Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma. Oklahoma Cooperative Extension Service.

CAUSAL ORGANISM: Brown patch is caused by the soil-inhabiting fungus Rhizoctonia solani and other rhizoctonia species.

HOSTS: Bentgrass, ryegrass, tall fescue and St. Augustinegrass; and occasionally hybrid bermudagrass and zoysiagrass.

SYMPTOMS: Brown patches up to three feet in diameter develop with dead leaves remaining upright on closely cut turfgrass (golf greens) and matted down on higher cut turfgrass (Figure 1). Leaves first take on a dark color, then wilt and turn brown. In early morning on closely cut turfgrass, a dark, "smoke ring" may appear at the edge of rapidly developing patches when humidity and temperatures are high (Figure 2). These symptoms disappear as the day progresses so that, by late morning, the patch appears a uniformly light brown or straw color. Symptoms on high cut turfgrass may resemble a ring with centrally located tufts of healthy grass observed within the patch.

Cool-weather strains of these fungi may also cause diseases from autumn through spring. The symptoms vary but are often expressed as light brown to yellow rings or patches on closely cut turfgrass. These symptoms have been called cool-weather brown patch, winter brown patch, or yellow patch.

DISEASE CYCLE: Rhizoctonia species are found in all soils and survive periods unfavorable for growth as dormant structures in plant debris. They can begin growth over a wide range of temperatures, depending on the different species involved. When favorable conditions exist, the fungi spreads radially in the upper soil or thatch to form a roughly circular colony. During humid, warm conditions, the fungi grow onto moist, turfgrass leaves and sheaths. The infected plant tissues lose their cellular integrity, becoming water-soaked and dark. When exposed to sunlight or drying wind, the infected spots or entire leaves die rapidly, shrivel and turn brown.

FACTORS FAVORING DISEASE DEVELOPMENT: Brown patch usually occurs in hot, humid weather when night temperatures are above 60°F and foliage remains wet for prolonged periods. "Winter" brown patch, which occurs on golf greens from autumn through spring, is favored by cold, wet conditions.
Poor soil drainage, lack of air movement, cloudy weather, heavy dew, overwatering and watering in late afternoon favor prolonged leaf wetness and increased disease severity. The application of high rates of nitrogen and/or deficiencies of phosphorus and potassium, especially when weather conditions are favorable for brown patch, can increase disease severity. Excessive thatch, mowing when wet, and leaf fraying by dull mower blades can also enhance the severity of brown patch.

CONTROL: When environmental conditions favor disease avoid application of excessive rates of nitrogen. Fertilizer should be applied judiciously, and adequate amounts of phosphorus and potassium are essential to ensure the highest possible levels of plant resistance. In general, cool-season turfgrasses should not receive more than one pound of actual nitrogen per 1000 square feet at any one time. Use very low rates or avoid applying nitrogen in late spring or summer to cool-season turfgrasses. Ensure adequate amounts of phosphorus and potassium by applying these nutrients based on soil test results.

Reduce prolonged leaf wetness by watering infrequently to a depth of 6 inches to 8 inches and at a time when the foliage is likely to dry quickly. Avoid watering in late afternoon and evening, and allow for better air movement by removing unwanted vegetation and selectively pruning trees and shrubs. Removal of morning dew reduces prolonged leaf wetness and exudates that favor disease development. This can be accomplished by dragging a hose across the turfgrass or by running the irrigation system for a short time period. Good surface and soil drainage must be present to reduce disease incidence.

Make sure mower blades are sharp to reduce the amount of wounded turfgrass in which the fungus can enter the plant. Collect and promptly dispose of clippings on infected areas or when conditions favor disease development. Avoid mowing turfgrass when wet, and do not mow too low so that the turfgrass will be better able to resist the disease.

Golf greens should be regularly core cultivated and topdressed to reduce thatch buildup. Applications of effective fungicides, when the first disease symptoms appear, will give good control of brown patch on highly maintained turfgrass. A preventive fungicide program should be considered on golf greens when conditions are favorable for disease development.

Figure 2. -- "Brown patch with 'smoke ring' around perimeter of patch", Courtesy of M. Kenna