Common Lawn Diseases: Identification Guide with Pictures

Your lawn looked perfect two weeks ago. Now there are strange patches, discolored rings, powdery coatings, or dead spots spreading in ways that don’t make sense. Before you reach for any product, you need an accurate diagnosis — because treating the wrong disease is at best a waste of money and at worst harmful to your turf.

This comprehensive lawn disease identification guide covers 12+ of the most common lawn fungal diseases in North America. For each one, you’ll find exactly what it looks like, which grasses it affects, what conditions trigger it, and how to treat it effectively.

Why Lawn Diseases Occur: The Disease Triangle

Every lawn disease requires three things to occur simultaneously — this is called the disease triangle:

A susceptible host — a grass type vulnerable to the specific pathogen
A virulent pathogen — the fungus, bacteria, or other organism that causes disease
Favorable environmental conditions — temperature, humidity, and moisture levels that allow the pathogen to thrive

Remove any one side of the triangle and disease cannot occur. This is why management strategies focus on modifying environmental conditions and choosing resistant varieties — you can’t eliminate pathogens from the soil, but you can deny them the conditions they need.

General Prevention Principles

Before diving into specific diseases, these universal practices reduce disease pressure across the board:

Water in the early morning so turf dries quickly during the day
Avoid evening irrigation, which extends leaf wetness periods
Maintain proper fertility — both deficient and excessive nitrogen promote disease
Keep thatch below 1/2 inch through annual core aeration or dethatching
Mow at the correct height for your grass type and keep blades sharp
Promote good air circulation by pruning nearby shrubs and trees
Choose disease-resistant varieties when overseeding or establishing new lawns
Get a soil test to ensure pH and fertility are in the optimal range — a soil test kit is a worthwhile investment for any serious lawn owner

1. Brown Patch (Rhizoctonia solani)

Visual Description: Circular to irregular patches ranging from 6 inches to several feet in diameter. Tan or brown blighted grass with a dark, water-soaked “smoke ring” border visible in early morning dew. Individual blades show tan lesions with dark brown margins.

Grass Types Affected: Tall fescue (most susceptible), perennial ryegrass, Kentucky bluegrass, bentgrass

Conditions: Hot, humid nights above 70°F; temperatures 85–95°F during the day; excessive nitrogen; evening watering

Treatment: Azoxystrobin, propiconazole, thiophanate-methyl, chlorothalonil. Improve watering timing and reduce summer nitrogen applications.

2. Dollar Spot (Clarireedia jacksonii, formerly Sclerotinia homoeocarpa)

Visual Description: Small, bleached spots roughly the size of a silver dollar (2–6 inches) in individual lawns maintained at lower heights. At higher mow heights, spots may merge into larger irregular areas. Look for distinctive hourglass-shaped lesions on individual grass blades — tan in the center with reddish-brown margins crossing the full width of the blade. White, cobweb-like mycelium may be visible in morning dew.

Grass Types Affected: Bermudagrass, creeping bentgrass, Kentucky bluegrass, perennial ryegrass, zoysiagrass

Conditions: Low nitrogen fertility, drought stress, temperatures between 50–80°F, extended periods of leaf wetness, low-cut turf

Treatment: Chlorothalonil, myclobutanil, propiconazole. Raise nitrogen levels slightly, avoid drought stress, improve air circulation. Watch for fungicide resistance.

3. Red Thread (Laetisaria fuciformis)

Visual Description: Pink to red patches 4–24 inches in diameter with a bleached tan appearance. The definitive feature is coral-pink or red thread-like fungal structures (called stromata) extending from the tips of grass blades — easily visible to the naked eye. Patches have a reddish tint from a distance.

Grass Types Affected: Perennial ryegrass (most susceptible), Kentucky bluegrass, fine fescues, tall fescue

Conditions: Cool, moist weather (40–70°F), low nitrogen fertility, slow growth periods in spring or fall

Treatment: Primarily cultural — increase nitrogen fertility, as low-N turf is the primary trigger. Fungicides (azoxystrobin, iprodione, propiconazole) are effective but often unnecessary if fertility is corrected. Red thread rarely kills turf.

