Brown Patch Fungus: How to Treat and Prevent It

What Is Brown Patch Fungus?

Brown patch is one of the most common and destructive lawn diseases in the United States. Caused by the soil-borne fungus Rhizoctonia solani, it affects virtually all cool-season and warm-season grass types. The disease thrives in hot, humid conditions and can destroy large areas of turf in just a few days if left untreated.

Understanding how to identify, treat, and prevent brown patch is essential for any homeowner who wants a healthy, attractive lawn. Left unchecked, brown patch can require complete lawn renovation — a costly and time-consuming process.

Identifying Brown Patch on Your Lawn

Look for these telltale signs:

• Circular patches: Brown or tan areas ranging from 6 inches to several feet in diameter
• Smoke-colored rings: A dark gray or purple "smoke ring" bordering the patch in early morning dew
• Irregular shapes: As patches grow, they merge into irregular brown areas
• Leaf lesions: Individual grass blades show tan spots with dark brown borders
• Rotted sheaths: The base of the grass blade (where it meets the stem) appears soft and water-soaked
• Pull test: Infected grass pulls out easily because the root crown has rotted

Conditions That Favor Brown Patch

Brown patch develops when these conditions align:

• Temperature: Daytime highs of 75–85°F (24–29°C) with nighttime lows above 60°F (15°C)
• Humidity: Extended periods of leaf wetness (12+ hours) from rain, dew, or irrigation
• Excess nitrogen: High nitrogen fertilizer applications in warm weather promote succulent growth that's vulnerable to infection
• Poor drainage: Waterlogged soil creates ideal conditions for the fungus
• Thick thatch: Thatch retains moisture and harbors fungal spores
• Tight soil: Compacted soil reduces root vigor and makes grass more susceptible

Grass Types Most Susceptible to Brown Patch

All grass types can be affected, but susceptibility varies:

• Highly susceptible: Perennial ryegrass, tall fescue, Kentucky bluegrass, bentgrass
• Moderately susceptible: Bermuda grass, zoysia grass, St. Augustine grass
• More resistant: Buffalograss, fine fescue (in suitable climates)

If you live in a brown patch-prone area, consider planting more resistant grass varieties when renovating or establishing a new lawn.

How Brown Patch Spreads

The fungus spreads through several pathways:

• Spore dispersal: Wind and water carry spores to healthy grass
• Mowing equipment: Fungal spores stick to mower blades and decks, spreading from infected to healthy areas
• Foot traffic: Shoes and pet paws carry contaminated soil
• Irrigation water: Sprinkler systems can distribute spores across the lawn
• Contaminated sod or seed: Infected planting material introduces the disease to new areas

Once established in the soil, R. solani can survive for years as resting structures (sclerotia), making prevention the most effective long-term strategy.

Treating Active Brown Patch Infections

When you spot brown patch, take immediate action:

1. Stop watering: Allow the lawn surface to dry. Water deeply but infrequently in the early morning only.
2. Avoid nitrogen fertilizer: Switch to slow-release or organic fertilizers until the disease is controlled.
3. Mow carefully: Mow infected areas last to avoid spreading spores. Clean mower blades afterward with a 10% bleach solution.
4. Apply fungicide: Use a systemic fungicide containing azoxystrobin, propiconazole, or myclobutanil. Follow label directions carefully.
5. Aerate: Once the disease is under control, aerate to improve soil drainage and root health.
6. Overseed: In severely damaged areas, overseed with resistant grass varieties after the fungus is eliminated.

Preventing Brown Patch: Cultural Practices

The best defense is good lawn management:

• Water correctly: Water deeply (1 inch per week) in the early morning (5–9 AM). Avoid evening watering.
• Mow at the right height: Never cut more than one-third of the grass blade. Keep cool-season grasses at 3–4 inches.
• Remove clippings during disease season: Infected clippings spread spores
• Dethatch annually: Keep thatch below 1/2 inch
• Aerate every 1–2 years: Improves drainage and reduces fungal habitat
• Use balanced fertilizer: Avoid quick-release nitrogen in summer. Use slow-release or organic alternatives.
• Improve air circulation: Trim trees and shrubs to reduce shade and increase airflow across the lawn

Fungicides for Brown Patch Control

For severe or recurring infections, fungicides may be necessary:

Fungicide Type	Active Ingredient	Best For
Systemic	Azoxystrobin (Heritage)	Curative and preventive
Systemic	Propiconazole (Banner MAXX)	Fast-acting treatment
Systemic	Myclobutanil (Eagle)	Preventive applications
Contact	Chlorothalonil (Daconil)	Surface protection

Apply fungicides according to label instructions. Most require reapplication every 14–28 days during disease-favorable conditions. Rotate between different modes of action to prevent fungal resistance.

Brown Patch vs. Similar Lawn Diseases

Brown patch is often confused with other lawn diseases:

• Summer patch: Similar circular patches but occurs on compacted soils and affects root systems. No smoke ring.
• Dollar spot: Small silver-dollar-sized spots, not large circular patches. Appears on nitrogen-deficient lawns.
• Pythium blight: Greasy, water-soaked appearance. Spreads rapidly in very hot, wet conditions. Leaves feel slimy.
• Fairy ring: Forms perfect circles or arcs. Often accompanied by mushrooms. Caused by different fungi in the soil.

If you're unsure which disease is affecting your lawn, send a sample to your local extension office for accurate diagnosis.

Frequently Asked Questions About Brown Patch