South Florida’s warm, humid, rain-soaked climate grows some of the most beautiful warm-season grasses in the country — and, unfortunately, some of the most aggressive lawn fungus in the country right alongside them. If you live in Palm Beach County and you’ve spotted a mysterious brown patch, a ring of mushrooms, or blades that pull up with rotten roots, a fungal disease is almost always the culprit.
The good news: nearly every lawn fungus in our region is identifiable by sight, and most are preventable with the right watering, mowing, and fertilizing habits. This guide walks you through the ten most common lawn diseases in South Florida — how to identify each one, what triggers it, and exactly how to treat and prevent it.
Quick Lawn Fungus Identification Table
Use this at-a-glance table to narrow down what you’re looking at, then jump to the full section below for treatment details.
| Disease | What you’ll see | Peak season | First move |
|---|---|---|---|
| Large Patch | Circular brown patches with yellow-orange edges, several feet wide | Nov–May (cool, wet) | Cut back water & nitrogen |
| Take-All Root Rot | Yellow, thinning patches; blades pull up with short black roots | Summer–early fall (wet) | Reduce stress; balance K & N |
| Rust | Orange-red powder that rubs off on shoes and hands | Fall, shady/low-nutrient areas | Fertilize; improve airflow |
| Dollar Spot | Silver-dollar-sized tan spots that merge together | Warm, humid, low-fertility | Fertilize; water deeply at dawn |
| Fairy Ring | Rings/arcs of dark-green or dead grass, sometimes mushrooms | Warm, wet, high thatch | Aerate; apply wetting agent |
| Gray Leaf Spot | Gray-tan spots with dark borders on blades | Hot, humid summer | Cut nitrogen; improve airflow |
| Cercospora Leaf Spot | Narrow purple-to-tan spots with dark borders | Warm, humid, stressed turf | Reduce stress; mow higher |
| Bipolaris Leaf Spot | Purple to reddish-brown lesions, worse in drought | Drought/heat stress | Fix nutrition & irrigation |
| Slime Mold | Crusty gray, yellow, or purple powder on blade surfaces | Warm, wet, shady | Brush or rinse off (harmless) |
| Pythium Root Rot | Greasy, thinning, discolored turf with blackened roots | Warm, wet, poor drainage | Improve drainage; cut watering |
The 10 Most Common Lawn Fungal Diseases in Palm Beach County
1. Large Patch (Rhizoctonia solani)
Grasses most affected: St. Augustinegrass and zoysiagrass (the two most common lawns in South Florida).
When it strikes: The cooler, wetter months — roughly November through May, whenever temperatures stay below 80°F and grass leaves stay wet for more than 48 hours.
Symptoms: Large, roughly circular brown patches with distinct yellow-orange or reddish edges. Patches often start around a foot wide and expand to several feet, sometimes with a ring of discolored turf around greener grass in the center. Blades near the patch edge pull away easily and may smell rotten.
What causes it: Cool, damp conditions combined with excess nitrogen fertilization, poor drainage, and prolonged leaf wetness.
How to treat and prevent it:
- Avoid overwatering and hold off on nitrogen fertilizer during the cool season — use slow-release nitrogen and balance it with equal potassium.
- Water only when needed, early in the morning (roughly 2–8 a.m.), so blades dry quickly.
- Improve drainage and reduce thatch through core aeration.
- For active or recurring outbreaks, apply a fungicide such as azoxystrobin, flutolanil, or propiconazole preventively in early spring and late fall.
2. Take-All Root Rot (Gaeumannomyces graminis)
Grasses most affected: St. Augustinegrass, and to a lesser degree other warm-season grasses.
When it strikes: Unlike a true cool-weather disease, take-all root rot flares during our warm, high-rainfall stretches — summer and early fall — when the turf is already under stress. This is a correction worth noting, because it’s often mistaken for a cold-season problem.
Symptoms: Irregular yellow or light-green patches from a few inches to several feet across. The tell-tale sign is underground: pull on the affected grass and it lifts easily, revealing short, dark, rotted roots.
What causes it: Almost any stress on the lawn can trigger or worsen it — poor drainage, heavy thatch, drought, scalping, compacted soil, and over- or under-fertilizing all play a role. High soil pH can make turf more susceptible as well.
How to treat and prevent it:
- Reduce stress first — this is the single most effective control, because fungicides work poorly once symptoms appear.
- Mow at the correct height and never remove more than one-third of the blade at once; fertilize with slow-release nitrogen balanced by equal potassium.
- Aerate to relieve compaction and top-dress with a thin layer of compost to encourage new root growth.
- If your soil pH is high, some turf specialists recommend gradually lowering it toward 6.0–6.5 with elemental sulfur or acidifying fertilizers. Test your soil before adjusting, and avoid adding lime.
- Preventive fungicides (such as azoxystrobin) can help when applied early, but pair them with the cultural fixes above.
3. Rust (Puccinia spp.)
Grasses most affected: St. Augustinegrass and zoysiagrass, especially in shaded or under-fertilized areas.
