Brown patch is a widespread turf disease that commonly presents as roughly circular areas of dead or discolored grass. These patches can range from a few inches to several feet in diameter, often appearing suddenly during warm, humid weather. The infected grass blades turn a light brown or tan color. Sometimes, a dark purplish-gray ring, known as a smoke ring, is visible on the outer edge in the early morning.
The Fungal Pathogen
The organism responsible for brown patch is the soil-borne fungus Rhizoctonia solani. This fungus survives in the soil and thatch layer, often remaining dormant for long periods as resting structures called sclerotia. It primarily feeds on dead organic matter, but it can quickly switch to a parasitic phase when conditions favor infection.
When the fungus becomes active, it produces mycelium that grows across the surface of the grass blades, leading to infection. Rhizoctonia solani attacks both cool-season and warm-season varieties. Highly susceptible species include perennial ryegrass, tall fescue, and bentgrass, though warm-season grasses like St. Augustinegrass and zoysiagrass are also commonly affected.
Climate and Soil Conditions Required for Activation
The activation of Rhizoctonia solani from a dormant state requires a specific set of environmental conditions, primarily heat and prolonged moisture.
Brown patch development is most severe when air temperatures are consistently high. Daytime temperatures should be in the range of 75°F to 85°F, and nighttime temperatures must remain elevated above 60°F. This warm temperature window allows the fungus to rapidly produce the mycelium necessary for infection.
The second requirement is extended leaf wetness and high humidity. The fungus needs the grass blades to be continuously wet for at least 10 to 12 hours for successful infection to occur. This prolonged moisture is caused by heavy rainfall, morning dew that does not quickly evaporate, or high relative humidity. Poor soil drainage further contributes by keeping the root zone saturated, which raises the humidity in the turf canopy.
Lawn Care Mistakes that Encourage Disease
Certain lawn management practices can inadvertently create the ideal environment for the Rhizoctonia solani fungus to thrive. One common error is improper watering, especially timing irrigation late in the day or evening. Watering after 7:00 PM allows water to sit on the grass blades throughout the night, significantly extending leaf wetness and promoting fungal activity.
Applying excessive amounts of nitrogen fertilizer is another factor that promotes brown patch during hot, humid conditions. High levels of nitrogen encourage a flush of succulent, lush leaf growth. This growth is biologically weaker and more susceptible to infection, and its density traps moisture in the turf canopy, prolonging leaf wetness.
The condition of the soil and the thatch layer also plays a role in disease severity. A thick layer of thatch—undecomposed organic matter between the soil and the grass blades—acts like a sponge, holding moisture near the base of the plant. Compacted soil similarly impedes water infiltration and air exchange, leading to poor drainage and waterlogged conditions that encourage fungal growth.
Cutting the grass too short, often called scalping, stresses the turf and reduces its energy reserves. This practice weakens the host plant, making it less able to resist the pathogen when temperature and moisture conditions are favorable. Correcting these cultural practices can reduce the likelihood of a brown patch outbreak.