Brown Patch vs Dollar Spot Diagnosis Decision Tree

Why this matters

Brown patch (Rhizoctonia solani) and dollar spot (Clarireedia jacksonii, formerly Sclerotinia homoeocarpa) are the two most commonly misdiagnosed turfgrass diseases in cool-season lawns and the warm-season transition zone. Both produce circular dead patches. Both peak in similar weather windows. Both look superficially identical at a quick walk-by. But the fungicide chemistries that control them are different, the cultural prescriptions are different, and the long-term agronomic response is different. Misdiagnose brown patch as dollar spot, apply a propiconazole single-mode product, and you treat the wrong pathogen while the actual brown patch escalates and takes out a 40 sq ft section of the lawn over the next 10 days. This decision tree walks the field tech through the differential.

Step 1 - Identify the grass species

The two diseases differ in host range:

• Brown patch hits cool-season turf (tall fescue, perennial ryegrass, Kentucky bluegrass) hardest, and also affects warm-season grasses (St. Augustine, zoysia) particularly in transition-zone climates.
• Dollar spot hits both cool-season (especially creeping bentgrass and Kentucky bluegrass) and warm-season (bermuda, zoysia) turf.

A tall fescue lawn with circular patches in mid-summer is the classic brown patch host. A bentgrass putting green with silver-dollar-sized spots is the classic dollar spot host. Identify the species first, then read the symptoms.

Step 2 - Measure the patch diameter

Brown patch patches are typically 6 inches to 3 feet in diameter, sometimes coalescing into larger irregular areas. Dollar spot patches are 1 to 4 inches in diameter (the namesake silver-dollar size), well-defined, and rarely larger than 6 inches.

If the patches are silver-dollar-sized and small, the diagnosis is dollar spot until proven otherwise. If the patches are foot-wide or larger, brown patch is more likely. If the patches are between 4 and 12 inches and you cannot make the call by size alone, advance to the next step.

Step 3 - Look for the smoke ring

Brown patch produces a "smoke ring" - a dark gray-purple band of wilted leaves at the advancing edge of the patch, visible in the early morning when dew is still on the grass. The smoke ring fades as the day warms and the dew evaporates. The smoke ring is diagnostic for brown patch.

Dollar spot does not produce a smoke ring. Dollar spot produces a tan to bleached-straw patch with sharp boundaries and no advancing-edge band.

If you see a smoke ring at sunrise, the call is brown patch. If you do not see a smoke ring, advance to the leaf-spot inspection.

Step 4 - Inspect individual leaf blades

Pull a few affected leaf blades from the patch edge and examine them with a hand lens:

• Brown patch leaf lesions are large, irregular, water-soaked at the edges, and span most of the leaf width. Lesions are tan to brown with a darker border.
• Dollar spot leaf lesions are sharply defined hourglass-shaped white-to-straw bands across the full leaf width, often with a reddish-brown border. Multiple lesions on a single leaf are common.

The hourglass lesion is diagnostic for dollar spot. The large water-soaked lesion is diagnostic for brown patch.

Step 5 - Check for mycelium

Both pathogens produce mycelium under wet conditions, but the appearance differs:

• Brown patch mycelium is sparse, gray-white, and fades within an hour of dew dry-off.
• Dollar spot mycelium is denser, cottony-white, and may persist longer into the morning.

Mycelium presence confirms an active pathogen but is not by itself diagnostic between the two. Use it together with the smoke ring and leaf lesion findings.

Step 6 - Cross-check the weather

Both pathogens have similar weather requirements but slightly different thresholds:

• Brown patch favors high humidity (over 95%), warm nights (above 70 F minimum), and prolonged leaf wetness (over 10 hours).
• Dollar spot favors warm days (75 to 90 F), cool nights (around 50 to 60 F), heavy dew, and lower-than-recommended nitrogen.

A mid-summer humid stretch with warm nights skews toward brown patch. A late-spring or early-fall stretch with heavy dew and a nitrogen-deficient lawn skews toward dollar spot.

Step 7 - Cross-check the nitrogen history

Dollar spot is the textbook nitrogen-deficient disease. Lawns under-fed for nitrogen and showing yellow-green color background are at high dollar spot risk. Adding nitrogen (the cultural intervention) reduces dollar spot severity even without fungicide.

Brown patch is the opposite - it is exacerbated by excess nitrogen, particularly soluble nitrogen applied during summer heat. A lawn that just received a heavy summer soluble-N feed and showed up two weeks later with patches is the brown patch profile.

Step 8 - Make the call and select the fungicide chemistry

If brown patch: reduce nitrogen; increase mowing height; improve drainage; no irrigation past 6 PM. Fungicide options include azoxystrobin (FRAC 11), pyraclostrobin (FRAC 11), flutolanil (FRAC 7), or fluxapyroxad (FRAC 7). Tank-mix or rotate FRAC groups to manage resistance.

If dollar spot: increase nitrogen if deficient; maintain consistent moisture (dollar spot is worse on dry-stressed turf); roll greens or top-dress to break dew formation. Fungicide options include propiconazole (FRAC 3), boscalid (FRAC 7), penthiopyrad (FRAC 7), or chlorothalonil (FRAC M5). Rotate FRAC groups - dollar spot has documented resistance to single-mode DMI and SDHI products in some markets.

Step 9 - Confirm with a soil or tissue sample if uncertain

If the field diagnosis is uncertain, send a tissue sample to the state extension turf diagnostic lab. Most land-grant universities operate plant disease clinics that culture the pathogen and confirm the identification within 5 to 10 business days. The sample-and-confirm step is cheap insurance on a high-value commercial property or a HOA contract where misdiagnosis costs the relationship.

Step 10 - Document the diagnosis

Photograph the patches with a ruler in frame for scale. Photograph the leaf lesions under a hand lens. Note the weather conditions, nitrogen history, host species, and diagnostic features that drove the call. Document the fungicide chemistry chosen and the FRAC group. Schedule a follow-up inspection at 7 to 10 days to verify the treatment is working - escalating patches after treatment means either misdiagnosis or a resistant pathogen population and warrants a chemistry rotation.

References

1. North Carolina State University TurfFiles, Brown Patch and Dollar Spot Identification Guides
2. Penn State Extension, Turfgrass Diseases and Their Management
3. FRAC (Fungicide Resistance Action Committee), Code List for Turf Fungicides
4. 7 USC Chapter 6, Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA)
5. EPA Pesticide Product Label System (PPLS), label-of-record search for current registrations
6. ANSI/ASABE Engineering Practice EP485 (referenced for irrigation scheduling impact on turf disease)