Integrated Pest Management

Necrotic Ring Spot

Necrotic ring spot (Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org)

Necrotic ring spot (Howard F. Schwartz, Colorado State University, Bugwood.org)

main host is Kentucky bluegrass but annual bluegrass and red fescue can also be infected
no spot or lesions on leaves; blackening of roots and stems; occasionally black fruiting bodies are found on infected tissue; roots are rotten

circular patches develop during cool wet weather that gradually enlarge to about 12 inches in diameter; during drought the spot can get as big as 2–3 feet
the fungus becomes active when the temperature at 3-inch depth reaches 60-70 degrees F on average
occurs throughout the growing season

Control of necrotic ring spot is the same as for summer patch:

Avoid drought stress.
During hot weather, light daily irrigation is necessary to reduce heat stress.
Maintain balanced and adequate fertility, especially nitrogen, potassium and phosphorus.
There are some cultivars that have shown some resistance in trials on the east coast. The National Turf Evaluation Program website lists the trials here.
Overseeding with perennial rye resolves the problem as perennial rye is resistant to the pathogen; other resistant cultivars are available in a variety of species.
Apply a fungicide (azoxystrobin; propiconazole; thiophanate-methyl) for control. Most products need to be applied by a commercial applicator.
Fungicide can be applied in the spring, but it needs to be applied before the fungus enters the roots (when the average soil temperature at 3 inches depth is between 60-70 degrees F).