This fungal disease is caused by a Rhizoctonia species. It commonly occurs in hot, muggy weather when night temperatures are at least 70 F and daytime temperatures are in the 80s and 90s. It is favored by heavy rains or watering and by grass that is dense and at least adequately fertilized.
Brown patch is easier to diagnose in the field than in the lab.
Symptoms usually stand out most vividly in hot, moist, overcast weather. The disease appears in patches, up to 2 or 3 feet across. These patches may be dark blue initially, as though turf is under drought stress. The color quickly changes to purple-brown, then light brown. Patches may develop green centers and resemble summer patch and necrotic ring spot.