4. Pythium Blight (Pythium spp.)

Visual Description: One of the most destructive lawn diseases. Greasy, water-soaked patches that rapidly collapse and turn tan or brown. White, cottony mycelium (resembling cotton balls or spider webs) is visible on the surface of affected areas in early morning — this is the most distinctive diagnostic feature. Patches are typically small initially (1–6 inches) but can coalesce and spread dramatically overnight along drainage patterns.

Grass Types Affected: Perennial ryegrass (highly susceptible), Kentucky bluegrass, tall fescue, creeping bentgrass

Conditions: Extremely hot, humid weather (daytime temps above 90°F, nights above 70°F), poorly drained soils, excessive irrigation, dense lush growth from high nitrogen

Treatment: Act immediately — Pythium blight can destroy large areas within 24–48 hours. Mefenoxam (Subdue MAXX), fosetyl-Al, propamocarb. Improve drainage, reduce irrigation immediately, improve air circulation. Do not mow or walk through infected areas — Pythium spreads easily on equipment and shoes.

5. Necrotic Ring Spot (Ophiosphaerella korrae)

Visual Description: Circular rings of dead grass 6 inches to 3 feet or more in diameter. The center of affected circles may remain green (creating a “frog-eye” pattern), while the ring surrounding it is dead or dying. Infected roots and crowns are dark brown to black. Symptoms appear most dramatically during summer stress periods.

Grass Types Affected: Kentucky bluegrass (primary host), annual bluegrass, creeping red fescue

Conditions: Cool-season soil temperatures (50–65°F) for fungal growth, but symptoms appear during summer heat stress; excessive thatch; compacted soils; early spring nitrogen

Treatment: Fenarimol, propiconazole, azoxystrobin applied preventively in fall or early spring. Core aerate to reduce compaction and thatch. Overseed with disease-resistant Kentucky bluegrass or ryegrass varieties. Recovery is slow — full renovation is sometimes needed.

6. Fairy Ring (Marasmius oreades and others)

Visual Description: Three types exist: Type I produces rings of dead, brown grass; Type II produces rings of dark green, stimulated grass; Type III produces rings of mushrooms or puffballs only, with no visible turf effect. Rings expand outward at 6–24 inches per year and can eventually reach 50+ feet in diameter. The zone below dead rings is often a dry, waxy, water-repellent layer of fungal mycelium in the soil.

Grass Types Affected: All turfgrasses

Conditions: Buried organic matter (old tree stumps, wood debris, construction debris) serves as a food source; well-drained, sandy soils; hot, dry summers worsen symptoms

Treatment: No fungicide completely eliminates fairy ring. For Type I (dead rings): aerate aggressively and apply a surfactant to break the hydrophobic layer; water deeply. Soil fumigation is effective but expensive and disruptive. Mask symptoms with extra nitrogen and iron fertilization to even out appearance.

7. Gray Snow Mold (Typhula spp.)

Visual Description: Circular, straw-colored patches 3–12 inches in diameter revealed when snow melts in late winter/early spring. Grayish-white mycelium may be visible on the surface. Small, orange to reddish-brown sclerotia (fungal resting bodies) are visible on infected blades. Patches are generally shallow — only leaves are killed, not crowns.

Grass Types Affected: All cool-season grasses; Kentucky bluegrass, ryegrass, and fine fescues most commonly affected

Conditions: Extended snow cover (especially on unfrozen ground), cool temperatures just above freezing under the snow

Treatment: Lightly rake affected areas to improve air circulation; turf usually recovers on its own as temperatures warm. Preventive fungicides (chlorothalonil, iprodione, propiconazole) applied in late fall before first snowfall are effective in high-snow climates. Avoid excessive fall nitrogen; mow until turf stops growing.

