Lawn Fungus Identification and Treatment: A Homeowner's Guide

Waking up to mysterious brown or yellow patches on your lawn is frustrating, especially when you have been watering and fertilizing faithfully. In many cases, the culprit is a fungal disease. Lawn fungus thrives in specific weather conditions, and understanding those conditions is the key to both treatment and prevention.

This guide covers the most common lawn diseases homeowners encounter, how to identify them, and which fungicide products will stop them.

Brown Patch (Rhizoctonia solani)

Brown patch is the most widespread lawn disease in the United States. It affects virtually all grass types but is most common in tall fescue, perennial ryegrass, and St. Augustine grass. It appears as circular patches of brown, thinned turf ranging from a few inches to several feet in diameter. On close inspection, you may see a dark, smoky ring at the outer edge of the patch, especially in the early morning when dew is present.

• Conditions that trigger it: Nighttime temperatures above 68 degrees Fahrenheit combined with humidity above 90 percent for 48 or more consecutive hours.
• When it appears: Late spring through early fall, with peak activity during hot, humid weather.
• Most susceptible grasses: Tall fescue, perennial ryegrass, St. Augustine.

Dollar Spot (Clarireedia jacksonii)

Dollar spot gets its name from the small, silver-dollar-sized bleached spots it creates in the turf. Individual spots are typically 2 to 6 inches in diameter. In the morning, you may see fine, cobweb-like mycelium on the grass blades when dew is present. On closer inspection, affected grass blades show characteristic hourglass-shaped tan lesions with reddish-brown borders.

• Conditions that trigger it: Warm days (60 to 85 degrees Fahrenheit), cool nights, and prolonged leaf wetness. Low nitrogen fertility increases susceptibility.
• When it appears: Late spring through fall. Often the first disease to appear each season.
• Most susceptible grasses: Bermuda, zoysia, bluegrass, fescue. Nearly all grasses can be affected.

Dollar spot is often a sign of low nitrogen. A timely fertilizer application can help your lawn outgrow the disease while you treat it with fungicide.

Gray Leaf Spot (Pyricularia grisea)

Gray leaf spot primarily affects St. Augustine grass and perennial ryegrass. It causes elongated gray to brown lesions on grass blades, often with a dark brown border. In severe cases, it can kill large areas of turf rapidly. Gray leaf spot is most active during hot, humid weather with frequent rainfall or overhead irrigation.

Pythium Blight

Pythium blight is one of the most destructive lawn diseases. It can kill turf overnight in hot, humid conditions. It appears as small, irregularly shaped patches of dark, water-soaked grass that feels greasy to the touch. In the morning, you may see white, cottony mycelium on the affected areas. Pythium spreads rapidly and follows drainage patterns, often appearing in streaks across the lawn.

Recommended Fungicide Products

Heritage (Azoxystrobin) — Broad-Spectrum Control

Heritage is one of the most widely used fungicides in professional turf management, and for good reason. Its active ingredient, azoxystrobin, provides broad-spectrum control of most major lawn diseases including brown patch, dollar spot, gray leaf spot, and pythium. It works both preventively and curatively. Heritage offers systemic protection, meaning it is absorbed by the grass plant and protects from within.

Banner Maxx II (Propiconazole 14.3) — Fast-Acting

Banner Maxx II is a go-to fungicide when you need fast results. Propiconazole is a systemic fungicide that is absorbed quickly and begins working within hours. It is particularly effective against dollar spot, brown patch, and many other common diseases. It has both preventive and curative properties, making it useful when disease is already present.

Eagle 20EW — Economical and Effective

Eagle 20EW contains myclobutanil, a systemic fungicide that provides excellent control of dollar spot, brown patch, and several other diseases. It is one of the more affordable fungicide options while still delivering professional-grade results. Apply at the first sign of disease or as a preventive when conditions favor fungal growth.

When to Apply Fungicide

Timing is critical with fungicides. Preventive applications are always more effective than curative treatments. Apply fungicide before disease appears when weather conditions favor fungal growth.

• Preventive: Apply when weather forecasts show extended periods of high humidity and warm nighttime temperatures. TurfTechAdvisor's Disease Risk predictor monitors these conditions for your location.
• Curative: Apply at the first sign of disease symptoms. The earlier you catch it, the less damage occurs.
• Repeat applications: Most fungicides provide 14 to 28 days of protection. Reapply according to the label if disease pressure continues.
• Rotate products: Alternate between fungicides with different modes of action (check the FRAC group on the label) to prevent the fungus from developing resistance.

Cultural Practices That Reduce Disease

Fungicides are important, but they work best when combined with good cultural practices that make your lawn less hospitable to fungal diseases.

• Water early in the morning so the grass dries quickly. Evening watering extends leaf wetness overnight, which is exactly what fungi need.
• Avoid over-fertilizing with nitrogen, especially in summer. Excess nitrogen creates lush, soft growth that is highly susceptible to disease.
• Improve air circulation by pruning overhanging branches and shrubs that trap humidity around the turf.
• Mow regularly at the proper height. Do not remove more than one-third of the grass blade at a time.
• Reduce thatch buildup. A thick thatch layer traps moisture and creates ideal conditions for fungal growth.

Use TurfTechAdvisor's Disease Risk predictor to get daily risk assessments for dollar spot and other diseases based on your local weather conditions. This takes the guesswork out of when to apply preventive fungicide.

This article is for general educational purposes only. Accurate disease diagnosis may require laboratory testing or professional assessment. Always read and follow the complete product label before applying any fungicide. Contact your local Cooperative Extension office for assistance identifying lawn diseases and for region-specific treatment recommendations.