When it strikes: Cooler, shorter days of fall, and any time growth slows in shady, moist, low-nutrient spots.
Symptoms: Reddish-orange pustules on grass blades that rub off as an orange powder on your hands, shoes, mower, and even pets. Heavily infected lawns take on an overall orange cast.
What causes it: Slow growth from low fertility, shade, poor airflow, and moisture — rust thrives when the grass isn’t growing vigorously enough to outpace it.
How to treat and prevent it:
- Fertilize appropriately to restore healthy growth (rust usually fades once the lawn is growing again).
- Maintain proper mowing heights and remove clippings while the disease is active.
- Improve sunlight and airflow by thinning nearby trees and shrubs.
- For severe cases, apply a triazole or strobilurin fungicide.
4. Dollar Spot (Clarireedia jacksonii)
Grasses most affected: A range of warm-season grasses, particularly those low on nutrients.
When it strikes: Warm, humid weather with heavy dew or evening irrigation, especially on under-fertilized lawns.
Symptoms: Small, round, straw-colored spots about the size of a silver dollar. As the disease spreads, spots merge into larger sunken patches.
What causes it: Low soil fertility, drought stress, and long periods of leaf wetness from dew or late-day watering.
How to treat and prevent it:
- Fertilize adequately — dollar spot is one of the few diseases that improves with a bit more nitrogen.
- Water deeply but infrequently, in the early morning, to limit how long blades stay wet.
- For persistent outbreaks, use a preventive fungicide such as thiophanate-methyl or propiconazole.
5. Fairy Ring
Grasses most affected: Any lawn, especially those with heavy thatch or buried organic debris.
When it strikes: Warm, wet weather, often where old roots, stumps, or construction debris are decomposing underground.
Symptoms: Rings or arcs of dark-green, fast-growing grass, dead grass, or mushrooms. In “hydrophobic” fairy ring, a water-repellent fungal mat forms in the soil, causing the ring to dry out and brown even when the rest of the lawn is watered.
What causes it: Decomposing organic matter, dense thatch, and compacted soil that a mushroom-forming fungus feeds on beneath the surface.
How to treat and prevent it:
- Core aerate through the ring to break up the water-repellent layer.
- Apply a soil wetting agent (surfactant) so water can penetrate and rehydrate the zone.
- Keep thatch in check and water the ring deeply; severe cases may benefit from a fungicide such as azoxystrobin.
6. Gray Leaf Spot (Pyricularia grisea)
Grasses most affected: St. Augustinegrass — this is one of its signature summer diseases.
When it strikes: Hot, humid summer weather, the peak of our rainy season.
Symptoms: Gray-to-tan spots with distinct dark brown margins on the leaf blades. Heavy infection gives the lawn a scorched, thinning appearance.
What causes it: Excess nitrogen fertilization, high humidity, extended leaf wetness, and poor air circulation.
How to treat and prevent it:
- Limit nitrogen applications during the summer months, when this disease is most active.
- Improve airflow by pruning surrounding trees and shrubs, and water early so blades dry quickly.
- If it persists, apply a fungicide for St. Augustinegrass such as azoxystrobin or pyraclostrobin.
7. Cercospora Leaf Spot (Cercospora fusimaculans)
Grasses most affected: St. Augustinegrass, particularly stressed or low-mown lawns.
When it strikes: Warm, humid conditions on turf weakened by drought or poor maintenance.
Symptoms: Narrow, elongated purple-to-tan spots on the blades, often with dark brown borders. Widespread infection can thin and yellow the lawn.
What causes it: Mowing too low, drought stress, and compacted soil that keeps the grass from growing vigorously.
How to treat and prevent it:
- Mow at the recommended height for your grass and keep blades sharp.
- Maintain consistent watering practices to minimize drought stress.
- Apply a fungicide such as thiophanate-methyl only if cultural fixes aren’t enough.
8. Bipolaris Leaf Spot (Bipolaris spp.)
Grasses most affected: Warm-season grasses under drought or nutrient stress.
When it strikes: Hot, dry, stressful periods — and it often lingers as the “melting out” phase that thins the lawn.
Symptoms: Purple to reddish-brown lesions on the blades, most noticeable during drought. Severe infections cause blades to yellow and die back.
What causes it: Drought stress, nutrient deficiencies, and inconsistent lawn care.
How to treat and prevent it:
- Improve lawn nutrition with balanced, slow-release fertilization.
- Keep irrigation consistent so the lawn never swings between soaked and parched.
- If cultural controls fail, treat with a fungicide such as azoxystrobin or iprodione.
9. Slime Mold
Grasses most affected: Any lawn; slime mold lives on the grass surface, not in the plant tissue.
When it strikes: After warm, wet, humid weather, especially in shady spots.
Symptoms: Crusty, powdery growth in shades of gray, yellow, or purple coating the blade surfaces. It looks alarming but does no real harm.