8. Pink Snow Mold (Microdochium nivale)

Visual Description: Similar to gray snow mold but patches tend to be larger (up to 12+ inches) and show a salmon-pink to copper-pink color at the margin. Unlike gray snow mold, pink snow mold does not require snow cover — it can occur during cool, wet weather above freezing. It also attacks crowns and roots, making it more damaging.

Grass Types Affected: All cool-season grasses; perennial ryegrass and creeping bentgrass are most susceptible

Conditions: Cool temperatures (32–55°F), extended periods of leaf wetness, overcast skies, high humidity, excessive fall nitrogen

Treatment: Preventive fungicides (iprodione, fludioxonil, propiconazole) applied in late fall. Avoid excess nitrogen in late fall. Improve drainage and air circulation. Allow affected areas to dry and recover naturally as weather warms.

9. Summer Patch (Magnaporthe poae)

Visual Description: Circular, sunken patches 1–18 inches in diameter with tan or straw-colored grass. Similar to necrotic ring spot, summer patch may display a frog-eye pattern — green grass in the center surrounded by a ring of dead turf. Infected roots and stolons are dark brown to black. Patches enlarge rapidly during July and August heat.

Grass Types Affected: Kentucky bluegrass (primary host), annual bluegrass, fine fescues

Conditions: Soil temperatures above 65°F (root infection begins in spring when soils warm to 65°F, but symptoms appear in midsummer); compacted soils; excessive thatch; acidic soils (below pH 6.0)

Treatment: Fungicides must be applied preventively in spring when soil temperatures reach 65°F — by the time symptoms appear in summer, fungicides have limited effect. Use propiconazole, fenarimol, or thiophanate-methyl. Core aerate, dethatch, and raise soil pH if below 6.5. Overseed with resistant varieties.

10. Leaf Spot / Melting Out (Bipolaris sorokiniana, Drechslera spp.)

Visual Description: Two phases:

Leaf spot phase (cool, moist weather): Small, oval lesions on leaves with dark purple or brown borders and tan centers. Heavily infected blades turn yellow and die.
Melting out phase (as temperatures rise): Crown and root rot causes thinning, irregular brown patches as the disease moves from leaf tissue to the plant’s vascular system.

Grass Types Affected: Kentucky bluegrass (highly susceptible), tall fescue, bermudagrass

Conditions: Cool, wet weather in spring and fall for leaf spot phase; drought stress combined with fungal crown rot during summer triggers melting out

Treatment: Chlorothalonil, iprodione, or mancozeb for the leaf spot phase. Reduce thatch, avoid evening watering, mow at correct height. Raise mowing height during stress periods. Choose resistant Kentucky bluegrass varieties.

11. Fusarium Blight / Crown and Root Rot (Fusarium spp.)

Visual Description: Scattered, irregular patches of blighted turf 2–6 inches wide that may coalesce. Crown and root tissues show pink to salmon-colored fungal growth under close examination. Affected plants pull up easily from the soil due to root rot. Often confused with summer patch or necrotic ring spot.

Grass Types Affected: Kentucky bluegrass, perennial ryegrass, tall fescue

Conditions: Hot, dry weather following lush spring growth; heat and drought stress; compacted, poorly drained soils; excessive nitrogen in spring

Treatment: Reduce stress through proper irrigation. Apply fungicides (thiophanate-methyl, propiconazole) preventively during spring. Core aerate, dethatch, and overseed with disease-resistant grass seed blends.

12. Powdery Mildew (Blumeria graminis)

Visual Description: Distinctive white to grayish powdery coating on the upper surface of grass blades — resembles a dusting of talcum powder. Heavily infected grass turns yellow and thins out. Rarely kills turf outright but significantly weakens it over time.

