Pathogen: Pythium aphanidermatum, Pythium graminicola and other Pythium species (water molds)
Also called: grease spot, cottony blight, Pythium foliar blight, Pythium, spot blight
Pythium Blight
The hot-night grease spot
Pythium Blight At-a-Glance
Grass Type Affected
Cool-Season Grasses
Recovers on Its Own?
Usually Not
Is Spraying Worth It?
Rarely Worth It
Disease Type
Water Mold
Active Season
Late Spring & Early Summer
Mid to Late Summer
Fall
Damage Potential
Can Be Fatal
Pythium Blight Quick Answers
What Pythium Blight Looks Like
From Standing Height
It starts as small, round spots 2 to 6 inches across that look dark and wet, as if someone spilled oil on the lawn. In hot, humid weather they grow and merge within days into irregular patches a foot or more across, often with a gray, orange or reddish-brown edge. On slopes and in low ground the damage runs in streaks along the way water drains, and sometimes in straight lines along mowing paths. Within a week the grass fades to straw, lies flat and matted, and can sink slightly below the healthy turf.
Up Close: Blades, Crowns and Roots
Unlike most lawn diseases, Pythium makes no spots or lesions on the blade. The whole leaf turns dark green to nearly black and looks water-soaked, then fades to reddish-brown and finally straw. Active leaves feel soft, slimy and greasy, and they twist, lie flat and stick together in clumps. There are no dark-bordered spots, no hourglass bands and no tip-only browning; the blade collapses from tip to base all at once.
The leaf bases soften and rot. In perennial ryegrass the rot moves into the crown, the growing point at the base of the plant, and kills it, so the plant cannot regrow. In tall fescue and Kentucky bluegrass the leaves may die back to the crown, but the crown and roots often survive. Roots are usually not involved. If a thinning, yellowing area in soggy soil has dark, rotted roots instead, that points to Pythium root rot, a separate, slower disease.
Signs of the Fungus
On humid mornings while dew is still on the lawn, look for fluffy white to gray threads, like cotton or cobwebs, tangled through the grass at the edge of a patch. This is the water mold itself growing out of the leaves. It vanishes within an hour or two as the sun dries the grass, so check at sunrise. The threads bind the blades together into matted, greasy clumps.
Could It Be Something Else?
Common Lookalikes
Brown patch strikes in the same hot, humid weather and also makes roundish brown patches, often on the same tall fescue lawns. The difference is on the blade: brown patch leaves tan spots with dark brown borders, while Pythium turns the whole blade dark and greasy with no spots at all.
Dollar spot also shows white, cobwebby threads on dewy mornings, which is why people mix the two up. Dollar spot patches stay small and dry, and its blades carry bleached, hourglass-shaped bands with tan to reddish-brown edges.
Drought and heat stress brown the lawn in the same midsummer weeks. But drought-stressed grass is dry, stiff and folded, it shows up first on high, sunny ground rather than in low wet spots, and it never shows morning webbing.
Field Checks Before You Treat
- Check at sunrise. Look at the edge of a patch while dew is still on the grass. Cottony white threads with greasy, dark blades underneath point to Pythium. Threads over small, dry patches with banded blades point to dollar spot.
- Feel the blades. Rub a few blades from the patch edge between your fingers. Soft, slimy, greasy leaves that stick together mean Pythium. Dry, crisp or papery leaves mean drought, brown patch or dollar spot.
- Look along the blade. Hold a few leaves from the edge of the patch up to the light. Tan spots with dark borders mean brown patch; hourglass-shaped bleached bands mean dollar spot; a whole blade that is evenly dark, wet-looking and collapsed, with no spot or border, means Pythium.
- Trace the pattern. Stand back and follow the damage. Patches that start in low spots and run downhill along drainage paths, or that form straight streaks along your mowing lines, point to Pythium. Damage on high, dry ground points to drought.
- Check the weather. Pythium appears after hot, humid nights above about 65°F, often right after an evening thunderstorm. Damage after a long dry spell is far more likely drought.
Grasses at Risk
Host Grasses
Which Grasses Suffer Most
Perennial ryegrass is the most vulnerable lawn grass, because the water mold reaches its crowns and kills plants outright. Annual bluegrass and rough bluegrass, common weedy grasses in lawns, are just as vulnerable, and so is creeping bentgrass. Tall fescue and Kentucky bluegrass are far less likely to be badly hurt. Their leaves can blight in softball-sized patches during a long hot, wet spell, but their crowns and roots often survive and regrow. Warm-season grasses aren’t documented hosts in home lawns. Purdue notes no proven differences between cultivars within a species, so NTEP ratings won’t turn up a resistant ryegrass. Where the disease recurs, switching species to tall fescue or Kentucky bluegrass is what works.
Why It Shows Up
Conditions That Trigger It
Pythium blight is a hot-night, wet-leaf disease. Risk starts when nighttime air temperatures stay above about 65°F and the grass stays wet for 12 to 14 hours several nights in a row. Outbreaks turn severe when nights stay at 68–70°F or warmer while daytime air temperatures run 85–95°F. Humidity matters because it keeps the leaves wet: this water mold needs a film of water on the grass to swim, infect and grow.
