Summer patch, Magnaporthe poae, and necrotic ring spot, Leptosphaeria korrae, are two separate diseases which attack turfgrass roots and previously were referred to as fusarium blight. Research continues to look for information on these diseases. Brown patch, Rhizoctonia solani, may also attack lawns. These "patch diseases" are similar in appearance and management.
Summer patch and brown patch tend to be most active in hot weather, while necrotic ring spot tends to be most active in late spring and in fall. Disease symptoms often show under lawn stress in summer, however. Crescent shaped or circular patches of dead grass, often with clumps of green grass inside, are a characteristic symptom (called frogeye). Lawns with advanced disease development may show irregular dead areas and streaks.
The best way to know if you are dealing with a patch disease and what type is to submit a sample to the University of Illinois Plant Clinic for lab testing.