Grass Types Affected: Kentucky bluegrass (most susceptible), fine fescues, bermudagrass

Conditions: Shaded areas with poor air circulation; moderate temperatures (60–72°F); overcast, humid conditions; high nitrogen fertility

Treatment: Primarily cultural — improve air circulation, reduce shade, lower nitrogen rates. Propiconazole, myclobutanil, and azoxystrobin provide control when needed. Long-term solution: overseed shaded areas with shade-tolerant grass species or transition to ground covers.

13. Rust (Puccinia spp.)

Visual Description: Orange, yellow, or reddish-brown powdery pustules on grass blades. Shoes and mower tires will be stained orange after walking through infected turf. Grass appears thin and off-color; heavily infected areas may thin significantly. Rust typically infects individual blades rather than killing plants outright.

Grass Types Affected: Kentucky bluegrass, perennial ryegrass, zoysiagrass, tall fescue

Conditions: Temperatures of 68–86°F, moderate humidity, slow turf growth due to low nitrogen or drought stress, late summer through fall

Treatment: Primarily cultural — fertilize to encourage active growth, which helps turf outpace the disease. Mow frequently to remove infected tissue. Azoxystrobin or propiconazole if chemical control is necessary. Rust rarely causes permanent damage in established lawns.

How to Use Fungicides Effectively

Choosing the right fungicide for your specific disease is critical. Using the wrong product wastes money and builds resistance. Here are the key principles:

Preventive vs. Curative Applications

Preventive: Applied before disease symptoms appear when conditions are favorable. Most effective strategy for high-value lawns.
Curative: Applied after symptoms develop. Can stop spreading but cannot revive dead tissue.

Resistance Management

Rotate between fungicide classes — never use the same mode of action consecutively
Key classes: Strobilurins (azoxystrobin), Triazoles (propiconazole, myclobutanil), Contact fungicides (chlorothalonil, mancozeb), DMI fungicides
A broad-spectrum fungicide with mixed active ingredients can simplify rotation for homeowners

Diagnosis First

Never apply a fungicide without a confident diagnosis. If in doubt, contact your local Cooperative Extension Service — many offer plant disease diagnostic labs that can identify the pathogen from a turf sample.

Quick Identification Reference

Disease	Pattern	Key Visual Clue	Peak Season
Brown Patch	Circular, large	Smoke ring in morning dew	Summer
Dollar Spot	Small spots (2–6”)	Hourglass lesions on blades	Spring/Fall
Red Thread	Pink-red patches	Red threads on blade tips	Spring/Fall
Pythium Blight	Greasy, spreading	Cotton-like mycelium	Hot summer
Necrotic Ring Spot	Rings with green center	Frog-eye pattern	Summer
Fairy Ring	Expanding rings	Mushrooms/dark green ring	Summer
Gray Snow Mold	Circular patches	Post-snowmelt, gray mycelium	Late winter
Pink Snow Mold	Circular patches	Salmon-pink border	Winter/Spring
Summer Patch	Sunken circles	Frog-eye, black roots	Midsummer
Leaf Spot	Scattered lesions	Purple-bordered spots	Spring/Fall
Powdery Mildew	Shaded areas	White powder on blades	Spring/Fall
Rust	Widespread thin areas	Orange powder on shoes	Late summer

Conclusion

Accurate disease identification is the foundation of effective lawn disease management. Misidentifying a disease — or treating a disease problem with a pesticide when the real issue is drought stress, soil compaction, or incorrect mowing — is one of the most common and costly mistakes homeowners make.

Use this guide as your starting point. Look at the pattern, the specific symptoms on individual blades, the time of year, your recent weather, and your recent lawn care practices. When in doubt, collect a sample from the edge of an active patch (where healthy and diseased tissue meet) and send it to a diagnostic lab.

Invest in the right tools — a soil test kit, quality disease-resistant grass seed, and a reliable broad-spectrum fungicide for when cultural controls aren’t enough — and you’ll be well-equipped to handle whatever your lawn throws at you.