Necrotic Ring Spot
Necrotic ring spot first appears as small patches 6 in. to 1 ft (15 cm to 0.3 m) in diameter. When the fungus is actively attacking the Kentucky bluegrass plants in the fall of the year, dark red blades of grass can be seen in the patches. The pathogen attacks root systems in the spring and fall, and in the summer, infected plants begin to wilt in patches. Patches eventually turn straw colored, and older patches (2 yrs +) appear as a "frog-eye" with a center of healthy turf surrounded by a ring of dead turf.
Necrotic ring spot occurs in the cool regions of the world where Kentucky bluegrass (a primary host) is grown. The fungus achieves maximum growth at temperatures between 68° and 82° F (20° and 28° C). The pathogen is active in the cool weather of the spring and fall, even though symptoms are seen in the warm weather of the summer. The fungus is thought to move among the turf stand by growing along the surface of roots and rhizomes.