Brown patch, caused by the soil-borne fungus Rhizoctonia solani, is a frequent lawn disease impacting many turfgrass varieties, especially cool-season grasses like tall fescue and perennial ryegrass. The fungus thrives in specific environmental conditions, leading to the rapid appearance of unsightly brown areas. Managing an outbreak requires immediate cultural modifications to stop the spread and, in severe cases, the selective application of chemical treatments. Long-term success relies on proactive maintenance that improves turf health and creates an environment less welcoming to the fungus.
Identifying Brown Patch and Its Triggers
Accurate diagnosis begins with recognizing the distinct visual symptoms. Brown patch typically appears as circular, sunken areas of damaged grass ranging from a few inches to over ten feet in diameter. The grass blades within the patch often appear rotted at the base, but the tips may remain intact, resulting in a distinctive, thinned appearance.
A telltale sign, particularly on closely mown turf, is a dark, grayish-black border known as a “smoke ring” visible around the edge of the patch in the early morning dew. The fungus is a foliar pathogen, attacking the grass blades without necessarily killing the plant’s crown or roots. This allows the turf to often recover once conditions improve.
The fungus becomes active when environmental conditions favor its growth. Brown patch thrives during periods of high humidity and prolonged leaf wetness, often requiring grass blades to remain wet for ten hours or more for several consecutive days. This occurs when nighttime temperatures remain above 68°F and daytime temperatures consistently exceed 80°F. Factors like poor soil drainage, excessive thatch buildup, and limited air circulation contribute to the moist conditions the fungus requires.
Immediate Cultural Control Methods
When brown patch is detected, the most effective initial response involves immediately altering the turf’s environment to make it inhospitable for the fungus. Watering adjustments are a primary focus; water deeply but infrequently to encourage deep root growth. Schedule irrigation for the early morning hours (ideally 6 a.m. to 10 a.m.) so the grass blades can dry completely before evening. Watering later in the day extends leaf wetness, which directly fuels the growth of Rhizoctonia solani.
Modifying mowing and fertilization practices also helps manage the disease. Avoid mowing when the grass is wet, as this spreads fungal spores across the turf. Clean mower blades after cutting an infected area to prevent contamination of healthy sections. During active disease periods, avoid applying high-nitrogen fertilizers, which promote lush growth highly susceptible to the fungus. Instead, use a slow-release, low-nitrogen fertilizer or temporarily suspend feeding until the outbreak is controlled.
When to Use Chemical Treatments
Fungicides are generally reserved for severe, recurring brown patch infections or for lawns with a low appearance threshold. Chemical control must be used alongside cultural methods, as fungicides alone cannot overcome poor lawn care practices. Homeowners should look for products containing active ingredients like propiconazole or azoxystrobin, which are effective against the Rhizoctonia fungus.
Fungicides are categorized by their mode of action, such as systemic or contact. Rotating these chemical classes is important to prevent the fungus from developing resistance. Systemic fungicides are absorbed by the grass and move through the plant tissues, offering longer protection. Contact fungicides remain on the surface of the leaf blade. For curative action against an active outbreak, a systemic product is typically more effective, but a combination of chemical classes in a rotation schedule offers the best long-term management. Always follow the manufacturer’s label instructions, paying close attention to application rates, watering requirements, and the interval between treatments.
Long-Term Prevention and Maintenance
Establishing a long-term strategy for brown patch prevention involves addressing underlying soil and turf health issues. Improving soil drainage is a significant step, often achieved through annual core aeration to relieve soil compaction and allow better air and water movement in the root zone. In areas with heavy clay soil, topdressing with compost or sand can gradually improve the soil structure.
Managing the thatch layer is also important. Excessive thatch—the layer of dead and living organic material between the soil and the grass blades—can harbor the fungus and retain moisture. Dethatching or verticutting reduces this layer, improving air circulation and water penetration. For persistent problems, selecting turfgrass varieties with documented resistance during overseeding or renovation is the most effective non-chemical solution.