposite ways. Dollar spot is strongly associated with nitrogen deficiency: lawns that are underfertilized are significantly more susceptible, and a timely nitrogen application in early spring can actually reduce dollar spot pressure on Bermuda by promoting the vigorous growth that helps the grass outpace infection. Helminthosporium, on the other hand, is worsened by excessive nitrogen. Lush, rapidly growing St. Augustine pushed with heavy summer nitrogen applications is more susceptible to helminthosporium infection, not less.
This inverse relationship means that if you treat halo lesions on a Bermuda lawn as helminthosporium and withhold nitrogen, you may actually make dollar spot worse. If you treat St. Augustine lesions as dollar spot and apply nitrogen to fight it, you may accelerate helminthosporium. Correct identification drives correct management.
Why Misdiagnosis Leads to Wrong Fungicide
Dollar spot is effectively controlled by DMI fungicides (propiconazole, myclobutanil, tebuconazole) and SDHI fungicides (boscalid, fluxapyroxad). Helminthosporium leaf spot responds to fungicides in the QoI (strobilurin) class such as azoxystrobin, as well as certain DMIs. While there is some overlap in fungicide classes that work on both, the best-fit products differ enough that using the wrong product produces sub-optimal results — and in North Texas summers, every day of ineffective treatment is more turf lost.
Reading about slimy black spots on grass blades can also help you rule out other causes of discoloration before committing to a fungicide application.
How Hamann Diagnoses Halo Lesions
When a Hamann technician arrives to evaluate halo lesions, the process starts with grass type, then blade-level inspection. We look at lesion shape, width relative to the blade, center color, border character, and morning mycelium before making any product recommendation. We also evaluate the irrigation schedule, recent fertilization history, and soil conditions, because cultural factors often explain why the disease is present in the first place. A correct diagnosis paired with a management plan is what produces lasting results — not just a spray and a hope.
Halo Lesions on Your Grass Blades in DFW?
Don’t guess between dollar spot and helminthosporium. Hamann Lawn Care & Weed Control has diagnosed and treated North Texas lawn diseases since 2006 — we’ll get it right.