Brown Patch disease is sometimes responsible for poor turf quality, but it is not the only cause of brown spots or bare patches in lawns. You may need to consider other possible causes of thinning or dead grass. Symptoms Areas affected by Brown Patch are initially roughly circular, varying in size from one to five feet or more. As an outbreak progresses and diseased patches coalesce, affected areas may lose the circular appearance and become irregular or diffuse. Brown patch in perennial ryegrass causes blades to wither and collapse.
Lesions initially are dark green or grayish green but quickly become tan as decayed leaves dry.
In Kentucky bluegrass, infected leaves exhibit elongated, irregular, tan lesions which are surrounded by a yellow or brown border. Figure 1 Figure 2 Disease Cycle Brown Patch is caused by infection of grass foliage and crowns by Rhizoctonia fungi. Rhizoctonia solani is a very common soilborne fungus and is the cause of Brown Patch symptoms in most instances. Rhizoctonia zeae can also cause Brown Patch in tall fescue under very hot, humid conditions. When environmental conditions are favorable for growth, the sclerotia germinate and produce cobwebby fungal mycelium, which is the active phase of the fungi.