Prior to the mid 1980's, these two diseases were part of an unexplained disease complex called Fusarium blight. We now know that necrotic ring spot (caused by Ophiosphaerella korrae) and summer patch (caused by Magnaporthe poae) are two distinct root rot diseases that can develop in the same turf area. They are widespread diseases on established Kentucky bluegrass turfs that are managed as amenity or sports turf. Other grasses susceptible to these diseases include annual bluegrass and fine-leaf fescues.
Summer patch and necrotic ring spot symptoms are virtually indistinguishable and both diseases are difficult to control. Kentucky bluegrass is generally not affected until the second or third year on newly laid sod and usually not for 4 years or more after seeding. First symptoms are scattered. light-green patches, typically 2 to 6 inches in diameter. In warm to hot weather, they soon enlarge and rapidly fade to a dull reddish brown, then a light tan, and finally to a light straw color. The patches may become elongated streaks, crescents, or roughly circular, and are 1 to 3 feet in diameter. Within the areas of dead or stunted grass there are often centers of green, apparently healthy grass resulting in characteristic "frog-eye" or "doughnut" patterns. The circular patches tend to increase in size for several years (in some cases up to seven years) and then disappear. The dead turf is commonly invaded by weeds. Diagnosis of this disease is difficult, and it is suggested that samples be sent to the Plant Clinic.
Compared to summer patch, the necrotic ring spot pathogen is more aggressive (less dependent on stressing factors) and the symptoms are often observed under cooler temperatures. Summer patch can become severe when turfgrass, especially Kentucky bluegrass, is under stress and entering summer dormancy. However, initial root infections occur in the spring when soil temperatures stabilize at 65 to 70 F at the 2-inch depth. The following stressful conditions cause summer patch to be more severe: prolonged periods of high humidity and warm to hot weather, moisture and heat stress, excessive watering, close mowing, a thatch exceeding 3/4 inch thick, soil pH above 7 or below 5, compaction, nematodes, and unbalanced applications of fertilizer.
Avoid pure stands of very susceptible grass varieties. Renovation or overseeding of affected areas with species mixtures or blends of resistant varieties is the most cost-effective way to minimize these diseases. Follow the cultural practices listed in Chapter 1 to reduce stress and promote root development and steady growth. In particular, the following practices are most beneficial: 1) core aerification, 2) increase the mowing height as high as the turf species and use will allow, 3) irrigate deeply, and 4) maintain a soil pH of 5.5 to 6.0, and syringe the turf (brief, mid-day sprinkling during very hot days) to reduce heat stress and prevent wilt symptoms.
Where cultural practices do not adequately control these diseases a fungicide application in the autumn followed by a spring application may be justified. Each application should be made when the average soil temperature (2 inch depth under sod at 10:00 am) reaches 68-70 F. Water the turf deeply the day before application. Since the effective fungicides can not move down in the plant to protect root system you will need to drench the fungicide into the soil immediately after the application using 0.5 to 1.0 inches (300 to 600 gal) of water per 1,000 square feet.