What causes it: Excess surface moisture, humidity, and shade — the mold simply uses the grass as a scaffold.
How to treat and prevent it:
- No chemical treatment needed. Simply brush, mow, or rinse it off with a hose.
- Improve sunlight and air circulation to keep it from returning.
10. Pythium Root Rot (Pythium spp.)
Grasses most affected: Any warm-season grass in poorly drained, waterlogged soil.
When it strikes: Warm, wet, humid weather with frequent irrigation or standing water.
Symptoms: Thinning turf and discoloration, sometimes with a greasy or matted look in the morning. Roots appear rotted and blackened.
What causes it: Poor drainage, overwatering, and shaded, humid conditions that keep the soil saturated.
How to treat and prevent it:
- Improve soil drainage and reduce how often you water — this is the foundation of control.
- Avoid mowing or walking through wet, infected areas to limit spread.
- For persistent cases, apply a fungicide containing mefenoxam.
The Real Secret: A Healthy Lawn Rarely Gets Sick
Notice how the same handful of triggers appear again and again — too much water, the wrong fertilizer at the wrong time, poor drainage, and stressed grass. Fungicides treat the symptom, but consistent lawn care removes the cause. Get these three habits right and you’ll prevent the majority of the diseases above:
Water deeply and early. One or two deep, early-morning waterings per week beat frequent shallow ones. Watering before dawn lets blades dry by mid-morning, which is the biggest single lever for fungus prevention. Follow a South Florida watering schedule rather than a fixed daily timer.
Mow high and sharp. Cutting at the correct height for your grass — and never removing more than a third of the blade at once — keeps roots deep and the plant strong. Dull blades tear grass and open wounds for fungus, so review proper mowing practices and keep the mower sharp.
Fertilize the right amount at the right time. Both too much and too little nitrogen invite disease. Overfeeding fuels large patch and gray leaf spot; underfeeding invites rust and dollar spot. Use slow-release nitrogen balanced with potassium and follow a Florida fertilizing calendar.
South Florida Lawn Disease Calendar
Because our seasons drive which diseases show up, timing your prevention matters.
Cool, dry season (roughly November–April): Watch for large patch and rust. Ease off nitrogen and overwatering; apply preventive fungicides to large-patch-prone lawns in late fall and early spring.
Warm, wet season (roughly May–October): Watch for gray leaf spot, take-all root rot, Pythium, dollar spot, and slime mold. Focus on drainage, early-morning watering, and stress reduction; limit summer nitrogen on St. Augustinegrass. Heat plus humidity drives disease pressure — the 150 rule for fungal outbreaks is a handy way to anticipate the riskiest days.
When to Call a Lawn Care Professional
You can handle many issues yourself with the steps above, but it’s worth bringing in a pro when:
- A patch keeps spreading or returns in the same spot every year despite your best efforts.
- Grass pulls up easily with rotted roots — root-rot diseases like take-all and Pythium are hard to diagnose and treat by sight alone.
- You’re not sure which disease you have, and guessing means buying the wrong fungicide.
- You want a soil test and a season-long prevention plan rather than one-off spot treatments.
Correct diagnosis matters more than most homeowners realize: the same brown patch can be caused by fungus, chinch bugs, drought, or dog urine, and the wrong treatment wastes money and time.
Frequently Asked Questions
What is the most common lawn fungus in South Florida? On St. Augustinegrass lawns — the most popular grass in Palm Beach County — large patch and gray leaf spot are the two you’re most likely to encounter, with take-all root rot a common third during the rainy season.
Will lawn fungus go away on its own? Some surface issues like slime mold clear up once conditions dry out, but most turf diseases will keep spreading until you correct the underlying cause (usually watering or fertility) and, in severe cases, apply a fungicide.
Can I fix lawn fungus without chemicals? Often, yes. Because most South Florida lawn diseases are driven by watering, mowing, drainage, and fertility, fixing those habits resolves many mild-to-moderate cases. Fungicides are best reserved for active or recurring outbreaks.
How do I tell fungus apart from insect damage or drought? Fungal patches usually have distinctive shapes (rings, spots, or defined edges) and telltale signs on the blades or roots, while insect damage (like chinch bugs) tends to appear in expanding sunny, dry areas, and drought browning follows the pattern of your irrigation coverage. When in doubt, a professional diagnosis is the fastest way to be sure.
Expert Lawn Care in Palm Beach County
If fungal diseases keep challenging your lawn — or you’d rather prevent them than chase them — Floridist is ready to help. Our Palm Beach County lawn care specialists offer comprehensive soil testing, accurate disease diagnosis, targeted fungus control and treatment plans, and proven preventive strategies to keep your grass thick, green, and healthy year-round.
Don’t let fungal problems ruin your beautiful lawn. Contact Floridist today and let our experienced team provide the professional, effective care your lawn deserves.
Sources for disease biology and management: University of Florida IFAS Extension — Large Patch (LH044) and Take-all Root Rot (LH079).