Brown patch is a turfgrass blight caused by the fungus Rhizocotonia solani.The disease usually appears during long periods of hot, wet, humid weather and can kill most species of cultivated turfgrass. Large blighted areas can appear within two days of infection.
Although it does not produce spores or mushrooms, Rhizoctonia solani is classified as a basidiomycete, a plant family that includes mushroom and club fungi. It survives by forming compact masses of sterile mycelia called sclerotia. Brown patch thrives on dead and decaying organic matter but will form on living plant tissue.
During warm, humid mornings, irregular patches of blight a few inches to 2 feet wide appear on turfgrass that is cut short. There may be a few blades of green grass in these patches; they may have a dark purplish gray border sometimes called "smoke ring."
If the grass is cut higher, the fungus may appear as larger circles 2 to 3 feet wide or there will be a general thinning in irregularly shaped areas. Leaves in these areas are gray and look dirty and rundown. Cool season turfgrass cut high may have patches of infection in which a circle of dead grass surrounds healthy grass. These patches are known as "frog eye."
Identifying Similar Fungi
The grower needs to be able to distinguish brown patch from dollar spot and pythium, two other common fungal infections that produce similar symptoms.
Dollar spot is light tan and affects the entire leaf blade in an hourglass shape. The hyphae or branches of brown patch mycelium divide at a 90 degree angle; dollar spot hyphae divide at smaller angles.
Brown patch hyphae are septate, meaning they are divided into small sections strung end to end. Pythium hyphae are non-septate.
The disease forms after at least 10 consecutive hours of wet leaves or humidity above 95 percent. Cultural practices for controlling brown patch are aimed at preventing these conditions.
Avoid applying excessive nitrogen fertilizer in warm, wet weather. Nitrogen produces soft, lush turf that brown patch likes.The increased growth removes the movement of air in the turf, increasing leaf wetness and humidity.
Irrigate early in the morning to removes large droplets of dew that evaporate slowly. Do not irrigate several hours before sunset; mowing then means the grass will stay wet longer. Mow the turf early in the morning to remove grass that is wet with dew.
Drag a hose, rope, chain or pole across the wet morning grass to remove the dew. This is called "poling" or "whipping."
Install soil drainage systems to prevent soil from compacting and thatch from accumulating. Prune or remove surrounding trees and shrubs to give turfgrass maximum sun and air.
The American Phytopathological Society recommends treating brown patch with fungicides containing the active ingredients benzimidazole, carbamate, carboximide, DMI, nitrile and Qo inhibitors. Horticulturalists at Cornell University recommend fungicides containing myclobutanil, quintozene, propiconazole, triadimefon or thiram.
Disease Resistant Turfgrass
The ability of turfgrass species to resist brown patch varies greatly. Kentucky bluegrass and fine fescue turfgrass are generally more resistant than perennial ryegrass or tall fescue. To learn the turfgrass species with the best resistance in your area, contact your county extension service or contact the National Turfgrass Evaluation Program, ntep.org.