Brown patches in lawns are commonly caused by the fungi called rhizoctonia, which has been dubbed the "brown patch disease." Although rhizoctonia is the common cause of brown patches in the lawn, it is not the only one out there. Environment as well as animal activity will cause similar symptoms as rhizoctonia.
The fungal pathogen Rhizoctonia solani is the cause of many brown patches in turf grass. According to Purdue University, symptoms of brown patch may appear overnight and is common in cool season turf grass species. Symptoms include the thinning of grass and circular olive-green stains, 4 to 12 inches in diameter with a gray or black ring around them, often called smoke rings. Brown patch is a summer disease, appearing once the fungal pathogen is activated by heat and humidity. Control includes cultivation practices, such as adding extra nitrogen to the turf to increase green blades and turf thickness, as well as application of a fungicide containing DMI.
Fertilization and Watering
Brown patches are often caused by poor cultivation practices. Overfertilizing a lawn may cause patches of grass to burn, which looks similar to the brown patch disease. Absence of a smoke ring helps indicate the patch is not fungal in origin. Overwatering a lawn will create brown patches also, especially if the water causes flooding of the lawn, which may cause root damage. Watering of a lawn is required only after the dew has dried in the morning, never at night, as this causes overgrowth and patchiness.
Brown patches may occur in localized dry patches in the lawn. Checking the moisture of soil under the brown patch and comparing it to other sections of the lawn will help determine whether the soil is dryer in that area.
Pets and Insects
Dog feces and urine will also cause brown patches if the dog prefers one area of the lawn to others.
Insects may cause small brown patches as well by causing a thinning of the lawn in one area. Such issues may be solved by the proper cultivation practices like mowing grass to its correct height (between 3 and 4 inches), proper watering and fertilizers, aeration and thatch removal.