Brown patch is a common turfgrass disease that can rapidly damage an otherwise healthy lawn. This condition is particularly prevalent in cool-season grasses like tall fescue and perennial ryegrass during the warmer months. The appearance of large, circular areas of discolored, dying grass indicates a complex interaction between a specific pathogen, the environment, and how the lawn is maintained. Understanding these factors is the first step toward preventing the characteristic damage from occurring.
The Pathogenic Culprit
The direct cause of brown patch is a soil-borne fungus known as Rhizoctonia solani. This organism is a natural inhabitant of nearly all turfgrass soils, existing harmlessly as dormant mycelium or resting structures within the thatch layer and soil. The fungus only becomes aggressive and begins to infect the grass when environmental conditions are exactly right for its proliferation.
Once activated, Rhizoctonia solani spreads by producing mycelium, which are fine, thread-like structures that grow across the grass blades. This fungus is primarily a foliar disease, meaning it attacks the leaf and stem tissues of the plant rather than destroying the roots or crown. The damage results in light brown circular patches ranging from a few inches to several feet in diameter.
In the early morning, especially when the grass is covered with dew, a narrow, dark, purplish-gray border, sometimes called a “smoke ring,” may be visible at the edge of the patch. This ring represents the actively spreading margin of the fungus. Although the leaves within the patch are killed, the grass crown is typically unaffected, allowing the plant to recover when conditions become less favorable.
Environmental Conditions That Encourage Disease
The fungus Rhizoctonia solani requires a specific set of environmental circumstances to shift from a dormant state to an active, disease-causing agent. The most significant trigger is a combination of heat and prolonged moisture on the grass surface. This disease is most aggressive in mid-summer when temperatures create an incubation environment for the pathogen.
Brown patch thrives when daytime air temperatures consistently reach 80°F or above, but the nighttime temperature is an even more critical factor. The fungus proliferates when overnight temperatures remain elevated, generally above 68°F. These warm nights prevent the grass from recovering from heat stress and keep the thatch layer warm, creating optimal fungal growth conditions.
The second, equally necessary condition is extended leaf wetness, often lasting 10 to 12 hours or more, coupled with high humidity. This moisture can come from heavy rainfall, dew, or irrigation. Poor air circulation, often due to surrounding shrubs or trees, can delay the drying of the grass blades, thus extending the leaf wetness period and promoting fungal spread.
Poorly drained or compacted soils also contribute significantly to the problem by holding excess moisture near the surface for longer periods. A thick layer of thatch, which is the layer of dead and living organic matter between the green grass blades and the soil, further exacerbates the situation. Thatch acts like a sponge, retaining moisture and providing the fungus with a hospitable environment to survive and germinate.
Maintenance Errors That Exacerbate Brown Patch
While environmental conditions provide the opportunity, certain turf maintenance practices can stress the grass and inadvertently provide the fungus with the perfect food source, making an outbreak more severe. One common mistake is the misuse of nitrogen fertilizer during the summer months. Excessive nitrogen application stimulates a flush of tender, lush new growth, which is highly susceptible to infection by Rhizoctonia solani.
This over-fertilization, especially with fast-release nitrogen sources, provides the fungus with readily available nutrients to fuel its rapid spread across the lawn. Lawns maintained with more moderate amounts of nitrogen or slow-release forms tend to show less disease severity.
The timing of watering is another factor that encourages the disease. Watering the lawn in the late afternoon or evening extends the period of leaf wetness overnight, which is exactly what the fungus needs to thrive. The water sits on the grass blades throughout the night, creating a saturated environment conducive to mycelial growth.
Improper mowing also stresses the grass, increasing its vulnerability to the pathogen. Scalping, or cutting the grass too short, removes too much leaf tissue, weakening the plant. Dull mower blades tear the grass instead of cleanly cutting it, creating open wounds that provide easy entry points for the fungus. These maintenance errors do not cause the fungus itself, but they create a highly vulnerable plant population in an environment where the fungus is already active.
Preventing Brown Patch By Managing Contributing Factors
Preventing brown patch involves directly mitigating the environmental and cultural factors that favor the fungus’s activity. The most effective change is adjusting the timing of irrigation to reduce the duration of leaf wetness. Watering should be performed in the early morning, ideally between 4:00 a.m. and 10:00 a.m., allowing the grass blades to dry completely before nightfall.
Improving air circulation and drainage is also essential to quickly dry the turf surface and eliminate pockets of high humidity. This can involve pruning surrounding shrubs or trees to increase light penetration and airflow, as well as aerating the lawn in the fall to alleviate soil compaction. Removing excessive thatch, which should be kept below a half-inch, also reduces the moisture-retaining layer where the fungus survives.
Fertilization practices need modification, focusing on moderate nitrogen application and avoiding high-nitrogen feedings during the hot summer months when the fungus is most active. Using slow-release nitrogen forms during the summer or reserving heavier nitrogen applications for the cooler growing seasons can help manage the growth rate and reduce susceptibility. Finally, maintaining an appropriate mowing height for the specific turfgrass species minimizes stress, which builds the plant’s natural defense against the pathogen.