What tips a lawn toward an outbreak:
- Evening rain or watering. Water that lands late in the day keeps the grass wet all night, which is exactly the long wet period the water mold needs. A late-afternoon thunderstorm on a hot night is the classic trigger.
- Low spots and poor drainage. Standing water lets the swimming spores travel, so the disease starts where water collects and spreads along the paths it drains.
- Too much summer nitrogen. Heavy feeding in hot weather grows soft, lush leaves that the water mold breaks down easily.
- Compacted soil and thick thatch. Both hold water at the surface instead of letting it soak in.
- Shade and still air. Trees, fences and buildings slow morning drying, so dew lingers for hours longer.
- New seedings. Young grass has soft tissue and no mature crowns, so it dies faster than an established lawn.
How It Survives and Spreads
Pythium survives from one summer to the next in the soil and thatch as thick-walled resting spores that can last for years. That is why the same wet spot tends to flare up summer after summer. When the soil warms and stays wet, these resting spores sprout or release tiny swimming spores that move through films of water and infect the grass directly.
Once inside, the water mold breaks down the leaf tissue, which gives the blades their dark, greasy look. On humid nights it grows out of the leaves as cottony threads that reach across to the next blades. It moves farther in two main ways. Surface water carries spores downhill along drainage paths. Mower wheels, mower blades and shoes drag wet, infected grass through healthy lawn, which is why damage sometimes appears as straight streaks along mowing lines.
Where It Hits Hardest: Regional Notes
Pythium blight is a problem where summer nights stay hot and humid, mainly from June through August across the transition zone, the Mid-Atlantic, the lower Midwest and the Northeast, where tall fescue, Kentucky bluegrass and perennial ryegrass lawns meet heat waves and evening storms. Warm-season lawns in the South aren’t documented hosts; the Southern risk is perennial ryegrass overseeded into bermudagrass for winter color, if it is overwatered or overfertilized during warm, humid spells. It’s rare in the dry West, where low humidity dries grass quickly, and in the far North, where nights seldom stay warm long enough.
Pythium Blight Diagnostic Profile
Check what you see against the full set of signs. The more that match, the more likely this is your problem.
What You’ll See
Symptom Color
Tan or Straw
Brown
Red, Pink or Purple
Orange or Rust
White or Gray
Black or Dark
Visible Signs
Cottony White Webbing
Lawn Pattern
Circular Patches
Irregular Patches
Small Spots
Streaks or Stripes
Patch Size
2 to 6 Inches
6 to 12 Inches
1 to 3 Feet
Over 3 Feet
Blade Symptoms
Rotted Leaf Base or Crown
When and Where
Weather Triggers
Warm & Humid
Hot & Humid
Long Leaf Wetness (Dew or Rain)
Wet or Poorly Drained Soil
Where in the Lawn
Shade
Low or Wet Spots
Compacted or High-Traffic Areas
New Seedings
Controlling Pythium Blight
- Fix the water first. Regrade low spots, point downspouts away from the lawn, and add drainage where water pools. Because the water mold travels and infects through standing water, NC State notes that fixing drainage usually eliminates the disease.
- Water before sunrise, and only when needed. The grass is already wet from dew at that hour, so morning watering adds no extra wet hours, and the rising sun dries the leaves. Evening watering stretches the wet period past the 12 to 14 hours the water mold needs. Water deeply and less often rather than a little every day.
- Don’t water or mow an active patch. When cottony threads are showing, both moves spread the water mold to healthy grass. Wait until the lawn is dry.
- Go easy on summer nitrogen. During hot, humid weather, give cool-season lawns no more than 0.25 lb of nitrogen per 1,000 sq ft per feeding, and do your main feeding in fall and early spring. Lush summer growth is soft and easy for the water mold to break down.
- Bag and clean up. Bag clippings from infected areas and send them out with yard waste or the trash. Don’t compost them, because the resting spores survive in soil and debris. Hose off the mower deck and wheels before mowing healthy areas.
- Relieve compaction and thatch. Core aerate in fall and keep thatch under about ½ inch, so water soaks in instead of sitting at the surface.
- Let in sun and air. Prune low branches and thin dense shrubs near trouble spots so morning sun and breezes dry the grass sooner.
- Seed at the right time. Don’t seed or overseed in summer heat, when seedlings rot quickly. Wait for late summer or early fall, once nights drop reliably below about 65°F.
- Switch grasses where it keeps coming back. In areas that flare up every summer, overseed with tall fescue or Kentucky bluegrass instead of perennial ryegrass.
Spraying is rarely worth it on an established home lawn: fixing drainage and watering timing usually stops Pythium blight, and extension specialists treat fungicides as a last resort.
Pythium is a water mold, not a true fungus, so most lawn fungicides don’t control it. Common lawn ingredients such as propiconazole, myclobutanil, thiophanate-methyl and chlorothalonil don’t work on water molds, so a product bought for brown patch can fail completely here. Use only a product whose label lists Pythium blight. The active ingredients that do control it, with NC State’s efficacy ratings:
| Active ingredient | FRAC group | Effectiveness | Cautions |
|---|---|---|---|
| cyazofamid | 21 | Excellent, even at high pressure | High resistance risk; rotate groups |
| fluopicolide + propamocarb | 43 + 28 | Excellent | Medium resistance risk |
| mefenoxam | 4 | Excellent | High resistance risk; rotate groups |
| picarbutrazox | U17 | Good, even at high pressure | High resistance risk |
| fosetyl-Al | 33 | Good, even at high pressure | Low resistance risk; works best as a preventive |
| azoxystrobin | 11 | Good at moderate pressure | High resistance risk; weaker under heavy disease pressure |
Spraying can make sense in two situations: protecting a new seeding or new sod during a stretch of hot nights, and protecting a perennial ryegrass lawn that blights every summer while a drainage fix is still pending. Treatment works best as a preventive. Start when nights stay above 65°F and the grass stays wet overnight, then repeat every 14 to 21 days as the label directs. Use lower label rates to prevent and higher rates once disease is active. Spray in 2 to 5 gallons of water per 1,000 sq ft to coat the leaves. Rotate FRAC groups between applications, because most of these ingredients carry a high resistance risk.
Many of these products are sold mainly to lawn professionals, and some potassium phosphite and propamocarb-only products are labeled for golf and commercial turf only. If you need to spray, Purdue recommends hiring a licensed lawn care service, because choosing the right product and timing it correctly matter more here than with most lawn diseases.
Recovery and Repair
Tall fescue and Kentucky bluegrass patches often fill back in over several weeks once nights cool, as long as the crowns survived. Give them time before reseeding: new green shoots at the base of the plants by early fall mean they’re coming back. Perennial ryegrass patches are dead and need reseeding.
Don’t reseed in summer heat. Seedlings sown into warm, wet soil rot almost immediately from the same water mold. Wait for late summer or early fall, when nights drop reliably below about 65°F, and then:
- Clear the debris. Rake out the matted dead grass so seed can reach the soil.
- Loosen the soil. Core aerate or slit-seed for good seed-to-soil contact; this also relieves the compaction that helped the disease.
- Seed a tougher grass. Use tall fescue or Kentucky bluegrass, especially in spots that blight every summer.
- Water lightly, then back off. Keep the seedbed moist until the seed sprouts, watering in the morning, then switch to deep, less frequent morning watering as soon as the grass is established.
Hold off on nitrogen for the surviving lawn until the disease has stopped and the weather has cooled, then feed normally in fall to thicken the stand.
When to Call a Pro
Send a sample to your extension diagnostic lab when dark, greasy patches appear or grow overnight in hot, humid weather and the field checks don’t settle it. Getting the diagnosis right matters because Pythium blight, brown patch and gray leaf spot need completely different fungicides. Cut a plug about 4 to 6 inches across, with soil and roots, from the expanding edge so it holds half sick and half healthy grass. Wrap it in dry paper towels, never a sealed plastic bag, and ship it early in the week. Call a licensed lawn care service if a ryegrass lawn or new seeding is collapsing and spraying is on the table.
Frequently Asked Questions
These answers are synthesized from plant pathology research and diagnostic guidance published by university extension services and plant disease clinics.
Because the storm handed it a long, warm, wet night. Rain late in the day soaks the grass right before dark, and on a night that stays above about 65°F the leaves never dry. That gives the water mold the 12 or more wet hours it needs to swim, infect and spread, so patches can appear by morning.
Your mower probably spread it. Mowing wet grass while the water mold is active picks up infected leaves and threads on the wheels and blades and drops them along the mowing path. Mow only when the grass is dry, and hose off the mower before mowing healthy areas.
Probably not. Most lawn fungicides are made for true fungi and don’t affect water molds, so a product bought for brown patch or leaf spot can do nothing here. Check the label: it must list Pythium blight, and the active-ingredient panel should show an ingredient such as cyazofamid, mefenoxam, fluopicolide with propamocarb, or fosetyl-Al.
Only if the same areas blight summer after summer. No ryegrass variety has proven resistance, so switching grasses works better than switching varieties. Overseeding with tall fescue or Kentucky bluegrass in early fall gives you a lawn whose crowns usually survive an outbreak, and fixing drainage at the same time removes the main cause.
Scientific Authority
This profile is built on plant pathology research and diagnostic guidance from university extension services and plant disease clinics. We prioritize accurate identification and cultural control first, recommending fungicides only where research shows they are justified, and naming them by active ingredient rather than brand.
Primary Resources
- NC State Extension — Pythium Blight in Turf
- NC State Extension — Disease Management in Landscape Turf
- Purdue Extension — Turfgrass Disease Profiles: Pythium Blight (BP-109-W)
- Purdue Extension — Common Diseases on Cool-Season Lawns (BP-218-W)
- University of Missouri Extension — Identification and Management of Turfgrass Diseases: Pythium Foliar Blight (IPM1029)
- University of California IPM — Pythium Blight (Grease Spot)
- Clemson Cooperative Extension HGIC — Overseeding with Ryegrass